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HANDBOOK ONINVESTMENT CASTING

THE LOST WAX CASTING PROCESS FOR CARAT GOLD JEWELLERY MANUFACTURE

WORLD GOLD COUNCIL

HANDBOOK ONINVESTMENT CASTING

THE LOST WAX CASTING PROCESS FOR CARAT GOLD JEWELLERY MANUFACTURE

By Valerio FaccendaConsultant to World Gold Council

with Chapter 3 written by Dieter OttFormerly at FEM, Schwäbisch Gmünd, Germany

WORLD GOLD COUNCIL

Copyright © 2003 by World Gold Council, London

Publication Date: May 2003

Published by World Gold Council, International Technology,

45 Pall Mall, London SW1Y 5JG, United Kingdom

Telephone: +44 20 7930 5171. Fax: +44 20 7839 6561

E-mail: [email protected]

www.gold.org

Produced by Dr Valerio Faccenda, Aosta, Italy

Editor: Dr Christopher W. Corti

Translated by Professor Giovanni Baralis, Turin, Italy

Originated and printed by Trait Design Limited

Note: Whilst every care has been taken in the preparation of this publication, World Gold Council cannot be responsible

for the accuracy of any statement or representation made or the consequences arising from the use of information

contained in it. The Handbook is intended solely as a general resource for practising professionals in the field and

specialist advice should be obtained wherever necessary.

It is always important to use appropriate and approved health and safety procedures.

All rights reserved. No part of this publication may be reproduced, translated, stored in a retrieval system, or transmitted

in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior permission in

writing of the copyright holder.

3 Alloys for Investment Casting 593.1 Yellow and red gold alloys 59

3.1.1. Metallurgy and its effects on physicalproperties 55

3.1.2 High carat golds with enhanced properties 64

3.2 White gold alloys 643.3 Influence of small alloying additions 67

3.3.1 Improving properties 673.3.2 Effect of individual additions 68

4 Equipment 754.1 Vulcanisers 764.2 Wax injectors 774.3 Investing machines 784.4 Dewaxers 794.5 Burnout ovens 794.6 Melting/casting machines 81

4.6.1 Comparison between centrifugal and static machines 81

4.6.2 Centrifugal machines 824.6.3 Static machines 83

5 Sources of equipment andconsumables 89

6 Further reading 97

7 Acknowledgements 102

8 World Gold Council technical

publications 103

Preface 6

Glossary 7

1 Introduction 131.1 Development of the modern process 131.2 The modern process and product quality 141.3 Choice of equipment and consumables 151.4 Health and safety 16

2 The process of investment casting 192.1 Design 202.2 Making the master model 21

2.2.1 Alloy of manufacture 212.2.2 Feed sprue 21

2.3 Making the rubber mould 232.3.1 Types of mould rubber 242.3.2 Making the mould 252.3.3 Cutting the mould 262.3.4 Storing and using the mould 282.3.5 Common problems 29

2.4 Production of the wax patterns 292.4.1 Types of waxes 292.4.2 Wax injection 302.4.3 Common problems 33

2.5 Assembling the tree 332.5.1 Bases and sprues 332.5.2 Tree design 35

2.6 Investing the mould 362.6.1 Flasks 36

2.6.2 Investment powders 362.6.3 Safety and storage of investment

powders 372.6.4 Checking the condition of the

investment: the ‘gloss-off’ test 382.6.5 Mixing the investment 39

2.7 Dewaxing the flask 412.8 Burnout 42

2.8.1 The burnout cycle 422.8.2 Behaviour of calcium sulphate-bonded

investment during burnout 442.9 Melting 452.10 Casting 46

2.10.1 Test for system temperature 48 2.10.2 Inspection criteria 492.10.3 Test for best feed sprue design 502.10.4 Casting with stones in place 51

2.11 Cooling and recovery of cast items 522.12 Summary of the basic guidelines for each

step of the process 532.13 Schematic list of possible defects 56

C O N T E N T S

6 Handbook on Investment Casting

PREFACEInvestment (or Lost Wax) Casting is one of the earliest processes developed by man,

dating back 6,000 years or more. Today, it is the most widely used process in

jewellery manufacture but probably the least understood by practitioners of the art.

It comprises a series of process steps, each of which must be performed properly, if

good castings are to result. It never ceases to surprise me just how many casters do

not realize what quality of casting it is possible to achieve consistently, if each process

step is done carefully in a controlled manner.

There are comparatively few good manuals on investment casting. Many date

back some years and focus on centrifugal casting. Our first WGC technical manual,

the Investment Casting Manual, was published in 1995 and has proved popular. Since

then, there have been substantial developments in the technology and our

understanding of the process. Thus, we considered it timely to update it, particularly

as stocks of the original are running out. This Handbook is the result.

It has given me great pleasure to work with Valerio Faccenda and Dieter Ott

(Chapter 3) in the production of this Handbook. Both Valerio and Dieter are well

known to many of you as experts in jewellery technology, especially in investment

casting, with each contributing several articles to Gold Technology magazine over

the years and presenting at several WGC International Technology Symposia at

Vicenza, Italy. Valerio, as a technical consultant to World Gold Council, has also

presented at many WGC technical seminars in countries around the world. He is, of

course, author of the Finishing Handbook. Dieter has made major contributions to

our understanding of the Investment Casting process and to the metallurgy of the

carat gold alloys and is author of the Handbook on Casting and Other Defects which

complements this Handbook. Both have presented at the prestigious Santa Fe

Symposia, Dieter on many occasions. I know that this Handbook will become a classic

in the jewellery field and meets a demand for a good comprehensive and

authoritative book on the subject. I know you will find it useful and enjoyable. I must

also mention Giovanni Baralis who translated this Handbook from Italian into English.

Whilst known to relatively few of you, Giovanni has been responsible for translating

Gold Technology into Italian over many years. He certainly makes my job easier.

This Handbook is the seventh in the range of technical publications published by

World Gold Council. These are designed to assist the manufacturing jeweller and

goldsmith to use the optimum technology and best practice in the production of

jewellery, thereby improving quality of the product, reducing defects and process

time which, in turn, results in lower costs of manufacture. We believe that it is

important for the practising jeweller to understand the technology underpinning his

or her materials and processes if he or she is to achieve consistent good quality. That

is one aim of these Manuals and Handbooks – not only to provide good basic

guidelines and procedures but to explain, in simple terms, why they are important

and how they impact quality. Armed with such knowledge, a jeweller should be

better able to solve problems that will inevitably arise from time to time.

Christopher W. Corti, London, April 2003

Handbook on Investment Casting 7

GLOSSARYNote: Certain technical terms exist in two spellings (e.g. carat or karat, mould or mold), reflectingEnglish and American common usage. In this Handbook, the English versions have been used,although both are given in this Glossary.

Accelerator: Compound which speeds up setting of investment, mainly to increase the productivity.In general it is based on crystalline substances such as sodium chloride, sodium citrate, Rochelle salt.

Alloy: A combination of two or more metals, usually prepared by melting them together. They aredesigned to have certain desired properties, e.g. strength, hardness, ductility, colour, etc.

Annealing: Restoration of softness and ductility to metals and alloys after cold working by heatingto a temperature that promotes recrystallisation.

Assay: The testing of items to determine their precious metal content, e.g. by fire assay or otheranalytical technique.

Base metal: The non precious metals in a jewellery alloy. For instance in a gold-silver-copper-zincalloy, copper and zinc are the base metals.

Binder: A substance used to hold together the investment powder, e.g. for casting jewellery, this canbe the Plaster of Paris (q.v.) or acid phosphate.

Burnout: The firing of the invested flask at temperature in an oven after dewaxing (q.v.), to conditionthe mould for casting and to completely eliminate any residual wax or other model materials.

CAD/Computer Assisted Design: A sophisticated software system for bi-dimensional or three-dimensional designing.

CAM/Computer Assisted Machining: A software system for automated machining of acomponent, driven by computer software, typically from a CAD system.

Carat/Karat: A unit for designating the fineness of gold alloys, based on an arbitrary division intotwenty-four carats. Pure gold is 24 carat or 100% pure. A 75% gold alloy is 18 carat and so on. (Thecarat is also a unit of weight for gemstones, equal to 0.2 grams).

Carat/Karat gold: A gold alloy which conforms to national or international standards of fineness andcan be legally marked or hallmarked.

Castability: The ability of a molten alloy to be poured into a mould, retaining sufficient fluidity to fillthe mould completely and to take up an accurate impression of the details of the mould cavity.

Casting: This word can have two different meanings: 1) the process of pouring a molten metal in amould; 2) the metallic object taken out from the mould, after solidification of the cast metal.

Casting grain: Metals or, more usually, alloys prepared for melting and subsequent casting bydividing the charge material into small particles (like gravel) by pouring a melt into water to form shotor grains.

Casting temperature: Temperature at which power is switched off and the molten alloy is pouredinto the mould.

Centrifugal casting: A method for casting metals in which the molten metal is driven by centrifugaland tangential forces from the crucible into a heated mould whilst both are rapidly rotated.

Chilling factor: Cooling capacity of a mould calculated from volume specific heat of the mouldmaterial and the mould/melt temperature difference. Value for gypsum binder - low; for silica -medium; for cold copper - very high.

Cold work/working: Deformation of a piece of metal or alloy to effect a change in shape attemperatures sufficiently below the annealing temperature to cause work (strain)-hardening, usuallywith a loss in residual ductility. The amount of cold work imparted is often measured in terms ofreduction in cross-sectional area (e.g. wire drawing) or in thickness (e.g. rolling of strip).

Cristobalite: The highest temperature phase of silica, stable and with high strength retention from1470°C (2678°F) to the melting point, 1700°C (3092°F).

De-airing: Removal of air bubbles from an investment slurry, to avoid bubble defects on the finalcasting. Assisted by vibration and/or vacuum.

Devesting: Separation of the cast tree from the refractory mould. This can be done by quenchingthe flask in water or by hammering or with high pressure water jets, depending on the refractory type.

Dewaxing: The removal of the largest part of the wax from the invested mould. It can be done dry,in an oven, or wet, with steam.


Dross: The scum that forms on the surface of molten metals, due largely to oxidation, but sometimesalso to the rising of impurities and inclusions to the surface.

Feeding: The necessary process of introducing molten metal through suitable channels (the sprues)and into the cavities of the mould to fill them and to compensate for contraction (shrinkage) ascastings solidify. Can be gravity assisted, or otherwise pressurised.

Feed sprue: A system of wax rods connecting the central sprue to the pattern to be cast. It formsthe channel for the melt to fill the mould cavity. It should be kept as short as possible and must notfreeze prematurely. Its junction with the pattern is called the “gate”.

Fineness: Precious metal content expressed in parts per thousands (‰). 18 carat is 750 fineness.

Flask: The outer metal container of an investment casting mould, used from the investment processthrough to extraction of the cast tree. It is available in standard sizes and reusable. It may be a solidcylinder or a cylinder perforated with holes to allow escape of air from the mould under vacuum.

Fluidity: Complex property describing the ability of a molten alloy to run into a mould and take upan accurate impression of the mould cavity. It generally increases with superheat, freedom fromoxidation and some alloying additions (such as zinc & silicon).

Flux: Inorganic mixture applied to melt surface to protect the melt from oxidation. It should melt ata temperature lower than melting temperature of the alloy.

Form filling: The ability of a molten metal to fill the mould cavity completely.

Fuel gas to oxygen ratio: The volumetric flow ratio matching the molecular ratio for completecombustion. With a hydrogen/oxygen flame a ratio of 2 gives a neutral flame with a sharp inner cone.A lower ratio gives an oxidising flame; a higher, a reducing flame.

Furnace: See Oven

Gate: The part of the feed system that controls the flow of metal from the feed sprue to the pattern.When it freezes, it is closed and no more metal can pass that point into the pattern.

Gloss time /Gloss-off time: The time between the addition of the investment powder to the waterand the moment where the slurry surface loses its “gloss”. It denotes the start of setting of theinvestment.

Gloss-off test: A test for determining the gloss-off time of a batch of investment powder. Useful indefining or eliminating possible causes of casting problems and defects.

Grain: See “casting grain”. It can also refer to the tiny crystals - or “grains” - forming the bulk ormicrostructure of a metal or alloy.

Graining: The process of preparing casting grain, normally by pouring the molten alloy into water.

Grain control/ grain size control: The metallurgical procedure to keep the grain (crystal) size of analloy under control, by addition of particular metals or compounds (grain refiners, q.v.).

Grain refiner: An addition of suitable metals or compounds to control the grain size of an alloyduring solidification or annealing (recrystallisation).

Grain size: Dimension of the crystalline grain of metals and alloys. In jewellery alloys, a fine grain sizeis usually preferred.

Gypsum: Calcium sulphate, used as a binder in investment.

Gypsum-bonded (investment): The traditional refractory investment based on silica powderbonded with Plaster of Paris (selected hemihydrated calcium sulphate) mainly used by jewelleryindustry for investment casting of gold alloys.

Hallmarking: The stamping of precious metal articles by an independent assay office to show thefineness of that article. Term derives from the U.K. for marks applied by Goldsmiths Hall -’marked bythe Hall’. Term is often used loosely to describe a mark applied by a manufacturer to show fineness innon-hallmarking countries.

Heat treatment: A treatment given to metals and alloys, involving a combination of temperature,time, heating and cooling, to effect a change in microstructure and other properties.

Hot shortness: Brittleness at high temperature, often intergranular, caused by either low meltingpoint segregates or other non-ductile grain boundary constituents.

Hygroscopic: A material possessing a marked ability to absorb water vapour from the atmosphere.Some compounds can react with atmospheric water vapour to form new compounds (e.g. calciumsulphate hemihydrate forms calcium sulphate dihydrate). Gypsum-bonded investment is hygroscopicand should not be left exposed to the atmosphere.


Inclusion: Non-metallic particle that is found in a metallic body. It can be generated from fragmentsof extraneous materials (e.g. refractory from furnace crucible or mould) or from the reaction of themetal with foreign materials (e.g. atmospheric oxygen, sulphur compounds generated frominvestment reaction, etc.).

Induction melting: Heating to above the melting point by generating eddy currents within aconducting material surrounded by a water-cooled copper coil carrying an alternating current at low(<150 Hz), medium (>500 Hz) or high (>100 kHz) frequency. Also creates a stirring effect in the meltby induced electromagnetic forces.

Investment/Investment powder/Investment mould: The investment is a mixture of fine silicapowder and a binder, formulated to withstand the high temperatures of burnout and casting. For theinvestment casting of gold jewellery, the commonest binder is gypsum, in its hemihydrated form(Plaster of Paris). Besides these main ingredients, commercial investment contains small amounts ofother chemicals (modifiers), designed to impart to the investment the required characteristics foroptimum performance. When mixed with water to form a slurry, the binder undergoes a hydrationreaction (like cement) to set the investment into a solid mould.

Liquidus temperature: The temperature above which an alloy is completely liquid, i.e. no moresolid metal is present. Below liquidus temperature there is an increasing proportion of solid phase untilat the solidus temperature no liquid remains in equilibrium.

Lost wax: Original name for investment casting. A wax model (or pattern) forms the cavity in theinvestment. Then the wax is melted out before firing of the mould. So the wax is "lost", from whichthe name of the process derives.

Master alloy: A premixed metal alloy (q.v.) that is added to fine gold to produce the final carat goldalloy. Generally contains silver and copper with other additions, e.g. zinc, nickel, palladium, deoxidisersand grain refiners.

Master model: The master model is the reference model for the design and can be made of wax orplastic or metal. Nickel silver or silver alloys are frequently used. Metal models can be rhodium platedto improve wear and corrosion resistance. CAD/CAM systems can also be used to produce mastermodels.

Melting range: The temperature interval between the solidus and liquidus temperatures (see Solidustemperature and Liquidus temperature).

Mould/Mold: A hollow object, containing a cavity that is the outer form of the piece(s) to bereproduced by wax injection or by metal casting. In the case of investment casting, the mould can bemade of various materials, e.g.: metal, rubber (for wax patterns) or refractory investment (for casting).

Mould clamp: A pneumatic device for maintaining a constant clamping pressure to a rubber mouldduring wax injection.

Mould/Mold Frame: A metal frame, usually rectangular (but can be circular), used to contain therubber layers and master model during production of the rubber mould in the vulcanising press.

Negative tolerance: Used in the context of standards of fineness. It implies a small allowance inprecious metal content below the specified minimum that is acceptable in some countries.

Oven: A furnace where a controlled and relatively uniform temperature can be held for the requiredlength of time. It can be heated by combustion of a suitable fuel (e.g. natural gas, propane, etc.) orby electrical resistance elements. The temperature is controlled through suitable regulators. Forburnout, the oven should be of the muffle type with a large volume to contain several flasks. It maybe fan-assisted and/or have a rotary hearth to aid temperature uniformity.

Overheating: When the temperature of the material becomes too high. Not to be confused withsuperheat (q.v.). Overheating is an unwanted and potentially detrimental occurrence. The overheatedmaterial can begin to decompose or react with other materials into which it comes in contact.Overheated melts can oxidise more readily.

Pasty zone: The pasty zone corresponds to the temperature range between the liquidus and thesolidus (q.v.). In this temperature range the metal is not fully liquid nor fully solid. It is in a "pasty" state.Compensating shrinkage by feeding liquid alloy under these conditions may be difficult. Pure metalsand eutectic alloys do not show a pasty zone.

Pattern: A master (usually metal) or consumable (lost wax process) model of a component that is tobe reproduced by casting. Pattern dimensions may need to allow for net shrinkage or expansion overthe whole casting process.

Phosphate-bonded (investment): Investment with acid-phosphate and magnesia, which first gelssilica flour and then bonds it by subsequent dehydration. It is used preferably for high melting pointalloys, e.g. palladium white gold and platinum.

Pickling: The process of dissolving away surface oxides and flux by immersion in a suitable dilute acidbath (‘pickle’). Normally used for cleaning cast trees, soldered or welded parts or scrap (before melting).


Plaster of Paris: White powder of calcium sulphate hemihydrate (2CaSO4.H2O). It is obtained bysuitable heat treatment at relatively low temperature of the mineral gypsum (calcium sulphatedihydrate - CaSO4.2H2O). It reacts with water to form the more stable dihydrate. This reaction is usedfor investment setting.

Porosity: Network of holes in castings, often at the surface, caused by entrapped or dissolved gas orby shrinkage on solidification.

Protective atmosphere: An oxygen-free or low oxygen gas atmosphere used to protect a materialfrom oxidation during melting, casting, soldering, welding or heat treatment.

Quenching: Fast cooling of a hot material by rapid immersion in a suitable fluid, such as water, oil, oreven air or other fluid mixture. The quenchant is usually water for carat gold alloys.

Rapid prototyping: Modern technique for producing prototypes with automated machines drivenby CAD/CAM systems. Many quite different techniques of rapid prototyping have been developed. Amodern method for producing a master model.

Reducing flame: A torch flame with excess fuel gas in comparison with available oxygen. Often usedfor shielding molten metal from oxidation.

Refractory: High melting point inorganic (ceramic) material used for furnace linings, crucibles ormoulds, usually based on graphite, oxides, nitrides or silicates. Often needs a suitable binder to holdthe refractory particles together. Preferably, it should also be resistant to thermal shock and chemicalattack.

Retarder: Many organic compounds and colloids retard the start of setting of gypsum-bondedinvestment. This increases the available working time (q.v.).

Scrap: Any redundant or reject metal/alloy from a manufacturing operation, that may be suitable forrecycling as feedstock to the primary operation.

Segregation: The non-uniform distribution or localized concentration of alloying elements,impurities or precipitates within the alloy microstructure, originating from solidification or heattreatment.

Setting time: The length of time the investment slurry requires to set, harden or cure.

Shot: See Casting grain.

Shrinkage: Volume contraction of a molten metal during solidification, typically about 5% for caratgolds. Can give rise to porosity in investment castings.

Silica: Silicon dioxide selectively processed for producing refractory and abrasive materials. Exists inthe vitreous state or as quartz, tridymite or cristobalite phases in equilibrium at increasingtemperatures.

Silicone rubber: A heat stable, flexible material containing organic radicals and silicon. Can be usedin the place of natural rubber for making rubber moulds. Can also be used for heat resistant sealinggaskets.

Silicosis: A serious lung disease caused by the inhalation of very fine silica (SiO2) particles. Precautionsmust be taken when handling investment powders.

Soaking: Holding the material in an oven at a constant temperature for the purpose of obtaining auniform temperature throughout the mass.

Solidus temperature: The temperature below which an alloy is completely solid, i.e. finishesfreezing on cooling or begins to melt on heating. Above the solidus temperature there is an increasingproportion of liquid phase until at the liquidus temperature no solid remains in equilibrium.

Spalling: The breakaway of the surface of the mould due to internal or external stresses, mechanicaland/or thermal. Can be a sign of a weak investment.

Sprue: Main, central pouring channel in the mould. It forms the stem of the tree and is connected tothe castings through the feed sprues (q.v.). It is obtained by melting the wax sprue used to build thewax tree (q.v.).

Sprue base: A pad, often of rubber, that makes a bottom for the flask during mould making. Thecone (or hemisphere) on a sprue base makes the recess that will be the pouring basin for the moltenmetal.

Superheat: Difference between the melting point / liquidus of an alloy and the casting temperature,required to allow the molten metal to fill the mould without premature freezing. Experience showsthat it should be as low as possible to avoid overheating (q.v.).

Third hand: Mechanical device, usually fixed to the workbench, to assist in rubber mould cutting.


Tree: See Wax Tree

Vacuum: A space in which the pressure is much lower than normal atmospheric pressure. Vacuumcan be applied to remove air from the mixed investment slurry or to "suck" molten metal in the flask.

Vulcanisation/Vulcanization: A chemical reaction of sulphur (or other vulcanising agent) withrubber to cause cross-linking of polymer chains. It increases strength and resiliency of the rubber.Performed as a step in making rubber moulds from the master model.

Vulcaniser/Vulcanizer: A piece of equipment used to carry out the vulcanisation, i.e. to produce arubber mould around a metal master model. Essentially, a press with heated platens.

Water blasting: Surface treatment in which high pressure water jets are used to remove theinvestment from a cast tree.

Water quality: The content of ionisable (dissolved) salts and organic matter in the water. Importantfor mixing of investment slurry, it should be accurately controlled, because it affects the working timeand gloss-off time (q.v.). Deionised water is the preferred water quality.

Water temperature: The temperature of the water mixed with investment powder. It should beaccurately controlled, because it affects the working time and gloss-off time (q.v.).

Wax: Any of a group of organic substances resembling beeswax. In general they are formed by estersof fatty acids with higher alcohols. Mixtures of different composition are used to obtain the requiredproperties for making patterns for lost wax casting (melting point, hardness, flexibility, etc.). Usually,the different wax types are differentiated by colour.

Wax injector/ Wax pot: Equipment containing molten wax under pressure for injecting into rubbermoulds to replicate the desired patterns. Often has a vacuum facility for removing air from the mouldprior to injection of wax.

Wax pattern: Wax replica of a master model, usually produced by injection of molten wax in a rubbermould. The solidified wax patterns are removed and used in the assembly of a wax tree, which is theninvested, to form an investment mould.

Wax tree: The assembly of wax patterns on a central sprue, from which the investment mould willbe made. Usually shaped like a tree, hence the name.

Wettability: The ability of a solid surface to be wetted when in contact with a liquid. Wettability ishigh when the liquid spreads over the surface. It is related to surface or interfacial tension.

Working time (investment): Time available for the preparation of the invested flask. It includes:mixing investment with water, de-airing, pouring the slurry in the flask and de-airing again. In thewhole it should be about 1 minute shorter than the gloss time (q.v.).

The oldest example of a gold investment casting: The Onager or wild ass, cast in electrum (the naturalalloy of gold-silver), part of the rein ring from thesledge-chariot of Queen Pu-Abi. From the royal tomb at Ur, Mesopotamia, dating to about 2,600 B.C.


1 INTRODUCTIONInvestment casting is probably the first process used by man for jewellery production,

dating back over 6,000 years. This happened long before man used the same process

for manufacturing weapons or other objects. Perhaps uniquely, investment casting is

the only manufacturing process that has been used first for jewellery production and

then subsequently for other production fields, like the mechanical engineering

industry.

Investment casting is also named lost wax casting: this latter name reminds us that

we start from a wax pattern that is invested with a refractory material to form a mould.

The wax pattern is then removed by melting (the wax is ‘lost’!) leaving a negative

impression in the mould, into which the molten metal is subsequently poured.

The word “investment” in the context of investment casting has nothing to do

with financial investment. It refers to the fact that the wax patterns are “invested”, i.e.

coated, with a refractory material. After setting of the refractory, the wax is melted

out and molten metal can be poured in the cavity that accurately reproduces the

shape and size of the wax pattern. The cast metal item accurately reproduces also

the fine details of the wax model.

1.1 DEVELOPMENT OF THE MODERN PROCESSAll past civilisations left us wonderful examples of investment cast jewellery. Such

jewellery specimens have been found in the treasures of the Pharaohs of Egypt and

in Atzec and Inca tombs of Central and South America. Also, in Europe, the ancient

Etruscans, the Greeks, Figure 1.1.1, the Romans and the Byzantines, Figure 1.1.2, left

us investment cast jewellery, and later, during the Renaissance, the great Masters

created wonderful masterpieces.

The starting point for the utilization of investment casting in industry has been the

application of the refractory investment in the form of a fluid slurry, invented near the

end of 1800. But, until the middle of the past century (I refer to the 20th century!),

investment casting has been used almost exclusively for the production of one-off

pieces for the very few persons who could afford it.

Around the middle of the past century, three major breakthroughs made

investment casting an industrial process, usable for mass production. The first

breakthrough has been connected to automatized chain making. Whilst this process

is not related to investment casting, it enabled production of large quantities of

jewellery (chain and bracelets) and favoured the access of jewellery to the field of

fashion and to an ever wider market.

The second breakthrough has been the invention of flexible rubber moulds, for the

mass production of wax patterns, by the Canadian, T.G. Jungersen. This invention was

rapidly patented in the USA, in 1944, Figures 1.1.3 and 1.1.4, and allowed goldsmiths to

reproduce intricate objects, with marked undercuts, without problems or limitations.

Finally, the third breakthrough has been the realization that the casting machines

developed for use in dentistry, with minor modifications, could be used also for the

industrial production of jewellery. These were spring-driven centrifugal casting

machines and explain why, even today, centrifugal casting machines are widely used

for jewellery production, in spite of the advent of static casting machines, particularly

in the last decade.

1I N T R O D U C T I O N

Figure 1.1.1 Greek ring, fourth century B.C.(Schmuckmuseum, Pforzheim)

Figure 1.1.2 Byzantine earring, sixth century A.D.(Schmuckmuseum, Pforzheim)

Figure 1.1.3 Patent for elastic rubber moulds,registered in USA in 1944


After flexible rubber moulds and casting machines were made available, a simple

optimisation of the consumable materials has been sufficient to allow a profitable

industrial utilisation of investment casting.

In particular, we refer to the wax and investment powder. The wax types used for

dental applications were too brittle and cracked easily during the extraction of the

wax pattern from the rubber mould, especially when marked undercuts were present

in the pattern. In this case, a correct balance of properties had to be sought, to

develop a product that could be used without particular problems.

The investment used in dentistry was too expensive for the goldsmith, who didn’t

need the high dimensional precision required for dental applications. Therefore, less

costly, but in no way inferior quality, investment types have been developed to meet

the requirements of the goldsmith. We refer here to silica-based, calcium sulphate-

bonded investment powder.

Since investment casting developed into an industrial process, it has become ever

more widely used. Today, we can say that at least 50% of jewellery worldwide is

produced by investment casting, as a result of the remarkable technical progress

made, whilst the ancient, time honoured basic concepts remain unchanged, Figures

1.1.5-1.1.7. As a result, investment casting has an aura of fascination, still preserving

the artistic and craft aspects of jewellery items.

1.2 THE MODERN PROCESS AND PRODUCT QUALITYInvestment casting is very versatile: both simple and intricate shapes can be

produced in small or large numbers. It is not costly: often, when we take into account

the cost of a good die, pieces that could be cold forged are more economically

produced by casting. However, investment casting is not a simple process. There are

many metallurgical principles we must consider and comply with in the many steps

of the process, if we are to obtain a good quality product. These steps are made

more complicated by the small size of the castings, which makes process control

somewhat difficult. Quite often, the goldsmith focuses his attention on the melting

and casting stages; these are only the final steps of a multi-stage process but, very

commonly, a defective or unsatisfactory product will be obtained, if all process steps

preceding the final ones have not been carried out correctly.

Some years ago, World Gold Council, with the Santa Fe Symposium, supported

research by the German Institute of Precious Metals into the defects occurring in the

production of jewellery pieces. This study showed that about 80% of defective

jewellery pieces had been produced by investment casting and that more than 50%

of the defects were due to porosity, a defect typical of the investment casting process.

The most important results of this research were collected together as case

studies in the Handbook on Casting and Other Defects, published by World Gold

Council, where the most common defect types are described, along with an

exhaustive explanation of their origin and useful recommendations for their

prevention. This Handbook is a very useful and essential complement to the present

Handbook, which is focused on the process.

Investment casting is a very ancient process; nevertheless, in its modern form it is

not an easy process to control. We mentioned that the small size of the castings we

want to produce is a problem. In Figure 1.2.1 we see the progress of solidification in

a ring with a large head. From the first to the last picture, only about 10 seconds have

elapsed. Solidification is completed in less than 1 minute. This experiment to observe

1 I N T R O D U C T I O N

Figure 1.1.4 Description of the mould and ofthe centrifugal wax injector, from the patent ofFigure 1.1.3

Figure 1.1.5 Modern investment castjewellery object: hinged pendant with clasp(Pomellato Spa, Italy)

Figure 1.1.6 Hinged bracelet: the single linkshave been investment cast (Pomellato Spa, Italy)

Figure 1.1.7 Investment cast pendants foryoung people. Their weight ranges from 1 to 3 g (Pomellato Spa, Italy)


the progress of solidification was conceptually very simple: molten metal has been

poured in the mould, and the liquid metal remaining after a pre-established time has

been removed by centrifuging. The shortest time has been about 1 second after

pouring. After centrifuging, the mould was opened and the amount of solidified

metal was evaluated.

These pictures show that the solidification process is very fast and, consequently,

its control is nearly impossible. Therefore, it is clear that the last steps of the overall

casting process should be carried out under the best possible conditions, in addition

to the correct execution of all preceding steps.

We would be foolish to believe that a completely automatised latest generation

melting/casting machine, centrifugal or static, with vacuum and pressure assist, can

compensate for carelessness in the preceding steps of the process. The machine will

help to achieve a consistent quality of the product, but it will never be able to attain

a good quality level, if errors have been made in the preceding steps of the process

or simple metallurgical principles have been ignored.

1.3 CHOICE OF EQUIPMENT AND CONSUMABLESThe modern goldsmith can choose from a wide range of equipment, from the

vulcanisers, wax injectors, investment mixers and burnout furnaces, to

melting/casting machines, which represent the largest capital investment.

With regard to melting/casting machines, two types of equipment are

commercially available that differ in the origin of the force that pushes the melt in

the mould: centrifugal machines and static machines. There are no special reasons to

prefer one type of machine to the other: both types can produce a high quality

product. The main differences between centrifugal machines and static machines will

be briefly summarized in Chapter 4, devoted to the equipment, but the final choice

should be made by the goldsmith, based on his needs.

This choice will depend on the amount of money he is willing to invest, on the

type and quantity of product to be produced and, particularly importantly, on the

level of technical after-sales service guaranteed by the supplier.

A fundamental consideration: a decision taken to purchase new equipment

because the current product shows too many defects can be a big mistake! Before

considering new equipment, it is absolutely necessary to make a thorough scrutiny of

the present production process. When (and only when) we are sure of obtaining the

best performance from the existing equipment, can we think to make an investment

in new equipment. At this time, when the market offers more and more automatised

equipment, there is the danger of committing the full responsibility for product

quality to the equipment. The results of such an attitude can be disastrous!

Therefore, the most important rule for achieving good results is always to engage

your brain and to scrutinize your current process constantly and accurately.

Investment casting should never be considered as a routine process. No detail of

the process should be neglected, even if, at first sight, it could appear unimportant.

In the course of the production process, the goldsmith uses not only equipment,

but also various consumable materials: the rubber for making the moulds, the wax

for the wax patterns, the investment for filling the flask and, lastly, the alloys.

All these materials are the outcome of careful study: they should be selected

and used correctly, carefully following the recommendations of the producer on

their use.


Figure 1.2.1 Development of solidification ina gold alloy ring: a - about 1 second aftermould filling

b - after 3 seconds

d - after 10 seconds

c - after 7 seconds

If the results are unsatisfactory, extemporaneous inventions should be avoided.

Please refrain from trying to transform your production workshop in a research

laboratory! There is the risk of a further worsening of the problem and of increasing

mental confusion! Time can be saved and results improved if we involve the

producers of the various materials directly in the problem: generally, the producer is

the first to be concerned about the results obtained by use of his product. Usually,

he will be able to detect possible errors and recommend suitable corrective action,

enabling you to save time and money.

1.4 HEALTH AND SAFETYWe have discussed the complex nature of the investment casting process and the

need to ensure the correct procedures are followed at each stage. There are health

and safety issues that need to be addressed. It is vital that the interests of the

workforce are protected if good quality and productivity are to be ensured. Some of

the materials may be hazardous or toxic. Of particular note is that related to handling

of investment powder and its removal after casting. This material causes silicosis!

Engineered control of investment dust or the use of a respirator, approved for silica

dust protection, is essential. Respirators must be properly fitted to each worker, who

should be trained in its care and use. Other hazards include hot metal handling,

electrical and chemical, etc. Suitable precautions must be taken, including provision

of protective clothing and implementation of rigorous safety procedures. These

issues will be discussed later in more detail.




2. THE PROCESS OFINVESTMENT CASTING

Investment casting is a typical example of a multistage process. We can list at least

13 separate steps from the initial design to the finishing of the jewellery:

1 – Design

2 – Making the master model

3 – Making the rubber mould

4 – Production of the wax patterns

5 – Assembling the tree

6 – Investing the mould

7 – Dewaxing the flask

8 – Burnout

9 – Melting

10 – Casting

11 – Cooling

12 – Cutting the cast pieces off the tree

13 – Assembly and finishing of the jewellery.

With the exception of the last two steps, all other steps directly or indirectly involve

metallurgical concepts that should be respected, if a good quality product is to

result.

The process does not tolerate errors: any careless operation, any apparently

innocuous shortcut is a potential source of defects in the finished product. Later, if a

defect is observed in the casting, very seldom is the root cause readily found and the

proper corrective action identified, because of the complexity of the process.

Temperature is an important process parameter in many of the process steps;

often the goldsmith tries to improve a situation by changing the temperature, for

example of the metal and/or the flask. Usually, a simple temperature change does

not solve the problem, but it certainly changes the operating conditions and makes

defect diagnosis more difficult.

When we have to deal with a defect in our castings, we should first consult the

Handbook on Casting and Other Defects to help determine the type and possible

causes. The number of defect types is not infinite and many of them, particularly the

most common ones, are described in the Handbook. In this way, it is usually possible

to identify the defect type and its possible causes correctly. The second step is to

scrutinize the process parameters to narrow the possibilities by elimination. Finally,

we can try to identify the root cause of the problem. Only at this point can we decide

the proper corrective action.

Because of process complexity, a defect does not usually originate from a single,

simple cause, but from a group of causes that are not necessarily located in a single

process step, but over several process steps. Therefore, systematic process data

recording is very useful and we never should take anything for granted. The human

factor is fundamental for achieving good results. I believe it to be not far from the

truth by saying that the contribution of the goldsmith to the achievement of good

quality is not less than 80%. The remaining 20% is represented by the equipment,

which should be well maintained and reliable.

2T H E P R O C E S S O F I N V E S T M E N T C A S T I N G


In this chapter we will discuss each step of the process, with particular attention

to the rules or guidelines to follow and to the most common problems that can arise.

Later, in separate chapters, we will describe the characteristics of the most commonly

used casting alloys and of the different equipment types. We will also give some basic

guidelines for making a correct choice.

2.1 DESIGNDesign represents the moment of creation, the birth of the idea for a new jewellery

product. Although we can cast very complex shapes, thanks to modern technology, the

designer should always have a good knowledge of the casting process, so that he/she can

design pieces that are easily cast. In the design phase, it is also important that the designer

be in regular contact with the caster on the shopfloor who will produce the casting.

Today, the design operation can be facilitated by the use of Computer Aided Design

(CAD) systems, which enable a dimensioned drawing to be obtained, used for making the

master model, Figure 2.1.1 (a – e). Such CAD software is not easy to use by inexperienced

persons. Specialised knowledge is required. Small workshops can seldom afford such

facilities, but it is possible to access CAD service through a reliable CAD service centre.

Considerable advantages can be obtained with the use of CAD systems, e.g. the ready

availability of a dimensioned drawing is a great help to the work of the model maker.

Moreover, if we use a CAD system, we can also use a Computer Aided Manufacturing

(CAM) system and/or one of the many available Rapid Prototyping (RP) methods, Figures

2.1.2 – 2.1.4, for making a first master model, typically in wax or plastic or even metal.

With regard to the creative design phase, we should remember that many

production problems originate from lack of communication between the designer

and the caster. This insular approach is no longer acceptable in a modern jewellery

company. A good ‘rule’ says that the relevant production staff should be involved

when a new jewellery design is discussed, to scrutinise for potential problems that

could arise in the production process. This should be done before the new jewellery

design is launched on the market. Good quality starts right from the very beginning!

At the Santa Fe Symposium of 1995, in a discussion on the way to shorten the time

between the idea and the realization of the product, J. Orrico, Director of Jewellery

Manufacturing at Tiffany & Co., said: “Sure a CAD machine will be great. But realize,

even though it is an extremely powerful tool, it can only facilitate the process. A

round table can do the same thing. If you can justify a CAD machine, great. If not,

everyone has a table. The process needs to cut across organisational boundaries to

be truly effective. Get started today!” This very simple, easily implemented

recommendation should be always present in our mind if we want to achieve a high

quality level: it is fundamental to establish a symbiotic relationship among the

different departments in the company.

2 T H E P R O C E S S O F I N V E S T M E N T C A S T I N G

Figure 2.1.1 a Design of a ring in 3 parts bymeans of CAD technique. (Courtesy ofPomellato Spa.)

b

c

d

e


2.2 MAKING THE MASTER MODELThe quality of the master model is of fundamental importance for the achievement

of good quality product: it should be perfect, with a perfect finish. It should not show

the slightest defect, because any surface defect will be replicated in the rubber

mould and, in turn, on the wax pattern, on the refractory mould and finally on the

castings. In most instances, a defect can be removed in the jewellery finishing stage,

to obtain the desired quality level, but it requires time and money. However, such a

defect limits the use of mechanized finishing. Such finishing is done by hand, with a

resulting waste of time and an increased production cost.

2.2.1 Alloy of manufactureThe use of an alloy with suitable high hardness is recommended for manufacturing

the master model: finishing of the model will be easier, with a better wear resistance.

We should remember that, if the jewellery design is a commercial success, the master

model will be used for making many rubber moulds. Therefore, good wear and

corrosion resistance are important characteristics for a master model.

The use of nickel silver (nickel 50%, copper 30%, zinc 20%) is recommended.

Many goldsmiths use sterling silver (silver 92.5%) to make the models, because they

are accustomed to cast and work this alloy. The only drawbacks to the use of sterling

silver are its low hardness and reactivity with the rubber during vulcanising.

No matter what alloy is used, rhodium plating of the finished model is strongly

recommended. For silver models, it is essential. Rhodium plating is bright and hard,

enabling better finishing, increased wear resistance and making it corrosion and

oxidation resistant, particularly in the vulcanisation stage, if conventional rubber is

used, Figure 2.2.1.

Up to now, we have discussed metal models. With the modern techniques of rapid

prototyping, it is now possible, with the aid of CAD-CAM systems, to manufacture

models in special plastics that can be used directly for making rubber moulds or for

casting a metal master model, in the place of a wax pattern, Figures 2.1.2, 2.1.3 and

2.1.4. Some jewellers use their wax or plastic model produced by Rapid Prototyping

to cast the master model in carat gold.

2.2.2 Feed sprueUsually the feed sprue is considered as an integral part of the model. It links the

pattern to be cast with the central sprue into which the molten metal is poured.

Function of the feed sprue

The feed sprue is a very important component of investment casting. It should

guarantee perfect filling of the pattern cavities in the mould. Even more important,

it should act as a liquid metal reservoir to compensate for the unavoidable volume

contraction of the gold during solidification of the cast items. If the feed sprue

cannot perform this second function, a defect will form - shrinkage porosity, with its

typical dendritic appearance, Figures 2.2.2, 2.2.3 and 2.2.4. This defect can be

entirely contained inside the casting and, if this is the case, there are no aesthetic

problems. However, as is more often the case, if it appears on the surface of the cast

piece, it must be repaired or the item scrapped. Repairing is a delicate operation that

can be difficult or sometimes impossible, Figure 2.2.5.

The criticality of the feed system changes in accordance with the type of casting

equipment. Feed sprue design is more critical with the traditional equipment for

Figure 2.1.2 Heads of a rapid prototypingmachine: the red head builds the supportingstructure, which will be removed later, whilethe green head builds the actual model

Figure 2.1.3 Operating diagram of the rapidprototyping machine shown in Figure 2.1.2Vista laterale = Side viewPasso della goccia = Spacing of the dropsDiametro della goccia = Drop diameterDirezione del movimento dei jets =Advancement direction of the jetsDirezione del deposito dei jets = Depositiondirection of the jetsAltezza di un layer = Thickness of a layerAltezza della parete = Thickness of the wholedeposit

Figure 2.1.4 Some models manufactured withthe rapid prototyping machine

Figure 2.2.1 Master model of a ring madefrom nickel silver, rhodium plated



static casting and a little less critical with vacuum assisted static casting. The difficulty

of feed sprue design decreases further with pressure and vacuum assisted casting,

the more recent evolution of static casting machine technology, and is minimum

with centrifugal casting. When we speak of criticality, we usually refer to form filling

in metal casting, because the feed system is never critical for wax injection.

Therefore, feed sprues should be carefully designed, Figure 2.2.6, as a function of

size and shape of the object to be cast. Given that solidification shrinkage, as a

physical characteristic, is unavoidable, the feed sprues, in addition to allowing complete

form filling, should be able to “drive” shrinkage porosity out of the cast object.

Design of the feed sprue

Basically, a feed sprue system is a tube or a set of tubes, wherein the metal should

flow as smoothly as possible. Turbulence should be reduced as much as possible: so

abrupt changes of cross-section, sharp angles, etc. should be avoided. Turbulence in

the flowing liquid metal can cause gas entrapment and gas porosity results from

entrapped gas in the casting. In all cases, turbulence causes a pressure drop, thus

hampering form filling. Therefore, it is always important to think in terms of fluid

mechanics and try to imagine the behaviour of liquid metal as it flows towards the

cavity to be filled.

Patterns with complex geometry or with abrupt changes of cross-sectional area

often benefit from multiple feed sprues. However, the best results are not always

obtained with a multiple feed sprue on the master model because, although

multiple sprueing can be beneficial during casting, it sometimes does not enable

high quality wax patterns to be obtained, in contrast to those obtained with a

simpler feed sprue.

In these instances, many workshops use models with a single feed sprue for wax

injection. Later, the single feed sprue is cut off and the wax pattern is fitted with a

multiple feed sprue. A set of rubber moulds of multiple feed sprues of different size and

shape can be used for this purpose. These multiple wax sprues can be fitted to the wax

patterns as required, in accordance with the type of casting to produce, Figure 2.2.7.

The “Y” feed sprue design is the simplest and, from the point of view of fluid

mechanics, the best type of multiple feed sprue. When the liquid metal gets to the

junction, where it splits into two streams, the metal will not favour one side or the other,

unless some other force is involved. Therefore a “Y” is a balanced fluid system. The stem

of the “Y” becomes the primary feed sprue and must have enough cross-sectional area

to supply ample metal to fill the two secondary feed sprues into which it splits.

Figure 2.2.2 Shrinkage porosity in a crosssection: the dendritic shape is evident

Figure 2.2.3 Shrinkage porosity in ametallographic microsection, observed underthe optical microscope

Figure 2.2.4 Dendrites in a shrinkage cavity,observed under the scanning electronmicroscope

Figure 2.2.5 Shrinkage defects in a ring with a large head in a vertical section cut through the ringhalf-way across the band width. Two defective zones are seen: a diffused one in the head andanother one in the opposite part of the shank, near the junction with the feed sprue. After pouring,the side parts of the shank solidify first, because they are thinner. Thus, when the thicker headsolidifies, feeding of more liquid metal is no longer possible. The defect on the opposite side isknown as a “hot spot”, because the sprue junction is heated by the flowing metal, causing a delayin solidification. This zone solidifies after the feed sprue and both sides of the shank are alreadysolid. So it is not possible to feed liquid metal to compensate for the shrinkage.



If there is the danger of investment erosion at the point of splitting of the

secondary feed sprues, or if the shape of the wax pattern requires a large

temperature difference between the liquid metal and the investment, the excessive

cooling expected where the metal splits off into the two secondary feed sprues of a

“Y” can be relieved by using a “V” design. The wax pattern can be produced with a

“Y” sprue, with the stem cut off to form a “V”; this junction is attached directly to the

main sprue. With all other parameters constant, the “V” feed sprue will deliver molten

metal to the pattern with less temperature drop than the “Y”, because the metal

path is shorter and less tortuous.

Size of the feed sprue

Another important point, also based on the principles of fluid mechanics, relates to

the constant cross-sectional area in primary and secondary feed sprues. If, for

example, the cross-sectional area of the primary sprue is 8mm2, then the cross-

sectional area of each of the two secondary sprues into which it splits should be

4mm2 and not 8mm2. The total cross-sectional area remains constant. In this way we

can reduce turbulence.

There are no formulae to calculate the optimum size of a feed sprue for a given

casting. As a practical rule, we can say that the cross-sectional area of the feed sprue

should range from 50% to 70% of the cross-sectional area of the pattern it will feed.

2.3 MAKING THE RUBBER MOULDThe correct design of the rubber mould is another important step in achieving a

good quality product. We can say that there are nearly no limits to the shape of

jewellery pieces that can be produced by investment casting with the presently

available materials. The only limit is the imagination and the creative power of the

person who should design and make the mould.

‘Mould engineering’ is an indispensable skill that should be cultivated inside the

jewellery company. By mould engineering, we refer to designing the mould, choosing

the correct material, deciding how many parts will form the mould and if metal inserts

will be necessary, deciding how the mould will be cut to facilitate the extraction of the

wax pattern, with minimum interference with the surface of the pattern itself.

In a Handbook such as this, we cannot teach mould-making technology, we can

only illustrate it through some examples. Mould-making should be learnt with

practice and prolonged, assiduous exercise. We recommend practitioners to attend

training courses on this particular subject, for example, those given by the producers

of mould rubber. In recent years, there has been a steady improvement in the

materials, as has occurred also for wax and investment powder. Therefore, regular

updating courses meet the need of understanding the new materials and refining the

basic technology.

Figure 2.2.6 Examples of split feed sprues(coloured in red) for correct feeding of liquidmetal in a ring. They should be connected tothe thicker part of the ring with a heavierhead; also, to the model with an inclinedangle, to reduce turbulence

Figure 2.2.7 a Moulds for making complexfeed sprues

Figure 2.2.7 b



2.3.1 Types of mould rubberMany different rubber types are commercially available, both natural and synthetic

and also including the silicone rubbers. Each type of rubber has a different balance

of properties and should be chosen for use in specific situations, consistent with the

objects to be cast. Usually, natural rubber is stronger and more wear resistant.

Silicone rubber is less strong, but enables a better replication of fine detail to be

obtained. Two component systems, that are not vulcanisable rubber, have been the

most recent to become commercially available. Apparently, they are simpler to use,

but they show significantly lower wear resistance compared with other rubber types.

The advantages and disadvantages offered by the most common rubber types are

listed in Table 1.

All types of rubber should be used with care and the recommendations of the

supplier should be followed accurately. In particular, vulcanisable rubbers have a

finite shelf life. Some of their characteristics can gradually deteriorate when this time

has elapsed.

The producers recommend storage of the rubber (before vulcanisation) away

from heat and light sources, at a temperature not higher than 20°C (68°F).

If these simple rules are followed, the rubber will keep its favourable properties

unchanged for one year at least. This is what producers guarantee. In practice, if

correctly stored, a rubber can last much longer, still giving very good results. All

batches of vulcanisable rubber are marked with a code number. In the case of

Type Advantages Disadvantages

Natural rubber Excellent tear resistance More difficult to cut(requires vulcanisation) Ideal for intricate models Requires more time for filling the frame

Requires only few release cuts It is relatively softVery limited shrinkage Gives a matt surface

Requires the use of spray or talcumTarnishing of silver models

Silicone rubber The frame is filled easily Requires more release cuts(requires vulcanisation) Easy to cut Shrinkage slightly higher than natural rubber

Different hardness levels available Good tear resistance but lower than Doesn’t require spray or talcum powder natural rubberGives a polished finishing

Room temperature silicone Very fine surface finishing Suitable only for simple wax or metal models,rubber (two components) Short time for preparation without undercuts

Negligible shrinkage Moderate tear resistanceDoesn’t require spray or talcum powder Difficult to burn (to enlarge feed sprue)

Liquid silicone rubber Very fine surface finishing Difficult to burn (to enlarge feed sprue)(two components) Doesn’t require spray or talcum powder Moderate tear resistance

Very easy to prepare High costCan be used with wax modelsNegligible shrinkage

Transparent, vulcanisable Good surface finishing Shrinkage not negligiblesilicone rubber Transparent Costly

Soft and flexibleEasily vulcanised

“No shrink” pink Very low shrinkage Vulcanising temperature (143°C +–1°C) mustVery good surface finishing be strictly complied with

Table 1 Advantages and disadvantages of different rubber types for mould making



complaints, the producer can trace the production date. Therefore, keeping a record

of the code number is important. Above all, we should not store large quantities of

rubber and we should use the older batches first (‘first purchased, first used’).

2.3.2. Making the mouldBefore making the mould, the master model should be carefully cleaned with a

degreasing solution in ultrasonic cleaning equipment. In the case of vulcanisable

rubber, the mould should be prepared by carefully packing the rubber layers inside a

suitable metal frame (preferably forged aluminium). The model is placed in the

centre of the rubber layers and is then covered with an equal number of rubber

layers, Figure 2.3.1 (a and b). The vulcanising press should have temperature-

controlled platens, preferably with independent thermostatic control. The calibration

of the temperature controller should be checked periodically with a reference

thermocouple or some other suitable device.

Two types of test should be done: with the first one, we verify that both heated

platens are at the same temperature. The test can be carried out by putting a small

wood block, the same size of the mould and with grooves on the upper and lower

surface, between the platens of the vulcaniser. The reference thermocouple is then

inserted in the grooves and temperature is measured at different points of the upper

and lower surface. The temperature readings should be the same in all positions.

The second test aims to verify the correct calibration of the temperature

controller. In this case we can use a small aluminium block, of the same thickness as

the mould, with a mid height hole for inserting the reference thermocouple. Then we

turn the vulcaniser on and we verify that the pilot light of the thermostat turns on

and off at the desired temperature of 152-154°C (about 305-309°F). If the light

turns on and off at a different temperature, we should adjust the temperature setting

knob until the correct temperature is obtained.

An incorrect vulcanising temperature is the most common cause of poor quality

moulds or of excessive shrinkage. The recommended temperature for vulcanising

natural rubber moulds is typically 152-154°C (about 305-309°F). For the silicone

rubber moulds, this rises up to 165-177°C (about 329-351°F). Vulcanising time varies

with the thickness of the mould: usually a time of 7.5 minutes per rubber layer is

recommended (a rubber layer is about 3.2mm/1/8 in. thick). Therefore, a mould

19mm (about 3/4 in.) thick will require vulcanising for about 45 minutes.

With particularly complex master models, if good results are not obtained under

the conditions cited above, we could lower the vulcanising temperature by about

10°C (18°F) and double the time. In this way the rubber will remain in a putty-like

state for a longer time and will have more time to conform to the model perfectly.

Figure 2.3.1 Steps for making a rubber moulda – The model is positioned in the centre ofthe mould

b – The mould is completed with other rubberlayers


Figure 2.3.2 Protective glove made fromstainless steel reinforced fibre for mould cuttinga – The glove fits either handb – Cutting with a protected hand


2.3.3 Cutting the mouldTo cut the moulds after vulcanising (or curing/setting, for non-vulcanising rubbers),

we use blades that should be sharpened or replaced frequently, because the cuts

must be sharp and perfect, otherwise we will have moulds that will produce defective

wax patterns. To make cutting easier, the blade should be wetted frequently with an

aqueous solution of surface-active agents.

Two important safety recommendations: the blades are very sharp and so we

work with the blade moving away from the hand holding the mould. A second

recommendation is to protect the hand holding the mould with a cut-resistant glove,

knitted with steel wire, Figure 2.3.2 (a and b).

As we proceed with cutting, the cut surfaces should be kept well open, by pulling

the rubber strongly apart: this is difficult to do with only one hand. For this purpose,

it is very helpful to use a simple, but effective device, called a “Third hand”: it will

facilitate your work significantly, Figure 2.3.3. The mould should be cut in different

ways, depending on the type of injector used for making the wax patterns. This is to

avoid the presence of air bubbles in the wax patterns, which will unavoidably lead to

the formation of defects. Presently, injectors are frequently used which exhaust the

air from the mould before injecting the wax. In this case, the moulds should be

vacuum tight. However, traditional injectors are still used in many workshops that do

not use the vacuum technique. In this case, the moulds should have suitable vents

cut, enabling the air in the mould to escape at the moment of wax injection.

In workshops where both vacuum and traditional injectors are used, problems can

arise if the moulds are interchanged between the two types, with unfavourable

consequences on the quality of the wax patterns.

Teaching how to build a perfect mould is quite difficult in a Handbook, but a few

examples are given to show what can be obtained from taking the ‘mould-

engineering’ approach. The importance of having a good mould maker in the factory

is clearly evident from the following example: the model, Figure 2.3.4, is apparently

very simple: a ring with a smooth surface, which has a marked undercut on its inner

side. At the insistence of the production department, the initial solution has been to

produce the wax pattern in two halves, Figure 2.3.5 (a and b). So there was a single

mould for each half of the ring. To produce an entire ring, either two wax patterns

are joined together or two half rings are cast in carat gold and soldered together. As

we can see from the figure, the mould had locating pegs for connecting the two

halves, which were removed after soldering. Both solutions showed considerable

disadvantages and required a long finishing operation to obtain an acceptable – but

never perfect – quality level.

A better solution was found later, thanks to a skilled mould maker, and is shown in

Figure 2.3.3 Bench fixture to facilitate mouldcutting (third hand)a – The “third hand”b – The third hand in use

Figure 2.3.4 Convex ring, with a pronouncedinternal undercut

Figure 2.3.5 a – A single rubber mould is used to producehalf of the ring shown in Figure 2.3.4

Figure 2.3.5 b – Two halves must be joined to make theentire ring



the Figure 2.3.6. It is a complex mould, formed in several parts, where the part

corresponding to the undercut has been cut in such a way as to be easily removed

without damaging the wax pattern. The wax pattern is obtained as a single piece, the

quality of the product is perfect and finishing labour has been reduced to a

minimum. We emphasize an important detail that should always be kept in our mind

when cutting a mould. The cut between the two halves of the mould has been done

to coincide with an edge of the ring: in this way there are no traces of separation lines

on the main surfaces of the wax ring and finishing operations of the casting are

simplified. So a significant improvement of product quality and a reduction in

manufacturing cost have been achieved.

Another example, similar to the one described above, is shown in Figure 2.3.7.

In this case, a metal insert has been used to prevent mould deformation during wax

Figure 2.3.6 Mould designedto produce the wax pattern ofthe ring in Figure 2.3.4 as asingle piece

Figure 2.3.7 Mould made of two types of room temperature-curing silicone rubber with a metalinsert, to produce a ring similar to the ring of Figure 2.3.4a – The metal master modelb to h – The mould. The metal insert prevents mould deformation during wax injection


a b c

d e

a b c

d e f

g h


injection, because two types of two component silicone rubber have been used for

making the mould, instead of natural rubber: one type for the inner part of the ring

and another one for the actual mould.

If we do not have a skilled mould maker in our factory, we can resort to a solution that

should never be considered as optimum, i.e. to use self-parting moulds. In this case, the

vulcanised mould will be opened with the simple action of the fingers. Before

vulcanising, the mould is assembled in the usual way, by packing the rubber layers in the

frame. When nearly half of the layers have been packed, we put small cubes of

vulcanised rubber or metal pegs at the outer edge of the mould. These rubber cubes or

metal inserts act as locating pegs for the two halves of the mould. Then the free surface

is dusted with talcum powder, Figure 2.3.8 (a & b), or is sprayed with a suitable silicone

product, or is covered with a thin plastic film. Then a further rubber layer is added, on

which the master model is positioned, Figure 2.3.9 (a & b). The previous operation of

dusting with talcum or silicone spraying or covering with plastic film is repeated.

Then we also repeat all other operations in an inverted order for the second half

of the mould, Figure 2.3.9c. The mould is then vulcanised. After vulcanising, the

mould will open by the simple pressure of the fingers and will comprise four parts.

Two outer parts - the mould shell - and two thin inner parts, formed by the two inner

layers, which are the true mould. These two parts will easily separate from the wax

pattern, without damaging it, Figure 2.3.10. In this mould type, the separation line is

in the centre and will always leave a ‘witness mark’, which must be removed later.

Moreover, this mould type is not suitable for vacuum injectors.

2.3.4 Storing and using the mouldAfter making, the mould should be numbered, referenced and stored in a closed

container - a drawer or a cupboard - away from sunlight and dust. The mould should

always be carefully cleaned after use. It is recommended that a register of the moulds

is maintained, where all parameters for the production of wax patterns are recorded

for each mould (wax type, wax temperature, injector temperature, vacuum, pressure,

cooling time). With some latest generation injectors, it is possible to record these

parameters on an electronic chip that is inserted in the mould and is “read” by the

injector at the moment of wax injection.

When a new mould is made, manufacturing parameters should be recorded with

care. If necessary, specific tests should be carried out to obtain a perfect mould. With

an optimised manufacturing process, mould shrinkage can be minimized.

Recently, some vulcanisable rubber types have become available on the market that

are claimed to be “no shrink”. The shrinkage of these rubbers can really be zero or nearly

zero, but to achieve this, the recommended vulcanising temperature should be

accurately adhered to. If the vulcaniser is not equipped with a very accurate temperature

control system, “no shrink” rubber can show some degree of shrinkage, maybe even

more conspicuously than with conventional rubber types. This can occur if the

temperature is only a few degrees higher or lower than the optimum temperature.

Figure 2.3.8 a Preparation of a self-partingmould, first half.

Figure 2.3.8 b – The red hatched zonesshould be dusted with talcum or protectedwith other means, because they should notbond during vulcanising

Figure 2.3.9 c – Covering the model

Figure 2.3.9 b – Positioning of the model inthe mould

Figure 2.3.9 Preparation of a self-partingmould. a – The model



Problem Causes Remedies

Mould is soft and sticky Too low temperature or too short time for Check with a suitable instrument the realvulcanising temperature of the vulcaniser

Comply with temperature and time recommended by the producer

The mould is hard and Pressure is too high, too long vulcanising Use lower pressuredistorted time and/or too high temperature Verify the temperature displayed by the

vulcaniserComply with time and temperature recommended by the producer

The different layers of Pollution of the surface of the rubber layers Reject the defective mould and improvethe mould tend to during mould making (dirty hands, grease, cleanlinessseparate talcum, etc.)

Bubbles or depressed Insufficient filling of the frame Improve filling of the frameareas on the larger surfaces of the mould

White dust on the Normal occurrence Do not mind itrubber surface before Do not try to remove itvulcanising

The rubber is hard and The rubber is already partially or completely Reject the rubber batch and verify that the doesn’t vulcanise vulcanised because of an accidental rubber is stored correctly

exposure to heat or because of aging

Rubber is hard and stiff The rubber is “frozen” after a prolonged Heat very slowly the rubber up to about storage at a too low temperature 38°C (100°F)

Excessive shrinkage Too high vulcanising temperature Check with a suitable instrument the real temperature of the vulcaniserComply with temperature and time recommended by the producerAlternatively, lower vulcanising temperature to 143°C (289°F) and double vulcanising time

The rubber doesn’t fill all The frame has not been correctly packed Insert small pieces of rubber in cavities and cavities and undercuts Rubber is too old undercuts

Check the temperature displayed by the vulcaniser

Table 2 Common problems in the production of rubber moulds

Figure 2.3.10 Details of the self-partingmould of the previous figures, aftervulcanisation

2.3.5 Common problemsSome of the most common problems we can meet when making a mould are listed

in Table 2, along with their causes and some simple remedies.

2.4 PRODUCTION OF THE WAX PATTERNS2.4.1 Types of waxThe use of a wax with a narrow melting range is recommended. A range of waxes

is available to the goldsmith: the wax type should be selected on the basis of the

object to be produced. Therefore, it is very important to know the physical

characteristics of the different types of wax as thoroughly as possible. Usually, the wax

producers give only one quantitative datum: the recommended injection

temperature. All other information given is purely qualitative. But this information

exists and should be available to the goldsmith if he is to make a correct selection of

the wax grade.



Information on hardness, density, ash content, viscosity, linear and volume thermal

expansion is important for making the correct choice.

For example, thermal expansion can be used for evaluating the cooling shrinkage

of the wax patterns and calculating the real dimensions of the cast pieces. Viscosity

will give information on the ability of the wax to fill the mould completely and the

density can be used for calculating the precise amount of precious alloy required for

casting. The values of some physical parameters for different wax types are shown in

Table 3. The values of these parameters should be available from all serious suppliers.

We can see that the values of some parameters can change by more than 20% from

one wax type to another. Therefore, the usual qualitative information offered, like

“high”, “low”, etc., should not be considered sufficient.

We should also note that wax grades are often differentiated by wax colour;

however, different suppliers use different colours for similar grades. Thus, a blue

grade from one will be different from the blue grade from another!

2.4.2 Wax injectionTemperature is also a fundamental parameter for the production of wax patterns.

Not only wax temperature, but also the injector nozzle temperature and mould

temperature are important. Injectors fitted with devices to monitor and control

nozzle temperature as well as wax temperature can be most effective in attaining

good quality wax patterns. While too low a wax temperature can cause incomplete

filling of the mould, too high a wax temperature can give rise to bubbles and

excessive pattern shrinkage.

It is recommended that the wax patterns of a given type produced throughout

the day are weighed systematically, to verify the operation of the wax department.

Too large a weight variability of wax patterns says that something is not running as

expected. The first thing to consider is the use of mould clamping devices for

Ring & Ball HardnessWax Softening Density -penetration Fluidity testtype point g/cm3 (100g load) mm

°C CPS RPM CPS RPM CPS RPM 48°C 60°C 72°C °C(°F) 118°F 140°F 162°F (°F)

A 70±3 .954 5,8 >50% 50°C 252 100 311 100 550 50 4% 9,1% 11,6% 68-71(158±5) (154-160)

B 70±3 .960 8,2 >50% 52°C 204 100 282 100 348 100 3,5% 10,0% 11,6% 71-74(158±5) (160-165)

C 72±3 .940 6,4 >50% 54°C 622 100 759 100 998 100 3,0% 7,3% 10,3% 71-74(162±5) (160-165)

D 68±3 .950 8,4 >50% 54°C 764 50 952 20 – – 3,3% 8,6% 12,8% 73-76(154±5) (163-169)

E 74±3 .955 9,6 >50% 54°C 217 100 267 100 400 100 3,5% 9,2% 11,6% 71-74(165±5) (160-165)

F 68±3 .960 7,6 >50% 52°C 248 100 307 100 413 100 4,6% 10,6% 11,8% 68±3(154±5) (154±5)

Cps = Centipoises Rpm = Revs. per min.

Table 3 Characteristics of some commercial wax types

Viscosity

Volume expansionFrom 24°C (75.2°F)

Injectiontemperature

77°C 71°C 66°C170°F 160°F 150°F



Table 4 Common problems in the production of wax patterns


Problem Causes Remedies

Bubbles Insufficient wax quantity in the injector Add wax in the wax potWax is too hot or too cold Calibrate wax temperaturePoor fitting between mould and nozzle Set the mould correctly or adjust the mould mouthToo high injection pressure Use lower pressureUsing vented moulds on vacuum injector Don’t use vacuum

Incomplete filling of the mould Injection pressure is too low Increase injection pressureWax temperature is too low Increase wax temperatureCold mould Heat the mould with repeated useThe feed sprue is too thin Use a wider feed sprueInsufficient vents (no vacuum injectors) Increase vents in the mouldVents are obstructed or dirty (no vacuum injectors) Clean the vents and keep them open with talcumObstructed injector Clean injector and nozzle

Excessive filling of the mould Pressure is too high Decrease pressureIncorrect clamping pressure on the mould Use correct clamping pressure

Make a new mould and cut it with improved toolsWax is too hot Lower wax temperatureToo long injection time Shorten injection time

Sticky wax pattern that is easily bent The mould has been opened too early Longer cooling timeWax is too hot Lower wax temperatureMould too hot Increase cooling time of mould before re-use

Excessive shrinkage Wax is too hot Lower wax temperatureInsufficient pressure Increase injection pressureInjection time too short Longer injection time Feed sprue too thin Use wider feed sprueThe mould is too cold Heat the mould with repeated useWax with excessive shrinkage Turn to low shrinkage wax

Sinks (depressions in large patterns) Incorrect selection of wax type Turn to a depression resistant wax typeInjection time too short Longer injection time Wax is too hot Lower wax temperatureInsufficient injection pressure Increase injection pressureFeed sprue too thin Enlarge the feed sprue

Poor surface finishing (also wrinkling) The mould is too cold Heat the mould with repeated useWax is too cold Increase wax temperature

Poor surface finishing (rough surface, cavities are present) Too low injection pressure Increase injection pressureToo much spray for releasing the patterns Clean mould and reduce spray quantityToo much talcum Clean mould and reduce talcum quantity (add a

layer of cloth to the linen bag)Spray and talcum have been used at the same time Clean mould and use spray only

Fins Too high injection pressure Lower injection pressureMould imperfectly cut Make a new mould and improve cuttingInsufficient clamping pressure Increase clamping pressureVents are insufficient or obstructed Clean the mould and the vents accurately

Cut additional ventsWax is too hot Lower wax temperature

Wax patterns tend to break Not enough spray for releasing Use more sprayThe mould has been incorrectly open or the pattern has been removed badly Improve mould opening and pattern

removal methodsThe mould has not been cut properly to facilitate pattern removal Make a new mould and improve cuttingCooling time too long Shorten cooling timeA brittle wax has been used Prefer a more flexible wax


controlling mould pressure during wax injection, to avoid the variability that occurs

when the mould is held by hand, Figure 2.4.1. Weight variation can exceed ±10% for

different operators or even for the same operator at different moments during the day.

An initial quality control should be done on the wax patterns, Figure 2.4.2. The

patterns should not be dirty (talcum powder, for example) and should not show

bubbles. When investing the flasks, bubbles can break open and fill up with

investment. So they can give rise to more serious defects in the castings. The

presence of bubbles can readily be detected by looking at the patterns against a light

source. Defective wax patterns should be immediately rejected and should never be

used for production. They will unavoidably give rise to defective castings, with

considerable loss of time and money.

The removal of fins and witness marks of the mould separation line, when very

evident, are the only repair operations acceptable for a wax pattern. We should record

the number of rejected wax patterns for each model type. High figures suggest that

the mould is badly made or has deteriorated. Otherwise, the injection parameters are

incorrect and should be modified, or the wrong wax type has been used.

Recycling of used wax and defective waxes should be totally avoided. It is a useless

and harmful ‘economy’ that will invariably lead to poor products. We should also

avoid using too much talcum powder to facilitate removal of the wax patterns from

the mould. The aim should be to use as little as possible. During the dewaxing

process, it is difficult to remove all the powder remaining on the surface of the

patterns or embedded in the wax. Certainly, talcum powder will not disappear during

burnout (talc is an inorganic silicate): it will lead to a poor surface or defects! It will

also accumulate in the rubber mould. To facilitate easy removal of the wax from the

mould, the use of a fine starch powder or a silicone spray is preferred. Excess starch

powder will burn in the burnout furnace, leaving no residues.

The main parameters involved in wax injection are temperature and pressure. For

vacuum injectors, a third one, vacuum, should be included. We can start by

discussing the last parameter, vacuum. To get a good effect from vacuuming, the

mouth of the mould must be perfectly matched with the nozzle of the injector,

Figures 2.4.3 and 2.4.4. If there is a gap between the mould mouth and the nozzle,

not only will we not exhaust the mould sufficiently, but in the subsequent step of wax

injection some air can be entrained by the wax and enter the mould. This air will be

added to the air already present in the mould cavity, with a considerable danger of

producing air bubbles in the wax pattern. We should keep in our mind that moulds

for vacuum injectors don’t have vents, so the air will find it rather difficult to escape

from the mould cavity during wax injection and will never be completely removed.

As far as temperature is concerned, usually we should work at the temperature

recommended by the supplier of the wax. A higher temperature can lead to wax

patterns with air bubbles, while at lower temperatures wax fluidity can be insufficient

and the model could be inaccurately replicated, with loss of fine surface detail. Lastly,

pressure is the only parameter requiring a real adjustment for each single model.

When we have found the correct pressure level, allowing the replication of the model

accurately, we should not change it. Changes of injection pressure can cause very

significant variation of the weight of the wax patterns and, consequently, of the

weight of the gold alloy castings.

Figure 2.4.3 Perfect seal between injectornozzle and mould mouth is very importanta – Correct geometryb – Incorrect geometry that can favourentrainment of air with the wax and does notensure a satisfactory vacuum in the mouldbefore wax injection

Figure 2.4.4 Mould frame with screwed-insprue former ensuring correct geometry ofmould mouth


a b

Figure 2.4.1 Mould clamp, allowing clampingpressure control during wax injection

Figure 2.4.2 Quality control of wax patterns.Generally different colours denote differentphysical characteristics of the wax


Figure 2.5.1 Two traditional rubber basetypes: with conical or hemispherical spruebutton

Figure 2.5.2 Possible problems with ahemispherical sprue button

Figure 2.5.3 Rubber base to reject: theconical sprue button shows wear on the tip

Figure 2.5.4 A worn out rubber base (seeFigure 2.5.3) forms an undesirable step in thesprue button

Each mould and each wax type are different and require a specific pressure level, a

specific temperature and an appropriate time for cooling. The best compromise among

these different parameters can be achieved only with experience and experimentation

on that specific mould with a specific wax type. Moreover, the characteristics of the

mould change as we continue with the injection of hot wax. Maybe the combination of

parameters giving good results initially (when the mould is cold) will no longer work well

when the mould has been heated by a prolonged use. Therefore, we should take into

account the time of cooling between subsequent injections.

Lastly, we should note that waxes should be stored on flat trays in a cool place and

covered to prevent dust settling on the surface by electrostatic attraction. They

should not be piled in heaps, as they are liable to distort or to surface damage.

2.4.3 Common problemsSome common problems that can occur in the production of wax patterns are listed

in Table 4, along with the possible remedies.

2.5 ASSEMBLING THE TREE2.5.1 Bases and spruesThe rubber base for the tree is the starting point for building the wax tree. It should

be selected with care. Usually, the rubber base includes the part that becomes the

sprue button of the cast tree.

We should check carefully that the base is clean and free from residues of used

investment. Residues of used investment can appreciably change the setting time of

the new investment, thus impacting on mould quality. Bases with a cone-shaped

sprue button are preferable to those rubber bases with a hemispherical sprue button,

Figure 2.5.1. A hemispherical sprue button can cause pressure losses and induce

turbulence during casting, Figure 2.5.2, with the consequent possibility of gas

entrapment in the liquid metal. These problems are more evident when we cast with

centrifugal machines rather than with static machines.

We should always check that the selected base does not show signs of wear on

the tip of the cone of the sprue button, where the main wax sprue is inserted, Figures

2.5.3 and 2.5.4. As before, the presence of a step between the rubber base and the

wax sprue can cause turbulence and pressure loss during casting. Each rubber base

should be identifiable with a code number and weighed.

It is considered better to use main sprues made from a wax with lower melting

range than the wax of the patterns. In this way, when dewaxing, the main sprue will

melt first and stress generation inside the invested flask will be avoided, when the wax

patterns begin to melt.

Slightly tapered main sprues are preferred to standard cylindrical ones. Tapering

gives a better heat balance: the solidification will progress from the top of the tree

(smaller diameter) to the bottom, favouring a directional solidification, Figure 2.5.5.

The danger of shrinkage porosity formation in the cast items is reduced.

Figure 2.5.5 Variation in temperaturedistribution in the tree resulting from the useof a cylindrical or tapered main sprue



A few years ago, a new patented system was developed, comprising an innovative

rubber base, onto which the tapered main sprue is screwed through a special device,

Figure 2.5.6. The main wax sprue includes a narrower conical sprue button that is

designed to facilitate mould filling with minimum turbulence, Figure 2.5.7. In this

way, the rubber base can be removed without stress or torque being applied to the

tree and the patterns, Figure 2.5.8, and any danger of cracks in the investment near

the sprue button and the main sprue is avoided. Such cracks can cause defects in the

castings.

In the author’s opinion, this system, tradenamed NeuSprue™, is one of the most

interesting new products to appear on the market in recent years, Figure 2.5.9. At

first sight, it is a very simple fixture, but its development was based on a rigorous

study, using finite element analysis. An optimised dimensioning of the main sprue has

been achieved, which enables a reduction in the weight of alloy required for each

cast and allows control of the progression of solidification.

In all cases, the cross-sectional area of the main sprue should be decided with

care, because it depends on the size of the tree and on the items we want to cast

(shape, size etc.). Some goldsmiths use a tubular main sprue. It is a tube, with a

diameter much larger than a conventional sprue, but it is hollow and its weight is

lower. This particular kind of main sprue is used for two reasons: it permits many

more pieces to be placed on the tree, because a larger surface area is available on

the main sprue, and a smaller amount of precious metal is required for casting,

because the main sprue is hollow. Therefore, a higher yield per flask can be obtained

and the amount of precious metal reduced. In the author’s opinion, even if the

reasons for choosing a hollow sprue are accepted, a hollow central sprue does not

allow directional solidification to be obtained in the best way, because of the

different distribution of heat release. Therefore, it may be better to stick to the more

traditional practice: a solid, slightly tapered, main sprue.

Figure 2.5.6 Assemblingsystem for the NeuSprue™sprue and base

Figure 2.5.7 The NeuSprue™sprue with its rubber base

Figure 2.5.8 Release system of the rubber base of theNeuSprue™ (right) eliminates stress caused by removal of oldstyle sprue base (left)

Figure 2.5.9 a Preparation of a tree withthe sprue shown in Figure 2.5.7.

Figure 2.5.9 b The sprue holder can betilted to facilitate attaching the wax patternsto the main sprue



2.5.2 Tree designAs far as possible, we should put wax patterns of similar shape, size and weight

together on the same tree. Thin patterns and thick patterns should not be cast on

the same tree. When the temperature is high enough to cast the thin patterns

beautifully, then the temperature will be too high to get good castings from the thick

patterns, if they are tree’d together.

In general, where different patterns are included on the same tree, thin or lighter

patterns should be put at the top of a tree, because pressure is higher there than

near the sprue button. If thin patterns will not fill at the bottom of the tree, then the

feed sprue may not be large enough nor attached to the main sprue in the best way

(presence of constrictions) or the temperature of the metal and/or of the flask may

be too low. Patterns that cast well at the same flask and metal temperature can be

mixed on the same tree with more challenging patterns at the top and easy to fill

patterns at the bottom.

The joints between the main sprue and the feed sprues must be smooth and well

filleted. Constrictions at the junction point should be carefully avoided, Figure 2.5.10.

When casting, the investment will protrude at this junction and can be eroded or

broken off by the flow of liquid metal. Such investment fragments could obstruct the

feed sprue and/or form non-metallic inclusions in the castings, Figure 2.5.11.

Traditionally, the angle between the feed sprue and the main sprue has been

recommended at about 45° – 60°. More recently, a larger angle of 70° – 80° has been

recommended for static vacuum casting Figure 2.5.12. Recent research has shown that

the best results are obtained when the wax patterns are welded perpendicularly to the

main sprue. So we obtain a double advantage: solidification takes place more

directionally - and the probability of formation of shrinkage porosity in the casting is

lower - and the escape of the gas from the mould cavity is easier, because there is a

thinner investment layer to go through to reach the outer surface of the flask. In this

way the probability of formation of gas porosity from trapped gas is reduced.

Figure 2.5.10 Joint constriction between themain sprue and the feed sprue: to be avoided!

Figure 2.5.11 Possible problems whenconstrictions are present (see Figure 2.5.10).During pouring, particles can break off fromthe investment, resulting in non-metallicinclusions in the cast piece

Figure 2.5.12 Optimum angle between themain sprue and the feed sprue. A 90o angle isnow preferred (see text)

Figure 2.5.13 Homogeneous treewith thin patterns



The length of the feed sprues should be such that the furthest part of the patterns

is no more than 10mm (0.4 in) from the wall of the flask, Figures 2.5.13 and 2.5.14.

Figure 2.5.15 shows the drawing of a tree, with the names we use for its different

parts.

Finally, assembled trees should be weighed to determine the weight of wax

(subtract the weight of the rubber base), as this allows the amount of carat gold for

casting to be calculated. Prior to investing, the trees can be washed in water

containing surfactant to remove any electrostatically attracted dust.

2.6 INVESTING THE MOULD2.6.1 FlasksSteel cylinders or ‘flasks’ are used to contain the investment mould. Stainless steel is

preferred. Before use, the flasks should be cleaned with a wire brush to remove all

traces of old investment, because residues of used investment can reduce the work

time of the new investment, thus influencing mould quality. The flask is placed

around the wax tree and sealed at its base.

Before filling, the perforated flasks, used in modern static casting machines,

should be wrapped or in suitable sleeves, made from rubber or special paper or

plastic, to seal the holes until the investment is fully set. In the case of solid flasks,

used mainly in centrifugal casting machines, the use of a wax net is recommended,

to assist in gas evacuation during casting. The wax net should be positioned near the

flask wall, Figure 2.6.1, and will be removed during dewaxing, leaving escape

channels for the gases present in mould cavities.

2.6.2 Investment powdersTwo basic types of investment are used for jewellery production. These differ in the

type of bonding material used, while the true refractory material is always the same:

a mixture of quartz and a-cristobalite. The bonding material can be calcium sulphate

(gypsum) or a mixture of one or more phosphate-containing materials. Calcium

sulphate-bonded (also known as gypsum-bonded) investment is used for casting gold

and silver alloys, while phosphate-bonded investment is used for alloys melting at

higher temperature, such as palladium white gold and, in particular, platinum alloys.

The investment powders contain also a small percentage of proprietary additives

to control the rate of setting and the properties of the set investment. There are also

special grades with additives that allow for casting with gemstones in place.

Alternatively, a standard grade of investment can be used for this purpose which is

mixed with water containing about 3.3 grammes (maximum 4 grammes) of boric

acid per 100 ml of water. Dissolve the boric acid in the water at 82°C (180°F) and

then cool it down before using. These investments must be dry dewaxed only, as will

be discussed later.

Of the 2 types, the goldsmith prefers calcium sulphate-bonded investment for two

main reasons:

(1) It is less costly.

(2) It is easier to remove. After solidification of the castings, it is sufficient to

quench the hot flask in water, which breaks the investment mould and allows

recovery of the cast tree.

Figure 2.6.1 Wax webs to facilitate gasevacuation from a solid flask


Figure 2.5.15 The tree and its different parts

Figure 2.5.14 Wax tree with heavypatterns


The most common investment type consists of a mixture of 25-30% bonding

material (Plaster of Paris (or gypsum), i.e. calcium sulphate hemihydrate:

CaSO4.1/2H2O) and 70-75% silica, the true refractory material, in the form of quartz

and a-cristobalite. The ratio between quartz and a-cristobalite varies with grade and

from producer to producer, Figure 2.6.2.

There are several grades of investment powders on the market, Table 5. The

quality of an investment powder depends on many factors, such as particle size and

purity of minerals. Cheaper grades often contain coarser, less pure powders. These,

together with the proprietary additives, affect the performance of an investment. In

recent years, research work has led to an improvement of quality and reliability of the

product. Investment is now stronger and more reliable and has a wider field of

application.

Nevertheless, making the investment mould is always the most critical step in the

investment casting process. It consists of a sequence of operations, requiring

adherence to some strict but simple rules. Unfortunately, these are often neglected,

maybe because of their simplicity, with adverse effects on product quality.

In the author’s view, there is no argument about using good investment powders,

produced by well respected companies, and on the necessity of accurately following

the procedure recommended by the producer.

2.6.3 Safety and storage of investment powdersTwo aspects must be highlighted before discussing the investment process. Firstly:

Safety! Fine silica dust, as used for the investment powder, is very dangerous.

When inhaled, it remains in pulmonary alveoli and can cause silicosis, a progressive,

irreversible lung injury. Silicosis is a serious disease that can result in

premature death and the warning labels, which are now a standard part of

investment containers, should be taken very seriously. The Materials Safety Data

Sheets, supplied by the investment manufacturers should be obtained and heeded.

Therefore, it is recommended that investment powder is handled in a separate area,

fitted with good exhaust ventilation and regularly cleaned to keep dust to a minimum.

When handling investment powder, the operator should always wear special

approved dust masks, rated for use with investment. Normal dust masks don’t stop

Figure 2.6.2 Investment structure. The largerprismatic crystals are calcium sulphate (thebinder) and the smaller crystals are silica (thetrue refractory material)


Ransom & KerrLab Hoben SRS,Randolph USA International UK

USA UK

Standard Ultravest Satin Cast 20 Gold Star Ultima Classicgrades for (Advantage) Kerrcast 2000 Gold Star XL (18 carat+)gold Supervest 20 Gold Star 21 Eurovest

Satin Cast regular Gold Star Plus (up to 14 carat)Investite

White Platinum Platinite PT Platincastgold/platinum Astrovest

Stone-in-place Solitaire Satin Cast 20 Gemset Stonecastcasting

Table 5 Typical grades of powders for investment casting


the fine silica particles, which are the most dangerous! Protective clothing, including

hats, should be worn and regularly laundered. Two operations are the most hazardous:

(1) Opening the container of investment powder and taking out the powder.

When the container of the investment powder is opened and investment

powder is scooped out, the finest particles become suspended and float in

the air

(2) Quenching the flask. When the flask is quenched after casting, the escaping

water vapour (steam) entrains fine silica dust into the surrounding

environment.

The second point refers to the method of investment storage. We should keep in

mind that Plaster of Paris (gypsum) used as the bonding material, is hygroscopic.

The Plaster of Paris absorbs moisture when it comes in contact with a humid

atmosphere, and becomes unable to play its function. Therefore, investment

powder must always be kept in dry conditions. The containers should be closed and

sealed after use. Where possible, the containers of the investment should be kept in

a room with controlled humidity and temperature, because investment temperature

is also an important parameter. Bulk investment powder is a bad heat conductor: if

stored in a cold or hot area, it can take a long time to reach the correct process

temperature, required for mixing. So the temperature of the investment should also

be checked. This can be done with a digital thermometer, now available cheaply.

If a room with controlled humidity and temperature is not available, the containers

should be preferably kept in a sheltered area, preferably on pallets, not resting on the

floor, rather than in open air. Air circulation will prevent the condensation of harmful

humidity.

Investment powder is the most perishable material used in the process of

investment casting. It has a typical shelf life of one year, when correctly stored.

Therefore, it is recommended not to store large quantities of investment powder in

the factory. The date of manufacturing is normally printed on the containers plainly

or in some easily readable code by the manufacturer and should always be checked.

In overseas locations, delivery of investment to the local wholesaler or agent by ship

can result in investment already well into its storage life.

2.6.4 Checking the condition of the investment: the ‘gloss-off’ test

Before using a new batch of investment for production, it is advisable to test it, by

measuring the ‘gloss-off’ time. This is a very simple test, requiring no special

instrument. Only a plastic coffee cup and a stop watch are required.

We weigh a small quantity of investment (30-50 g) and a quantity of water at

room temperature (20°C/68°F) in the ratio recommended by the producer. We add

the investment powder to the water in the plastic cup and start the stopwatch. We

mix with a glass rod for the recommended time and then observe the surface of the slurry.

The moment when the mixture starts setting is denoted by a change in appearance

of the surface from a bright gloss or shine to a dull matt. This is the gloss-off point.

With a good quality investment, with water at 20°C (68°F), the gloss point is reached



after 9-10 minutes (all commercial investment types fall in the range 7-10 min.). If a

considerably longer time is required to reach the gloss-off point, the investment is

not behaving properly (probably due to the hydration of calcium sulphate) and has

deteriorated.

The ‘working time’ of an investment is the gloss-off time less 1 minute. The ‘gloss-

off’ test is useful for checking the condition of a batch of investment, if problems

(defects) occur in casting, attributable to a poor mould. It is a way of checking if the

problem is due to the investment or to the burn-out cycle.

2.6.5 Mixing the investmentThe setting time is very important, because it is the basis for performing all the

operations involved in creating the invested flask (mould). If we do not respect the

required time, weak or poor moulds will result, leading to various defects such as

watermarks, sandy surfaces and fin formation.

Setting of the investment slurry is due to hydration of calcium sulphate

hemihydrate; this is a chemical reaction, so it is strongly influenced by the

temperature of both water and investment powder, Figure 2.6.3. Therefore, it is very

important to use water at the recommended temperature, typically about

20°C/68°F, to ensure a consistent behaviour of the investment. Investment made

with water that is too hot will set faster. Water that is too cold will slow down the

setting time and lead to weak moulds and defects such as watermarks.

Recent work shows some tap waters can substantially extend the setting time,

Figure 2.6.4. As for water quality, it is preferable to use deionised water, because the

setting time can be appreciably changed (lengthened) by the substances dissolved in

tap water. The ‘gloss-off’ test will demonstrate this difference if batches of

investment are made with both deionised and tap water. Clearly, we can assume that

for most locations, the tap water composition will be nearly constant, but we cannot

be certain. In some locations, it can change significantly with the seasons. The use of

deionised water will remove such uncertainty and variability and thus contribute to

the most profitable use of investment powder in quality terms.

We should note that the producers of investment powder develop their powders

for use with deionised water and their advice on its use is based on deionised water

at 20°C/68°F. Should it be difficult to obtain deionised water, the measurement of

“gloss-off time” is even more important, because it is the base for determining the

time available for all investing operations.

The sequence of steps for investing the flask is as follows:

1. Weighing investment powder and water

– This must be done accurately. A measuring cylinder should be used for

the water, scales for the powder

2. Mixing the powder in the water

– Always add the powder to the water to ensure good mixing without

‘lumps’

3. Vacuuming the mixture

– This removes entrapped air


Tim

e (m

in)

Temperature Effect

8 10 12 14 16 18 20 22

60 65 70 75 80 85 90 Temperature (F)

Pour Time Set Time

Figure 2.6.3 Effect of temperature on pourtime and set time

-4 -3 -2 -1 0 1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8 9 1 0 11

Water Source Number

Water Quality

Pour Time Set Time

Del

ta T

ime

(min

)

Figure 2.6.4 Effect of water quality on pourtime and set time


4. Filling the flask

– To fill the flask around the wax tree

5. Vibrating the flask under vacuum

– To remove any remaining air bubbles that may stick to the wax surface

and ensure good surface replication

6. The flask is left to stand for investment setting

– The investment is weak at the setting point and it strengthens over time.

Any movement at this stage risks cracking the investment.

Time is a critical parameter: the first five operations must be carried out before the

slurry starts setting. This is known as the ‘working time’. A good rule is to vibrate the

flask until 1 minute before the slurry starts setting (hence the importance of

measuring the “gloss-off time”!). We should keep in our mind that we deal with a

liquid-solid mixture, not a solution. If we don’t mix enough or if we let the slurry rest

for too long a time between vibrating and final setting, the water will tend to

separate at the interface between wax and investment, forming watermarks, Figures

2.6.5 and 2.6.6, a kind of veining that accurately replicates the tiny water streams

creeping between the wax surface and the investment. The watermarks will be

faithfully reproduced on the castings and will be superimposed on the surface details

of the cast item, which will be ruined.

The slurry can be prepared by hand, Figure 2.6.7, with very simple equipment, like

kitchen mixers, Figure 2.6.8, bell jars for vacuuming the slurry, or rotary vacuum

pumps, etc. But, if we want to obtain a consistently good quality, it is advisable to use

investment mixing and pouring units, where the whole process, up to filling, vibrating

and vacuuming the flask is carried out in an automatic and programmed way.

It is important that the recommendations of the invesment manufacturer on

powder/water ratio, mixing times, temperatures, etc. are followed. Just as an

indication, the data for an investment powder with about 9 min. gloss-off time are:

1. powder to water ratio: 100:38

2. mixing time: about 3 min.

3. vacuuming: about 1.5 min.

4. pouring the slurry in the flask: about 1.5 min.

5. vacuuming and vibrating the flask: 2 min.

total working time: 8 min.

According to most recent theory, after vacuuming we should let the flasks sit

undisturbed from a minimum of 1 hour to a maximum of 2 hours, before dewaxing.

The flasks should never become thoroughly dry: if this happens, they should be

abundantly sprayed with water before dewaxing.

It is not advised to prepare batches of filled flasks and use them in subsequent

days. If the flasks become completely dry, there is a high risk of crack formation,

rupture or even major blowouts during casting, Figure 2.6.9. The preferred practice

is to prepare the flasks, to let them set and to send them directly to dewaxing and

burnout. The programmed burnout cycle will be carried out overnight and, on the

following day, when equilibrated at the casting temperature, the flasks will be cast.


Figure 2.6.7 Hand mixing in air

Figure 2.6.8 Machine mixing in air

Figure 2.6.5 Watermarks on a cast item asseen under the scanning electron microscope

Figure 2.6.6 Watermarks on a ring, as cast

a

b


2.7 DEWAXING THE FLASKRecent research carried out by the producers of investment powder suggests that

after complete setting of the investment, i.e. 1 to 2 hours after flask filling, the wax

of the patterns should be removed, to empty the mould cavities where the liquid

metal will be poured.

Dewaxing can be carried out in two ways: dry - it is the older method - or by steam.

Dry dewaxing is often done in the burnout oven as part of the burnout cycle, but can

be done in a separate dewaxing oven, prior to the main burnout cycle. Originally, steam

dewaxing was introduced for ecological reasons, to avoid air pollution from the smoke

generated by large scale burning of wax, particularly in places where many jewellery

factories were operating in close proximity. Later it has been realized that steam

dewaxing can lead to better product quality, with reduced gas porosity in the castings.

Research performed on this subject, particularly by the German Research Institute

for Precious Metals (FEM) of Schwäbisch Gmünd, has shown that there are two types

of gas porosity: from trapped gas and from reaction gas. The first type comes from

the gas present in mould cavity, in combination with metal turbulence during

casting. The second type comes from the decomposition of calcium sulphate

(investment binder), which produces gaseous sulphur dioxide, which largely remains

in the metal filling the form. Under normal conditions, this decomposition reaction

begins around 1140°C (2084°F), Figure 2.7.1, but it is accelerated by silica and, even

more, by reducing substances like carbonaceous residues from wax, Figure 2.7.2. In

this case, the decomposition temperature of calcium sulphate is lowered to values

near to the investment temperature at the moment of casting.

The studies have also shown that, with dry dewaxing, the wax impregnates

investment surface pores, Figure 2.7.3, and is difficult to remove completely. So,

during burnout, carbonaceous residues are formed that favour calcium sulphate

decomposition both during burnout, with reduction of investment strength, and

during casting, with formation of gas porosity, Figures 2.7.4 (a & b), 2.7.5 and 2.7.6.

On the contrary, with steam dewaxing, humidity saturates the porosity of investment

and inhibits wax absorption. So the probability of calcium sulphate decomposition is

reduced. For this reason steam dewaxing has been preferred or, at least,

recommended for some time.

Figure 2.6.9 Burst flask, arising from processerrors

Figure 2.7.1 Thermal decomposition curve ofcalcium sulphate

Figure 2.7.2 Thermal decomposition curve ofcalcium sulphate when reducing substancesare present

Figure 2.7.3 Evidence of wax penetration intoinvestment porosity during dry dewaxing (palerhalos)

Figure 2.7.4 Gas porosity observed under theoptical microscope. a – Surface


400µm

Figure 2.7.4 b – Cross section. It can be seenthat porosity affects not only the surface butalso the inner part of the object


More recent research has introduced some doubt: steam dewaxing could modify

the morphology of the components of the investment, reducing investment

permeability, important in removing air from the mould. Research on this subject is

still under way, so at present we cannot clearly recommend one method over the

other unless gas porosity is a significant problem.

Irrespective of the preferred dewaxing method, the flask should not be allowed to

cool down between dewaxing and burnout. The investment will suffer thermal stress

and its strength will be decreased.

We should note that steam dewaxing should not be used when casting with

gemstones. The investment used for this special purpose contains boric acid to

protect the stones. Boric acid is dissolved and removed by steam and no longer

available to protect the stones.

A warning about steam dewaxing! It is important that the steam be vented

out, preferably upwards, before removing the flasks from the chamber. Steam burns

are nasty and should be avoided!

2.8 BURNOUTBurnout, as the name implies, is carried out to burn out the last traces of wax and to

give the investment mould the refractoriness and characteristics required for casting.

The final characteristics of the mould will depend strongly on the burnout cycle

selected and particularly on the heating rate and temperature homogenisation in the

holding periods. Therefore, it is important to accurately follow the burnout cycle

recommended by the producer of the investment. The ratio between quartz and

a-cristobalite varies with investment grade and manufacturer and, consequently, the

optimum burnout cycle may change.

2.8.1 The burnout cycleThere are two critical points in the heating cycle. The first one is at about 100-120°C

(212-248°F), when absorbed water and part of the gypsum crystallisation water

evaporate. This is a slow process, taking place with volume contraction. Therefore,

the temperature should be increased slowly, to avoid the creation of stresses that

could cause cracks in the mould, with consequent formation of fins on the cast items,

Figure 2.8.1.

The second critical point is around 250°C (482°F), when a-cristobalite transforms

to b-cristobalite. This transformation takes place with a volume increase. In this case

temperature should be held constant for sufficient time to ensure that the

transformation occurs uniformly in the whole mould.

Lastly, with gypsum-bonded investment, we should not exceed the maximum

temperature of 750°C (1382°F). Above 750°C (1382°F), because of the presence of

silica, calcium sulphate decomposition can start, with consequent degradation of

investment strength. This can result in the formation of a sandy surface on the

castings, Figure 2.8.2.

Figure 2.8.1 Fins on cast rings, caused bycracks in the investment


Figure 2.7.5 Usually, the sprue button shows abulge in the centre when calcium sulphatedecomposition occurs

Figure 2.7.6 In some cases, the sprue buttonshows a single inner cavity, produced by strongreaction gas evolution (see also Figure 2.7.5)


On the other hand, to guarantee complete combustion of carbonaceous residues

left by the wax, we should exceed 690°C (1274°F). A nearly universally accepted

compromise gives 730°C (1346°F) as maximum burnout temperature. The critical

role of temperature comes out clearly from what has been said. Therefore, it is very

important to check the temperature control equipment of the burnout oven

periodically with a calibrated thermocouple, Figure 2.8.3.

The following is a typical burnout cycle. After dewaxing, ramp slowly to 250°C

(482°F) in 1 hour, hold at 250°C (482°F) for 2 hours, ramp to 450°C (842°F) in 1

hour, hold at 450°C (842°F) for 2 hours, ramp to 730°C (1346°F) in 11/2 hours, hold

at 730°C (1346°F) for 3 hours, then slow cooling to the selected flask casting

temperature and equilibrate at the casting temperature for at least 11/2 hours. The

casting temperature of the mould is chosen as a function of the pattern being cast

and the alloy used. The timing given for the cycle will vary, depending on the size of

the flask. Larger flasks require longer cycle times, Table 6.

It is very important to keep the flask at the holding temperature long enough to

equilibrate temperature in the whole volume of the mould. We should remember

that investment is a poor conductor of heat. Temperature measurements carried out

by inserting thermocouples in different points of the moulds have shown that,

independently from temperature level, at least 11/2 hours are required for the centre

of the mould to reach oven temperature. The same holds for the heating and the

cooling part of the burnout cycle. Flasks should not be allowed to cool down to room

temperature during the burnout cycle and then be reheated. They will crack and be

of poor quality. If the burnout oven fails or there is a power failure and the

temperature of the flask falls below about 250°C (482°F), throw the flasks away!

The oven atmosphere must be strongly oxidising, to guarantee complete burning

of carbonaceous residues. For the same reason, overfilling the oven with flasks

touching each other should be avoided. Sufficient space should be left for air

circulation among the flasks.

As with investing, the investment manufacturer’s recommended burnout cycle

should be followed.

Where stones-in-place casting is being done, the burnout cycle must be modified

to prevent damage to the stones. Maximum temperature is only 630°C (1166°F) but

times may be longer to ensure wax burnout. Follow the recommendation of the

investment manufacturer. An example is shown in Figure 2.8.4.

Figure 2.8.2 Sandy surface on a cast item,caused by investment crumbling duringcasting

Figure 2.8.3 Reference thermocouple

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

800

200

400

600

0

Hours

Tem

pera

ture

(°C

)

Figure 2.8.4 Example of a burnout cycle forstones-in-place casting. (Courtesy SRS Ltd.)


Table 6 Flask size and typical burnout cycle time

Flask size Total cycle time Times to & at step temperatures* (hrs)

2.5 x 2.5 in. (63 x 63 mm) 5 hours 1 + 1 + 2 +1

3.5 x 4 in. (89 x 100 mm) 8 hours 2 + 2 + 3 + 1

4 x 8 in. (100 x 200 mm) 12 hours 2 + 2 + 2 + 4 + 1

*300°F/150°C; 700°F/370°C; (900°F/480°C); 1350°F/730°C; Casting temperatureSource: KerrLab


2.8.2 Behaviour of calcium sulphate-bonded investment duringburnout

After dewaxing, when the temperature of the flask rises above 100°C (212°F), free

water evaporates and gypsum (CaSO4.2H2O) begins to lose its water of hydration, but

the complete transformation of gypsum into the anhydrous form of calcium sulphate

(anhydrite) occurs over a wide temperature range, through complex transformations

of the crystal lattice.

From the point of view of the goldsmith, it is important to note that these

transformations take place with a considerable volume contraction, which is particularly

severe at 300-450°C (572-842°F). If gypsum alone were used to produce investment

for lost wax casting, the moulds would crack in service and would also produce castings

a great deal smaller than the original patterns. Silica is used to compensate for this

gypsum shrinkage and to regulate the thermal expansion of the mould.

Silica exists in several crystalline forms, and two of them are used in the production

of investment powders. Quartz is the most readily available form and its conversion

from a to b crystal forms is accompanied by an increase in volume at around 570°C

(1058°F). Cristobalite is the other major constituent of investment powder and

this form of silica also undergoes a significant increase in volume as it transforms

from its a to b crystal structure at around 270°C (518°F). Thus, these

two allotropic forms of silica are used to override the shrinkage effect of the

gypsum binder.

A typical thermal expansion curve of a jewellery investment powder, Figure 2.8.5,

shows how the cristobalite provides the expansion between 250 and 300°C (482-

572°F). Then, there is a temperature band up to about 570°C (1058°F) where the

gypsum shrinkage dominates. Above 570°C, we see the contribution of quartz

transformation.

It is important to remember that, when the investment mould cools, it will pass

through the silica transformations which, being reversible, will contract an equal

amount to the original silica expansion. But the contraction of the plaster is

permanent, so there is no more volume compensation. This cooling curve can be

used to understand the final casting size and explains why the flasks cannot be

cooled too much between burnout and casting. After casting and on cooling

the plaster becomes very weak and, coupled with the disruption caused by the

cooling contraction of silica, enables the cast investment to be readily removed

during quenching.


Figure 2.8.5 Thermal expansion curve of atypical calcium sulphate bonded investmentfor jewellery casting


2.9 MELTINGNot all jewellery workshops buy ready-made carat gold alloys from precious metal

alloy producers, so very often melting coincides with the formation of the alloy for

casting. Generally fine gold is added to a suitable master alloy. This is generally

preferable than producing carat gold alloys in situ, starting from pure metals. The use

of reliable master alloys, produced by reputable companies and correctly utilized,

can help to avoid many problems and guarantee a consistently good quality of the

end product.

The alloy to be melted should preferably be used in the form of grains of similar

size. This gives an advantage in temperature control. When the alloy is in small

pieces, melting is easier and faster and the risk of overheating can be avoided. Many

goldsmiths prefer to make the carat gold alloy in a preliminary melt, casting it into

water to make grain. Graining is carried out by pouring the molten metal from

suitable crucibles, preferably with bottom pouring, into stirred water.

Basically, there are three methods for melting: the gas torch, the electric

resistance furnace and induction heating. The torch is the most ancient method and

finds little use for melting in modern jewellery factories. Propane or natural gas is

preferred for heating, supposedly because they give a cleaner flame than acetylene.

The flame for melting should be reducing: a reducing flame has an irregular contour,

is bright blue and makes little noise. A reducing flame has a low oxygen content, so

it captures oxygen from the surrounding atmosphere and shields the melt from

oxidation. Nearly all alloy types can be melted with a torch.

Electrical resistance heating has been largely used for melting until the more

recent introduction of induction heating. Resistance heating allows working in a

closed environment, where atmosphere control is possible. Melting can be

performed in inert gas (nitrogen or argon) or in slightly reducing atmosphere

(forming gas). With resistance heating, it is difficult to obtain the high temperature

required for melting some white golds. In all cases melting is rather slow.

Induction heating is the most modern method and is used in nearly all latest

generation casting machines. Induction melting is very fast and induces stirring of

the molten metal, with rapid thermal and chemical homogenisation. The stirring

effect is greater, the lower the frequency of the induction heating.

Melting is probably the step of investment casting with the highest “metallurgical”

content. Therefore, it is very important to follow some basic rules or guidelines.

1. Before melting, the required amount of precious metal alloy should be

calculated: the weight of the wax tree multiplied by the density of the alloy

gives the minimum weight of alloy required for melting. A further amount

of alloy will be added to allow for the sprue button.

2. The amount of recycled scrap in the melt charge should be kept to a

mininum but never more than 50% scrap metal should be used for the

charge.

3. Any scrap metal to be remelted must be perfectly clean and free from

oxides and investment residues.



4. For preference, grained alloys should be used. When scrap from preceding

operations is used, it is recommended to remelt and grain it prior to use for

casting.

5. The melt should be stirred after melting, to ensure complete

homogenisation. In modern induction heated casting machines, stirring is

induced by electromagnetic interaction. In open furnaces, torch or electric

resistance heated, stirring should be done manually with a suitable

refractory rod, to avoid pollution of the melt.

6. The metal should be kept in the molten state for the shortest possible time,

to limit oxidation and the loss of alloying elements by evaporation.

7. Before casting, the molten metal should be heated to a temperature higher

than the melting temperature of the alloy (superheat). The required

amount of superheat depends on the alloy, on the type of items to cast and

also on the casting equipment. In all cases, the degree of superheat should

be kept as low as possible: it could range from about 50°C (122°F) with a

bottom pouring crucible in a modern casting machine to typically 75-

100°C (167- 212°F) in an open top pouring crucible.

2.10 CASTINGIn modern melting/casting machines, pouring of the molten metal into the mould is

carried out automatically. Melting and casting are controlled by the machine through

dedicated software. For most current static machines, pouring takes place through the

bottom of the crucible, so metal temperature loss is reduced to a minimum. If the

machine has a tilting crucible, an additional temperature loss up to 80-100°C (144-

180°F) should be considered when determining the casting temperature of the molten

alloy (i.e. amount of superheat). The liquid metal and flask temperatures should be kept

as low as possible to minimise the formation of defects, in particular gas porosity.

Therefore, before starting the production of new items, a set of tests should be

performed to find the optimum system temperature. The term ‘system temperature’ is

used to indicate the set formed by the molten metal and flask temperatures.

Solidification starts immediately after the liquid metal has filled the cavity of the

mould. The temperature difference between the liquid metal and the flask is always

considerable (about 400°C/720°F or higher). Therefore, the liquid metal filling the

mould cavity will start freezing from the investment mould surface, Figure 1.8 (a - d),

and solidification will progress rapidly to the inner part of the pattern. If the tree has


Pattern Size Total Heat Increase Percent Increased Surfacein mm Surface Area, Over 1mm Area increase

mm2 thick pattern (over 1 mm Pattern)

15 x 15 x 1 510 0 0 %

15 x 15 x 2 570 2 x 11 %

15 x 15 x 4 690 4 x 27 %

Table 7 Pattern size and relative surface area

Figure 2.10.1 Patterns with different shapefactor

Figure 2.10.2 Experimental patterns withdifferent shape factor


been assembled correctly, possibly with the patterns making a 90° angle to the main

sprue, if the feed sprues have been properly designed and if a main sprue of the right

diameter has been used to perform correctly, without exceeding its duty as a heat

reservoir, solidification will take place directionally towards the sprue and shrinkage

porosity will collect in the main sprue and in the sprue button.

If, on the other hand, shrinkage or gas porosity is present in the cast items, the

process parameters should be modified in a rational way, after due consideration of

the situation.

To find the optimum temperature combination for the liquid metal and the flask,

it is necessary to clarify some concepts concerning the shape of the items to be cast

and, specifically, the shape factor (surface to volume ratio).

If we cast three patterns that are 15 x 15 mm x 1, 2 and 4 mm thick respectively,

Figure 2.10.1, on the same tree, we could say that the casting conditions were the

same for all three patterns because the investment and the metal were at the same

temperature when the metal was cast. The surface area on the top and bottom of

all the patterns is constant; the only increase in surface area on the larger patterns is

on the sides; thus, the volume increases much faster than the surface area, Table 7.

All the heat lost to the investment from the metal must go through the mould-

metal interface. Investment is a poor conductor of heat and measurements show

that after the metal is cast, only 1 to 1.5mm thickness of investment material next to

the metal will experience any temperature change; naturally, as the metal cools, the

adjacent investment heats.

The temperature of the metal may have been the same when it was cast, but

each pattern holds a different amount of metal and, therefore, a corresponding

amount of heat energy. The 4mm thick pattern will discharge 4 times the heat to the

investment relative to the 1mm pattern. This means the temperature rise of the

investment will be much greater around the 4mm pattern than around the 1mm

pattern and the 2mm pattern should lie in-between.

If the metal temperature and the flask temperature are correct for the 1mm

pattern (this is the hardest to fill and requires higher temperature), then the

temperature will be too high for the larger patterns and gas porosity is likely.

Casters have a practice of classifying their patterns for flask temperature in terms

of heavy, medium and light. Most casters would classify two of the patterns on the

tree in Figure 2.10.2 as heavy and one each as medium and light. Therefore, the

concept of system temperature is also useful for taking into account the effect that

surface area and volume (surface area to volume ratio) have on the cooling of the

metal and the subsequent increase in the temperature of the investment at the

metal interface for a specific pattern, flask and metal temperature and alloy. The

pattern with the grooved surface has less volume of metal as the other 4mm thick

pattern and the surface area is somewhat larger. Because of that, it might cast better

at the ‘medium flask’ temperature. It can be concluded from this that:

a) System temperature is pattern specific. When considering which patterns can

be on the same tree, the surface to volume ratio should be noted, not just the

cross-sectional thickness.



b) When the pattern has high surface area and low volume (thin patterns), the

flask temperature influence is greater than that of the metal temperature. As

volume increases in ratio to surface area (thick patterns), the flask temperature

influence on the system temperature decreases.

c) Flask temperature is controlled by the hardest to fill pattern on the tree.

d) When thin and thick patterns are on the same tree, the flask temperature has

to be high enough to fill the thin patterns, and would be too high to cast the

thick patterns at their best system temperature.

e) System temperature is alloy specific. The casting temperature for a metal has to

be above the liquidus temperature and since various alloys melt at different

temperatures, the casting temperatures will vary as well. For a particular alloy, the

casting temperature will generally be lower for thick section patterns and higher

for thin section patterns, but in every case the casting temperature of the metal

is strongly influenced by size, shape and attachment point of the feed sprue.

Better-designed feed sprues will allow casting at a lower system temperature.

2.10.1 Test for system temperatureA simple experiment can be used to quickly find the best system temperature for a

range of patterns cast with a specific alloy. Build five trees alike with five or six

different patterns on each tree, as seen in Figure 2.10.3. The selection of patterns

should represent the variety of patterns you cast, for example thin, medium, thick,

large and small. Inspect all the wax patterns before using them and attach them the

same side up. The patterns are attached in a vertical row at the top, centre and

bottom of the main sprue. Do not expect all the different patterns to cast well on any

one tree; rather, the purpose is to find out how each pattern casts at a temperature

combination. If there are five patterns on the tree, one cast will give a good idea how

each of these different patterns will cast at a given temperature set and, therefore,

five experiments are performed in one cast. This is called a designed experiment,

whereby the normal methodical testing process is shortcut.

A set of test trees, as described above, are cast using a grid of flask and metal

temperatures. The grid should note the alloy and the patterns being cast. Put the

presumed temperature ‘sweet spot’ in the centre of the grid as shown, Table 8.


Date:Alloy: 18KY Patterns Tested: A, B, C, D, E

Flask Temperature, °C (°F)

Metal Temperature, °C (°F) 500 550 600(932) (1022) (1112)

960 (1760)

980 (1796) Flask 2

1000 (1832) Flask 1 Flask 3 Flask 5

1020 (1868) Flask 4

1040 (1904)

Table 8 System Temperature Test Grid

Figure 2.10.3 Experimental tree for systemtemperature selection


In this case, the flask temperature is 550°C (1022°F) and metal 1000°C (1832°F).

Cast one flask at each temperature combination on the grid above, below and at

each side of the sweet spot. Make sure all the flasks are well soaked at the casting

temperature before casting. Holding the flask for three or four hours at casting

temperature is considered prudent to get good experimental results.

After casting, inspect the castings in the as-cast condition, record the results and

send any promising casting through finishing and normal quality inspection. A simple

inspection criterion can be used to grade the castings for evaluating test results.

2.10.2 Inspection criteriaAll inspected castings are rated as a 1, 2 or 3 where

1 = any casting that can be finished and would pass internal quality control

2 = any casting that can be repaired, finished and would pass internal quality

control

3 = any casting that is rejected, not economic to repair

In most cases, the castings graded #3 will be sorted out in the as cast condition.

Some #2 castings may be identified in the as cast condition, or subsurface defects

may show up later. Wax patterns must be free of any powder. By careful inspection

of wax patterns before casting, defects attributed to the mould and wax pattern can

be eliminated. Care should be given to identify any defect that can be attributed to

investment or burnout. Fins from cracked investment, or voids caused by investment

inclusions, for example, are not temperature related casting defects and should be

excluded from this test grading. A short list of defects that should be attributed to

wrong system temperature are incomplete filling, gas porosity, shrinkage porosity,

rough surface (where the wax was smooth), and cracks.

After the castings are graded, the score (1, 2 or 3) for each pattern number is

recorded on a test results chart, Table 9.

The test data are easy to understand in this form and trends can quickly be seen.

The example in Table 9 clearly shows the best flask and metal temperature for

casting pattern A in alloy 18KY (18 carat yellow).


Date:Alloy: 18KY Pattern A



980 (1796) 1/1/1

1000 (1832) 2/3/3 1/2/2 3/3/3

1020 (1868) 2/2/2

Top / Centre / Bottom

Table 9 System Temperature Test Results Chart


Pattern A was picked to represent a larger selection of patterns that were judged

to have similar surface-to-volume ratios and, therefore, would be expected to cast well

at similar flask and metal temperature. So all patterns that are represented by pattern

A in the test should be cast at metal 980°C (1796°F) and flask 550°C (1022°F).

The goal is to get all grade one castings and it is possible that that is not achieved

for a pattern in the temperature grid that was picked for the test. In Table 10, pattern

B is used to show how the chart can identify trends.

Metal 1000°C (1832°F) and flask 600°C (1112°F) is the best combination, but not

good enough. Since metal 1020°C (1868°F) and flask 550°C (1022°F) is much

better than metal 980°C (1796°F) and flask 550°C (1022°F), the trend to improve

would be to increase metal temperature to 1020°C (1868°F). This could be done as

a single cast test, or a new grid could be formed with a new presumed sweet spot.

2.10.3 Test for best feed sprue designAfter the system temperature is found and applied to the range of pattern styles

produced, it may become evident that not all the patterns are casting with the desired

quality at the system temperature chosen for it. This leaves two options: find a new

temperature set for that pattern, or experiment with the feed sprue. If the casting

surface is rough and such things as powder in the wax, or a rough wax coming from the

mould are eliminated, then the temperature may be too high for that pattern and a

lower temperature can be explored. If the surface is very fine but details such as prongs

are not filling, the feed sprue may be to blame. Another designed experiment can be


Figure 2.10.4 Selection of best feed spruedesign

Figure 2.10.5 Often it is necessary to considerthe model as an integral part of the feedsystem, to position feed sprues correctly

Figure 2.10.6 Examples of cast trees

a

b

a b c

Date:Alloy: 18KY Pattern B



980 (1796) 3/3/3

1000 (1832) 3/3/3 2/2/2 1/1/2

1020 (1868) 1/2/2

Top / Centre / Bottom

Table 10 System Temperature Test Results Chart


used to find the feed sprue design that works best for any pattern. This time, only one

pattern design will be used on the tree, but it will be attached with five different feed

sprue configurations. Using wax wire (or wax feed sprues made in a rubber mould, Figure

2.2.7), attach feed sprues to the patterns in different locations. Build a tree in the same

manner as the system temperature and test with three patterns on the tree with each

of the five or six feed sprue configurations, Figure 2.10.4. One flask may be all that is

required to solve the defect, but if the results are not satisfactory, then make and cast

additional flasks on a new temperature grid. In some cases also the pattern should be

considered as part of the feed system, Figure 2.10.5. Some examples of successfully cast

trees are shown in Figure 2.10.6.

2.10.4 Casting with stones in placeThe technique of producing jewellery by investment casting with stones in place

(stones are set in the wax pattern) is no longer a novelty, but its use has increased

considerably in the last 10 years. At the beginning, this technique has been used for

large scale industrial setting of synthetic stones, mainly cubic zirconia, where the cost

of manual setting was not justifiable, but later its use has rapidly been extended to

natural stones, like diamond, ruby, sapphire, etc., Figure 2.10.7.

The same steps of the conventional investment casting process are used for

stone-in-place casting, but some modifications are required. The master model

should be suitably designed for positioning the stones correctly and the stones

should have a small groove, just below the girdle, to favour firm clamping by the

metal. Wax patterns must be flexible and springy and the stones are set in the wax.

This operation is much simpler and faster than setting in the metal. The use of a

special vacuum tweezer is advised to facilitate handling of the stones.

Invisible setting is the most suitable technique for stone-in-place casting. The use of

the special investment grades or classic gypsum-bonded investment, but with a very fine

grain, is recommended. In the latter case, boric acid should be added to the investment

slurry, to protect the stones during burnout and casting, as described earlier. Dewaxing

should be performed dry, to avoid dissolution of the boric acid by steam.

The maximum temperature in the burnout cycle should be lower than usual, to

avoid spoiling the stones. Consequently, holding time at maximum temperature will

be longer than usual, to remove carbonaceous residues left from wax completely.

Maximum burnout temperature and recommended holding time should be

approximately:

• for diamond and emerald: 630°C (1166°F)/6 hours – flask casting temperature

480-530°C (896-986°F),

• for zircon, ruby, sapphire and synthetic stones 680°C (1256°F)/5 hours – flask

casting temperature 550-600°C (1022-1112°F).

Figure 2.10.7 Example of a cast tree withstones in place



The cast flasks must not be water quenched immediately, to avoid cracking of the

stones by thermal shock. The flasks set with diamonds can be water quenched after

at least 20 min. from casting. Flasks with other types of set stones can be quenched

after 60-120 minutes.

2.11 COOLING AND RECOVERY OF THE CAST ITEMSThe flasks cast with simple yellow or red gold should be water quenched about

3 minutes after casting but this time will depend on other factors, such as the flask

temperature on casting and specific alloy composition. With a longer cooling time,

cast items in 18 or lower yellow and red carat gold can harden, because of the

precipitation of intermetallic gold-copper phases in the gold matrix. If we want to

have the alloy in the condition of maximum softness (e.g. if heavy cold working is

required) it is necessary to heat to a high temperature (600-700°C (1112-1292°F))

and then water quench the castings. Flasks cast in low carat golds containing silicon

must be cooled longer to avoid quench cracking, preferably to 400°C (750°F) before

quenching. Flasks cast with nickel white gold should cool for slightly longer time

(5-6 minutes) before water quenching. Nickel white gold can crack if cooled too fast,

because of strong internal stresses.

The higher is the quenching temperature of the cast flask, the easier is the recovery

of the cast tree. The investment crumbles into pieces because of the thermal shock.

Safety note: As said earlier, quenching of the flask must be done in a well

ventilated area and the operator should wear special protective masks, approved for

protection from silica dust. Inhalation of fine silica dust is dangerous and must be

avoided. The steam produced by quenching hot flasks entrains very fine silica

particles that remain airborne and can be inhaled by an unprotected operator or

passer by!

The recovered tree should be thoroughly cleaned of investment residues adhering to

its surface. Cleaning is performed with high pressure water guns or by wet grit-blasting.

The above mentioned process refers only to calcium sulphate-bonded investment.

In the case of phosphate-bonded investment, the separation of the cast tree from

the investment can be realised only by mechanical means.

Subsequently, if the surface of the tree is oxidised (a frequent occurrence), it

should be pickled carefully in an acid bath. The most frequently used pickling solution

is 20% sulphuric acid in water at a temperature of 50°C (122°F). The cast tree is

dipped in the solution for about 2 minutes. Some workshops use “safety pickle” as an

alternative to storing and mixing sulphuric acid. This is sodium hydrogen sulphate

that, when dissolved in water at a concentration of 220 g/litre, gives what is

essentially a dilute solution of sulphuric acid.

If phosphate-bonded investment has been used, good results are obtained with a

50% water solution of hydrofluoric acid at 50°C (122°F). The cast tree is dipped in

the solution for about 5 minutes.

Safety note: Acids can be dangerous: they are strongly corrosive and can

cause serious problems, if they come into contact with the skin or the eyes.



Hydrofluoric acid is more dangerous than sulphuric acid and must be handled

with considerable care under an exhaust system and avoiding contact with the skin.

Glass containers or beakers cannot be used; it should be contained in

plastic containers and bottles.

When diluting a concentrated acid to make a pickle, the acid must be added

slowly to water, while stirring, and not the other way round. The exothermic reaction

that occurs when adding water to concentrated sulphuric acid produces intense heat

and may cause instantaneous boiling and spillage. The operator should always wear

protective clothing when handling acids and, most important, suitable eye

protection! In the case of contact of an acid with the skin or the eyes, wash

immediately with abundant water, then see a doctor.

Acids and spent pickle solutions can be polluting and should not be discharged

into the drainage system without treatment: all requirements for safety, health and

environmental protection should be complied with.

After pickling, the cast tree is washed with water and dipped in a sodium

carbonate solution, to neutralize acid residues. Then it is carefully washed, to remove

all traces of pickle solution, and dried, preferably with a pressure jet of steam.

After drying, the cast tree is subjected to visual inspection. Possible defects, like

incomplete filling, shrinkage or gas porosity, etc. should be accurately described and

the position of the defective castings on the tree should be recorded. The more

information collected on the defects, the higher will be the probability of being able

to explain what happened and to take corrective action.

The subsequent step is cutting the castings off the main sprue. This can be done

with hand cutters or pneumatic sprue cutters that largely eliminate physical strain,

Figures 2.11.1 and 2.11.2.

After a second and deeper quality inspection, the cast items are sent for assembly

and finishing. The recommended finishing procedures are described in the Finishing

Handbook, published by World Gold Council in 1999.

2.12 SUMMARY OF THE BASIC RULES FOR THE DIFFERENTSTEPS OF INVESTMENT CASTING

In this section, a summary is given of the basic rules and guidelines to be followed in

the different steps of investment casting, necessary to obtain good quality cast

product. These rules have been distilled from what has been discussed in the

preceding sections.

Design (2.1):

• A good knowledge of the whole process is required.

• The designer should be in continual contact with the production staff.

• “Castable” objects should be designed.

• Sharp changes of cross-section (e.g. thick-thin-thick) should be avoided.

Otherwise adequate feed sprues should be provided.

• Possible production problems should be discussed before launching a new model.

Master model (2.2):

• Prefer alloys with suitable hardness.


Figure 2.11.1 Bench sprue cutter

Figure 2.11.2 Hand held sprue cutter


• Finishing must be perfect.

• Rhodium plating is recommended.

• Also rapid prototyping techniques should be considered.

• The design of the feed system must take into account size and complexity of the

model.

• In the feed system, bottlenecks (thick-thin-thick patterns) and abrupt changes of

direction should be avoided. The principles of fluid mechanics should always be

considered.

Rubber mould (2.3)

• You should cultivate the skill of an expert mould maker.

• Knowledge of the characteristics of materials (natural rubber, silicone rubber, etc.)

should be the basis for selection of the correct material.

• Store the products for mould making as recommended by the producer.

• The geometry of the mouth of the mould should be accurately designed (it must

fit exactly on the nozzle of the injector).

• Vulcanisers with a reliable temperature monitoring and control system should be

used.

• The temperature in the vulcaniser should be frequently checked with a calibrated

instrument.

• The moulds should be kept perfectly clean and stored away from heat and light.

They should be numbered for identification.

Wax patterns (2.4)

• Prefer wax types with a narrow melting range.

• Understand the properties of different wax types to allow correct selection.

• Prefer injectors that apply vacuum in the mould prior to injection.

• Use a mould clamp with controlled clamping pressure.

• Record production parameters for each model.

• Weigh the wax patterns to evaluate weight variability for a single model.

• Verify wax quality accurately before use. Reject faulty wax batches.

• Don’t use too much talcum powder to facilitate pattern extraction from the

mould.

• Don’t use recycled wax.

Assembling the tree (2.5)

• Prefer a main sprue designed for optimum performance.

• In any case, prefer a rubber base with a conical sprue button (not hemispherical!).

• The rubber base should not show signs of wear.

• The rubber base should not contain residues of investment from preceding flasks.

If necessary, clean the base thoroughly!

• The welds between the main sprue and the feed sprues should be well filleted.

Avoid constrictions!

• Prefer a 90° angle between main sprue and feed sprue.

• The outer end of the wax patterns should be at about 10 mm distance from the

flask wall.



Investing the flask (2.6)

• Use investment powders produced by reputable companies.

• Store the investment in a well-sealed container and in a dry place.

• Check the production date of each new batch.

• Before use, clean the flask with a wire brush, to remove all traces of used

investment.

• Use powder and water at the recommended temperature.

• Mix the powder with the water in the ratio recommended by the producer.

• Prefer deionised water.

• Check the gloss-off time of each new batch of investment.

• Let the flasks set for at least 1 hour and no more than 2 hours before dewaxing.

Dewaxing (2.7)

• There is not clear understanding if dry or steam dewaxing should be preferred.

The most important thing is to start the burnout cycle immediately after

dewaxing, without letting the flask cool.

Burnout (2.8)

• In the case of electric resistance heating, prefer ovens with forced ventilation.

• Verify that temperature is uniform throughout the oven also during heating.

• Avoid loading too many flasks in the oven. Enough space should be left for air

circulation.

• Follow the burnout cycle recommended by the producer.

• Observe the holding times in the heating ramp.

• Don’t exceed 750°C (1382°F) maximum temperature (for gypsum bonded

investment).

• Temperature should be allowed to homogenize in the whole flask before casting.

• Preferably, the oven should be equipped with a double control system, with a

thermocouple in the work chamber and another one near the heating elements.

Melting (2.9)

• Calculate the weight of alloy required for casting (from the weight of the wax

tree).

• Use grained alloy or alloy cut in small pieces.

• Use clean metal.

• Don’t make a charge with more than 50% scrap metal.

• Don’t remelt the alloy more than three times.

• Avoid unnecessary overheating.

• Stir the molten metal for perfect homogenisation.

Casting (2.10)

• Keep the alloy molten for the shortest possible time.

• Use the minimum degree of superheat consistent with good casting.

• Cast in the shortest possible time.

Cooling (2.11)

• Water quench 3 minutes after casting (yellow and red gold castings) or 6 minutes

after casting (nickel white gold).

• After recovering the tree, clean it accurately and make a visual inspection. Type

and position of defects should be recorded.



2.13 SCHEMATIC LIST OF POSSIBLE DEFECTSAs mentioned in the introduction, this Handbook will not deal with defects in detail.

The analysis of most common defects, along with a description of their causes and

possible remedies to avoid their occurrence, has been dealt with in the Handbook on

Casting and Other Defects in gold jewellery manufacture, published by World Gold

Council in 1997. The reader should refer to this Handbook.

Here it is emphasized that, in most cases, defects don’t have a single cause.

Frequently, there are many causes acting together to cause a specific defect.

Consequently, corrective action will require compromises, in order to minimize

defect formation and improve end product quality.

A schematic list of most frequently observed defects is given below, along with

most common causes. These causes can act separately or in combination.

Shrinkage porosity

• Pattern is improperly sprued. Sprues may be too thin, too long or not attached in

the proper location.

• Not enough liquid metal reservoir after filling mould cavity.

Gas porosity: it can consist of trapped or reaction gas. Distinguishing the two causes

is very difficult:

• Too much turbulence during pouring.

• Incorrect assembling of the patterns on the tree.

• Too much distance between the extremity of the patterns and the outer surface

of the flask.

• Too high metal and/or flask temperature.

• Metal is contaminated with gas.

• Too much moisture in the flux, if used.

• Too much recycled scrap has been used. Always use at least 50% new metal.

• Poor mould burnout.

Incomplete filling

• Insufficient feeding system.

• Too low metal and/or flask temperature.

• Pattern was improperly sprued, creating turbulence when casting in a centrifugal

casting machine.

• Centrifugal casting machine had too high revolution per minute.

Fins on the edges

• The investment has absorbed humidity before the preparation of the slurry.

• Flask was disturbed while investment was setting.

• Rubber base was removed too soon.

• The flask has been allowed to partially dry before dewaxing.

• Too high burnout temperature.

• The flask has been allowed to cool between dewaxing and burnout.

• Flask was improperly handled or dropped.

• Speed was set too high on centrifugal casting machine.

• Flask was placed too close to heat source in burnout oven.

• Flasks were not held at low burnout temperature long enough.



Bubbles or nodules on the surface of the cast items

• Air bubbles in the wax patterns because:

– Vacuum pump is leaking air.

– Vacuum pump has water in the oil.

– Vacuum pump is low on oil.

– Investment not mixed properly or long enough.

– Invested flasks were not vibrated during vacuum cycle.

– Vacuum extended past investment working time.

Depressions in the surface of the cast items

• The defect was already present in the wax patterns (see Table 4).

Watermarks

• The correct water to investment powder ratio has not been observed.

• The flask has been vibrated for too short a time (too long time between end of

vibration and investment setting).

Inclusions (Foreign particles: oxides, investment, graphite) in castings

• Patterns were improperly sprued to wax base or tree or not filleted, causing

investment to break at sharp corners during casting.

• Flask was not sufficiently cured before placing in the burnout oven.

• Improper dewaxing cycle was used.

• Flask was not cleaned from prior cast.

• Loose investment in sprue hole.

• Molten metal contains excess flux or foreign oxides.

• Crucible disintegrating or poorly fluxed.

• Improperly dried graphite crucible.

• Investment was not mixed properly or long enough.

• Flask was not held at low burnout temperature long enough.


• Contaminants in wax patterns.

Rough surface

• Too much talcum powder has been used to facilitate the extraction of the wax

patterns.

• Talcum powder and spray have been used at the same time.

Sandy surface: often associated with investment particles enclosed in the surface of

the metal:

• Too high burnout temperature.

• The investment has absorbed humidity before the preparation of the slurry.

• Flask was not sufficiently cured before placing into burnout oven.

• Flask was held in steam dewaxer too long.

• Metal, flask or both were too hot.

• Patterns were improperly sprued.


Shiny castings

• Carbonaceous residues have been left in the mould, creating a reducing condition

on the mould surface.


3 ALLOYS FOR INVESTMENTCASTING

The result of the casting process depends strongly on the properties of the alloys

used. The alloy composition should be selected to suit the casting process. That

means that alloys tailored for casting should be used. In the past, mainly ‘general

purpose’ alloys have been used. This was (and still is, in part) possible for yellow carat

gold based on gold-silver-copper due to the beneficial working properties. However,

the situation is different for white gold alloys.

The steadily increasing requirements for quality and economy have led to the

development of alloys modified for casting in recent decades. Such development is

difficult because the modifications have to be achieved without any change in

fineness and colour. Because of this, the modifications have been restricted to

relatively small alloying additions. The development of white gold alloys suitable for

casting has been somewhat different. General information on jewellery alloys is

available in the literature.

3.1 YELLOW AND RED GOLD ALLOYS3.1.1 Metallurgy and its effect on physical propertiesThe properties of the ternary alloy, gold-silver-copper, are strongly influenced by the

binary systems, especially gold-copper and silver-copper, Figures 3.1.1 and 3.1.2. The

low melting eutectic in the silver-copper phase diagram influences the melting (and

casting) behaviour of yellow gold. A relatively small variation in the silver/copper ratio

changes the melting range of the alloy considerably. In addition, a separation into

two phases, a silver-rich and a copper-rich phase, will occur (especially in 14 carat

alloys, see later).

Age hardening can occur below approximately 410°C (770°F) due to the ordering

process originating from the gold-copper system.

3A L L OY S F O R I N V E S T M E N T C A S T I N G

10 20 30 40 50 60 70 80 90 1000

300

400

500

800

900

1000

1100

600

700

200

0

100

Atomic percent copperAu Cu

Tem

pera

ture

°C

(Au, Cu)

10 20 30 40 50 60 70 80 901000

Weight percent copper

1064.43°C 1084.87°C

910°C

44

L

AuCuII

AuCuI

Au3Cu

AuCu3I

AuCu3II410°C

390°C

385285

64

38.6

240

Figure 3.1.1 Gold-copper phase diagram

Weight per cent copper

100 30 50 70 9020 40 60 80 100

10 30 50 70 9020 40 60 801100

1000

900

800

700

600

500

400

300

200

Atomic per cent copper

Tem

pera

ture

°C

Ag Cu

960.5°

14.1 ????

39.9 ????

95.1 ????

108.3

??????

???

??????

10 0 30 50 70 90 20 40 60 80 100

1000

800

600

400

200

Composition weight, percent copper

Tem

pera

ture

°C

Ag X Cu

960°

1083°

G

D

F

100 20

1000 960°

800

600

400

200

a

Ag

Temperature

°C

1083°

30 40

D

F G

A

Composition weight, per cent copper

b

Solid a+b

Liquid L

92.0CB

8.8 28.1

EL+b

L+a 779°C

50 60 70 80 90Cu100

Y

X

Y

A

b

Solid a+b

Liquid L

92.0

C B

8.8 28.1

E L+b L+a 779°C

a

Figure 3.1.2 Silver-copper phase diagram


3 A L L OY S F O R I N V E S T M E N T C A S T I N G

It is not within the scope of this chapter to discuss the ternary alloy system in detail

(this is more fully discussed in the literature – see Further Reading at the end of this

Handbook).

As an example, Figure 3.1.3 shows the influence of the composition on the liquidus

temperature. There is a deep ‘valley’ in the liquidus temperature, starting from the

eutectic composition on the silver-copper side and continuing in the direction of the

gold-copper side (for more practical diagrams see Figures 3.1.7 and 3.1.8).

Figure 3.1.4 shows the phase distribution at a temperature of about 300°C

(572°F). The formation of a heterogeneous, two phase field and the formation of age

hardening intermetallic compounds can be recognised. The diagram gives the

situation for an ideal equilibrium state, which is never fully attained under the

practical conditions of investment casting. However, it conveys an idea what might

happen in yellow gold, with consequences for mechanical properties and tarnishing

behaviour. The separation into two phases decreases the tarnishing resistance; the

formation of so-called ordered intermetallic compounds increases hardness and

strength but reduces the ductility (increases tendency to embrittlement).

Table 11 give some examples of the compositions of yellow gold alloys at various

finenesses (caratages). Data for melting range, density and standard colour are

included as far as available. The data for the melting range are not very reliable in

some cases and have to be used with care.Table 11 Examples of yellow gold alloys used in the jewellery industry

Solidus LiquidusGold Silver Copper Zinc temperature temperature Density

Carat ‰ ‰ ‰ ‰ °C °C g/cm3 Colour*

14 585 90 320 5 860 890 13,1 5N14 585 100 277 38 835 865 13,1 3N14 585 140 270 5 835 865 13,25 4N14 585 200 200 15 825 835 13,5 2N14 585 260 140 15 830 845 13,7 1N

18 750 20 220 10 897 917 15,4518 750 45 205 0 890 895 15,15 5N18 750 90 160 0 880 885 15,3 4N18 750 90 155 5 880 895 15,3 4N18 750 125 125 0 885 895 15,45 3N18 750 140 90 20 865 903 15,3618 750 155 90 5 870 900 2N18 750 160 90 0 895 920 15,6 2N18 750 210 40 0 960 990 15,7 1N

21 875 0 125 0 926 940 16,7 Red21 875 17,5 107,5 0 928 952 16,8 pink21 875 45 80 0 940 964 16,8 yellow-pink

22 916,6 21,4 62 0 959 982 17,822 916,6 62 21,4 0 1010 1035 1822 917 32 51 0 964 982 17,822 917 55 28 0 995 1020 17,9

* Based on ISO 8654 classifications

Weight per cent copper10

Weig

ht p

er c

ent s

ilver

Weight per cent gold

90

80

70

60

50

40

30

20

10

0 30 50 70 9020 40 60 80

950°

900°

85

0°

800°

10

00°

950°

900° 1000°

1050°

1050°

90

80

70

60

50

40

30

20

Au

Cu Ag

Figure 3.1.3 Liquidus surface of gold-copper-silver system

100

80

60

40

20

0 100 20 40 60 80

80

60

40

20

Au

Cu Ag

α

α1/AuCu

α1/AuCu3

α1 + α2

Figure 3.1.4 Two phase region in gold-copper-silver alloys



All the alloys are based on gold-silver-copper. Most of the 14 ct alloys contain an

addition of zinc. About 50% of the 18 ct alloys also have small additions of zinc. Zinc

additions are not common in higher carat alloys. The influence of zinc additions on

properties are considered separately in section 3.3. Low carat alloys (10, 9 and 8 ct)

are, with few exceptions, based on copper- gold-(silver)-zinc (in this order). Zinc can

be considered as a main alloying element in this case.

In addition to these ‘traditional’ alloying elements, in recent times small additions

of other elements are in use, e.g. for grain refining (iridium) and ‘deoxidation’ (silicon,

boron). The influence of these additions is also discussed separately in section 3.3.

Recently, to improve the mechanical properties of high carat alloys (20 ct up to

micro-alloyed ‘pure’ gold) special alloy systems have been developed. They are not in

frequent use and shall be mentioned only briefly.

The density of 14 and 18 ct yellow gold is strongly influenced by the silver/copper

ratio (at constant gold content), Figures 3.1.5 and 3.1.6. Small additions of zinc have

a small influence on density, especially with 14 ct alloys where zinc is added more

frequently (Note: Figure 3.1.5 also contains alloys with small additions of zinc).

The densities of 21 ct alloys (875‰ gold) lie in the range of 16.7-16.8 g/cm3; for

22 ct (917‰ gold), the values lie between 17.8 and 17.9 g/cm3. Variations in the

silver/copper ratio are very limited and have no significant influence on density.

The solidification range of a yellow gold alloy depends on the composition in a

rather complicated way. In principle, it can be read from the ternary phase diagram.

However, for practical purposes, diagrams which demonstrate the melting range for

the most important 14 and 18 ct alloys as a function of silver content are more useful,

Figures 3.1.7 and 3.1.8.

For the 18 ct alloys, an increasing silver content mainly influences the liquidus

temperature, which also increases, but has less influence on the solidus temperature.

For the 14 ct alloys, the liquidus temperature is increased at higher silver

concentrations to a limited extent, but the solidus temperature is significantly

decreased. Therefore, the solidification range itself is increased for both 14 ct and for

18 ct alloys at higher silver contents. The consequence of a larger solidification range

is increased (micro-) segregation and a more pronounced dendritic structure.

The solidification behaviour of an alloy during casting is not only influenced by

the temperature range of solidification but also by the heat introduced by the melt

into the flask.

Table 10 shows some examples for the heat of solidification and the specific heat

of jewellery alloys and the pure alloying metals. The values are based on mass (as it

is common). Gold has the lowest and copper has the highest heat of solidification

Gold Silver Copper Heat of solidification Specific Liquidus Solidus SuperheatHeat Temp. Temp. 100 K (°C)

% % % J/g J/cm3 J/(g*K) °C °C J/g

91.7 6.2 2.1 60 1002 0.174 1032.8 1009 1775.0 16.0 9.0 72 1123 0.212 933.3 902.8 2158.5 30.0 11.5 76 1048 0.242 891.4 850.9 24

90 10 111 0.320 901.6 779.8100 65* 0.157*

100 107* 0.310*100 205* 0.494*

* Source: Edelmetall Taschenbuch

Table 12 Data of thermal analysis for some typical jewellery alloys

Density

60 100 140 180 220 260

80 120 160 200 240 280

300

13.8�

13.7

13.6

13.5

13.4

13.3

13.3

13.2

13.1

Silver (‰)

Den

sity

(g/c

m3 )

Figure 3.1.5 Density of 14 carat yellow goldas a function of silver content

Density

20 100806040 120 140 160 180

15.7

15.6

15.5

15.4

15.3

15.2

15.1

Silver (‰)

Den

sity

(g/c

m3 )

18 ct YG Density as a function of silver content

Figure 3.1.6 Density of 18 carat yellow goldas a function of silver content

10080 90 110 260 280 300 320 340 360

960

940

920

900

880

860

780

800

820

840

Silver (‰)

Tem

pera

ture

Influence of silver content on the solidification range of 14 ct yellow gold

SOLIDUS LIQUIDUS

Figure 3.1.7

6020 40 80 100 120 140 160 180

915920925930935940945

910905900895

880885890

Silver (‰)

Tem

pera

ture

(°C

)

Solidification range of 18 ct YG as a function of silver content

SOLIDUS LIQUIDUS

5N

4N

3N

2N

Figure 3.1.8


with silver lying in between. Therefore, alloys with more silver and copper deliver

more heat at solidification related to the mass. For practical purposes, it is more

important to know values related to the volume. The difference between different

jewellery alloys are now equalised to some extent. The table also shows that the heat

introduced by a melt superheat (casting temperature above liquidus temperature) of,

for example, 100°C (180°F) adds approximately a third of the solidification heat to

the total amount of heat introduced in the flask.

The sudden decrease of volume at solidification is responsible for the occurrence

of shrinkage porosity in jewellery castings

Table 13 presents some estimates for solidification shrinkage of some pure metals

and a typical jewellery alloy.

The overall porosity in a complete casting will be smaller than these values

because the shrinkage can be compensated to some degree by supplying additional

melt through the sprue and the gating system. However, in certain critical areas of

the casting, the porosity can be concentrated and exceed the mean value.

The interfacial tension between the melt and the investment is a critical factor

influencing the degree of form filling, the reproduction of fine surface details and the

surface roughness.

Yellow gold, based on gold-silver-copper, has a relatively high interfacial tension if

the formation of oxides is avoided (i.e. if casting is performed in oxygen-free

surroundings, e.g. in vacuum or a reducing atmosphere). However, the formation of

copper oxide reduces the interfacial tension considerably. Some values of interfacial

tension for 14 ct yellow gold are given in Table 14.

The relatively low value obtained with argon may indicate that oxygen was not

completely removed before filling the chamber with argon.

A high interfacial tension produces a characteristically rough surface structure,

mainly on thick walled items. This is due to dendritic solidification and shrinkage. The

microstructure is strongly dendritic. At solidification, initially a framework of dendrites

is formed. At final solidification, shrinkage sucks up the interdendritic melt from the

surface, leaving a dendritic relief. If the interfacial tension is low, the wall of the

investment is wetted and its smooth structure is reproduced, as long as no

decomposition of the investment occurs.

As seen in Table 14, the tension can be simply reduced by casting on air. However, no

improvement in surface quality will be achieved. The benefits of low surface (interfacial)

tension are compensated by the detrimental influence of oxidation and scaling. The

more effective approach is through varying the alloy composition (see later).

Casting atmosphere Contact angle (deg) Interfacial tension (N/mm2)

Vacuum 0.1mbar 144 1210Forming gas (N2+H2) 148 1330

Argon – 660Air <50 (low value) investment wetted by melt

Table 14 Influence of atmosphere on interfacial tension (14 ct yellow gold)


Metal Shrinkage at Solidification, volume,%

Gold 4.8 Silver 7.3 Copper 5.4 18ct yellow gold 6.0

Table 13 Shrinkage at solidification, examples


Table 15 gives some approximate values of hardness for yellow gold alloys of

different fineness in the as cast state.

The hardness at a given fineness varies strongly with silver/copper ratio and also

with the treatment of the flask after casting (cooling conditions). Therefore, the

hardness may vary over a wide range.

The strong influence of the silver/copper ratio on the hardness of 18 carat alloys

is shown in Figure 3.1.9. Values ranging from hard (and brittle) to relatively soft

(ductile) are possible. The main reason for the increase in hardness with increasing

copper content is the age hardening (ordering) effect, as mentioned earlier. Copper-

rich alloys can form the ordered state very quickly and the hardness will be increased

considerably, with loss of ductility. Silver-rich yellow alloys undergo, firstly, a

separation into copper-rich and silver-rich phases, followed later by age hardening.

However, the amount of hardened phase is smaller. The hardening process is less

pronounced and the cast material remains soft and ductile.

The embrittlement of copper-rich pink and red alloys frequently causes cracks,

especially when the items are subsequently deformed, e.g. for widening or during

stamping. Fig. 3.1.10 shows a broken shank of a red gold ring. Theoretically, the

embrittlement can be avoided by quenching from a temperature of about 600-

700°C (1112-1292°F). In practice the flask cannot be quenched fast enough to avoid

the problem with some red/pink gold alloys. The only way to get a ductile material

in this case is by subsequently annealing the cast items at approx. 600°C (1112°F)

and quenching them quickly into water.

Composition (‰) HardnessCarat Gold Silver Copper HV

14 585 300 115 130 -14718 750 160 90 13518 750 125 125 17021 875 45 80 9622 917 55 28 65

Table 15 Typical hardness (as cast) of gold-silver-copper alloys


6040 80 100 120 140 160

200

220

260

240

280

180

160

120

140

Silver (‰)

Har

dnes

s H

V

Hardness of 18 ct yellow gold (as cast) as a function of silver content

HV

Figure 3.1.9

Figure 3.1.10 Cracks in red gold ring shank


3.1.2 High carat golds with enhanced propertiesTraditionally, high caratage gold jewellery is in great demand in the Middle and Far

East markets. High carat alloys (21 carat and higher) alloyed only with copper and/or

silver additions are soft, so in recent years, improved strength high caratage gold

alloys have been developed. Obviously, these are yellow gold alloys, because of the

very high gold content. The main challenge has been to find small alloying additions,

which increase the strength. However, many of the new alloys are more difficult to

make and use, needing more sophisticated melting and working facilities, compared

with traditional alloys.

Gold-titanium

The gold – 1% titanium alloy, Au990Ti, has a hardness of approximately HV180 in the

age-hardened state. The hardness is comparable with standard carat alloys. Also the

ductility and wear behaviour are excellent. The disadvantages are:

• The high reactivity of titanium requires a protective atmosphere (argon) for

melting and annealing.

• The high strength is only obtained after age hardening. Subsequent soldering will

weaken the material again.

• Frequently, a pale greyish colour is observed after finishing. Investigations showed

that this effect is not a property of the alloy but caused by non-optimum polishing

conditions.

Gold-gallium

A gold – 1% gallium alloy, Au990Ga, is easy to work with, but the hardening effect in

the as cast state is moderate. It is more pronounced after deformation.

Gold-cobalt-antimony

This patented alloy, 99.5% gold-0.3% antimony-0.2% cobalt, Au995Sb3Co2 (‰),

recently developed by Mintek, can be hardened by cold work plus age hardening up

to HV140. Investment casting is possible.

Micro-alloyed ‘fine gold’

24 carat, fine gold alloys of 99.5% + purity, alloyed with very small amounts of

calcium, rare earth etc., have been developed and patented. In the annealed or as

cast condition, the hardness is slightly increased, but more significant increases can

be attained by working and aging. However, as noted above, these new alloys are

more difficult to make and use, needing more sophisticated melting and working

facilities. The use of these alloys is restricted to special applications.

3.2 WHITE GOLD ALLOYSWhite gold alloys have an extremely wide composition range. They are essentially 3 types:

the nickel whites, the palladium whites and the mixed (nickel and palladium containing)

white golds. More recently, another class of nickel-free ‘alternative’ white golds have been

developed, based on use of metals such as manganese and chromium as the primary

whitener. In each class, a large variety of alloys are possible. In general, high concentrations

of nickel or palladium (circa 12%+) are required for a good white colour. Many commercial

alloys are thrifted in nickel (less hard) or palladium (less expensive), often with copper

additions, and are not a good colour, so requiring rhodium plating.



In Table 16 and Table 17, some examples of 18 and 14 carat ‘classical’ alloys are

given. Many other combinations are possible.

Nickel white gold

Nickel-containing white gold alloys tend to be hard and less ductile. Table 18

illustrates how the hardness of 18 ct gold increases with increasing content of non-

precious metals. These values are for the soft annealed state.

The mechanical properties of investment cast nickel white gold are not

predictable. At a low cooling rate, a nickel-rich phase segregates, causing

embrittlement. Fast quenching can cause cracking. As it is almost impossible to cool

the tree in a flask under defined conditions, the properties are not fully predictable.

This disadvantage is not only true for mechanical properties, but also for corrosion

resistance and, consequently, for nickel release. The corrosion resistance decreases if

segregation of nickel-rich phases occurs. Alloys with such segregation will release

more nickel than homogeneous alloys.

A relatively high zinc content is necessary to avoid undue brittleness. However,

zinc-containing alloys should not be melted in a vacuum due to excessive

evaporation of zinc. Because of this, frequent re-melting of scraps will cause an

unwanted change in composition, again resulting in increased brittleness.

Concentration, ‰ Temperature, °C*Au Ag Pd Cu Zn Ni Solidus Liquidus

Nickel white gold750 0 0 55 50 145 895 945750 0 0 10 75 165 888 902

Palladium white gold750 100 150 1240 1300750 150 100 1180 1225751 118 130 1180 1235751 80 170 1300 1315750 40 170 40 1200 1290

750 60 130 58 2 1090 1185

Mixed white gold750 135 75 20 20 1050 1110750 110 50 30 60 950 1025

Au – gold, Ag – silver, Pd – palladium, Cu – copper, Zn – zinc, Ni – nickel * approximate values

Table 16 Typical 18 carat white gold alloys

Concentration, ‰ Temperature, °C*Au Ag Pd Cu Zn Ni Solidus Liquidus

Nickel white gold585 270 50,0 95 920 990585 185 75,0 155 915 1020

Palladium white gold585 215 150 50 1080 1165

Mixed white gold

585 180 140 65 10,0 20 1010 1080585 180 140 45 50 995 1090

Au – gold, Ag – silver, Pd – palladium, Cu – copper, Zn – zinc, Ni – nickel * approximate values

Table 17 Typical 14 carat white gold alloys

Au+Ag+Pd Cu+Ni+Zn HV% % soft annealed

100 0 6590 10 18075 25 220

Au – gold, Ag – silver, Pd – palladium, Cu – copper, Zn – zinc, Ni – nickel

Table 18 – Influence of composition onhardness of 18 ct white gold



Whereas melting of clean alloys in graphite crucibles is possible, the use of

gypsum-bonded investment is problematic. An increased reaction with the

investment is likely. This has a detrimental effect on surface quality and can increase

gas porosity. This effect depends strongly on the mass of the cast item and on the

flask and melt temperatures.

A further disadvantage of nickel-containing alloys is the pronounced affinity

of nickel for sulphur, present in the gypsum. Nickel sulphide can be formed which

segregates on grain boundaries and causes embrittlement. For this reason,

re-melting of scraps and pieces is especially critical, due to adhering old (gypsum-

bonded) investment. Sulphate (gypsum) will be reduced to sulphide on melting in a

graphite crucible. Nickel sulphide segregation results.

Additionally, silica (the main component of investment) can form silicide

compounds, with a similar embrittling effect as sulphide, if melting unclean scrap

under reducing conditions.

Nickel white gold can cause allergic skin reactions in nickel-sensitised people.

Therefore, the European Community has promulgated a Directive, EN 1811, to

protect the consumer. This Directive forbids the use of nickel only in the jewellery

used for piercing or in a healing wound. In all other cases where jewellery is in direct

and prolonged contact with the skin, the use of nickel is not forbidden, but a

maximum nickel release rate has been defined. This is determined with a specific

test in an artificial sweat solution. Alloys releasing nickel exceeding the limit

are not allowed.

Palladium white gold

The most important properties of palladium white gold of relevance to the caster

are:

• High melting temperature, which requires suitable casting equipment.

• High casting temperature, which might exceed the thermal stability of the

investment.

The wide use of induction melting techniques reduces the problem of high melting

temperatures. However, measuring the temperature with nickel/nickel-chromium

thermocouples is not possible; platinum/platinum-rhodium thermocouples are

necessary. The question of the use of graphite crucibles is discussed frequently.

Palladium in pure or low-alloyed form reacts with carbon (solubility of carbon in

palladium). However, the small concentration of palladium in gold alloys does not

give problems if the alloy is melted in graphite crucibles.

The great amount of heat, which is introduced into the mould by the melt, can

decompose gypsum-bonded investment, which leads to surface defects and gas

porosity. The danger of decomposition depends strongly on the mass of items. Thin

walled items may be cast using gypsum-bonded investment without too much

problem. On the other hand, heavy items can produce problems. In this situation, the

only solution is the use of phosphate-bonded investment.

Another disadvantage is the softness of the alloys for many applications. Some

addition of nickel (i.e. ‘mixed alloys’) will improve hardness and strength.



Alternative alloys

As mentioned earlier, ‘alternative’ white gold alloys have been developed as a nickel-

free substitute for the expensive palladium white alloys. Additions with a bleaching

effect used in such alloys are manganese and chromium.

Alloys with small additions of manganese have been known for many years. Higher

concentrations of manganese are necessary for a sufficient bleaching effect if nickel

and palladium are to be substituted completely. Such alloys have proved brittle and

susceptible to corrosion.

Chromium-containing alloys with a narrow specified composition show a good

colour and a good workability. However, casting and annealing of such alloys is

difficult due to the high reactivity of chromium. Chromium reacts not only with

oxygen but also with nitrogen and carbon. Investment casting must be performed in

a very clean argon atmosphere and in a special ceramic crucible. Phosphate-bonded

investment should be used. At present, these requirements cannot be fulfilled in

standard casting shops.

3.3 INFLUENCE OF SMALL ALLOYING ADDITIONSOver recent decades, efforts have been undertaken to improve the casting behaviour

of carat gold alloys and/or the properties of cast jewellery items. Any modification to

alloys has ensured that neither the gold content nor the colour will be influenced.

Therefore, only small additions are appropriate. Depending on the addition, the term

‘small’ ranges from less than 100 ppm to several percent. It is essential that

recommended maximum concentration limits are followed. Also, the processing

conditions may have to be adapted to suit the modified alloys.

3.3.1 Improving propertiesIn the following, a brief overview is given about the properties where improvement

is desirable:

• The interfacial tension between the melt and the investment is a critical factor

influencing form-filling, the reproduction of fine surface details and surface

roughness.

• Form-filling is strongly influenced by casting conditions; however, some additions

have a beneficial effect (e.g. zinc, silicon).

• Reducing shrinkage porosity is highly desirable. Unfortunately, no addition can

influence this. Gas porosity might be reduced to some limited extent by a suitable

addition of zinc (perhaps also with silicon).

The grain size of investment cast items often tends to be rather coarse (large). The

consequences are:

• Strong segregation and a pronounced dendritic structure, with inferior corrosion

resistance and mechanical properties.

• Increased sensitivity to cracking.

• Formation of a rough surface (‘orange peel’) if the cast items are deformed

subsequently.

• Poor polishing behaviour.

The influence of coarse grains on shrinkage porosity is not unambiguous.



Investigations have shown that a smaller grain size apparently does not reduce all the

porosity, but does reduce the formation of large pores and nests of pores, Figure

3.3.1. Whilst the maximum amount of porosity in a specific region of a cast item is

reduced dramatically with decreasing grain size, the mean value is not influenced

significantly.

Grain refining improves the deformation and polishing behaviour (as mentioned

above) and inhibits cracks in critical cases. Zinc at higher concentrations reduces the

sensitivity to age hardening (makes the alloy ‘softer’, important for wire drawing).

Unlike the high carat golds, no special additions are in use to improve the strength

or the age hardening properties for 18 or lower carat gold alloys.

In alloys consisting only of gold-silver-copper, copper can be oxidised with

formation of copper oxide. Casting of such yellow gold usually results in a black

copper oxide layer at the surface, formed while cooling down the flask. This

‘blackening’ can be avoided or, at least, reduced by the addition of zinc or silicon

(see later).

On the other hand, the formation of copper oxide inclusions can be caused by

re-melting of dirty scrap material, by use of oxygen-containing copper for alloying or

by melting and casting in an oxygen-containing atmosphere. The oxide, on its part,

can cause gas porosity through a rather complicated reaction.

However, copper oxide can be reduced easily by melting in a reducing atmosphere

or by melting in a graphite crucible in a neutral or reducing atmosphere. Time and a

sufficiently high temperature are necessary for completion of

this reaction.

No special deoxidising addition is necessary for removing oxides from the melt if

the melting and casting conditions are correct.

3.3.2 Effects of individual additionsThe effects of the elements most commonly used as small additions to carat gold

alloys are discussed in the following paragraphs.

Zinc

Some quantity of zinc can be alloyed in yellow gold without changing the

microstructure. The specific amount of zinc which can be tolerated depends on

fineness (caratage) and the silver to copper ratio. Gold can dissolve approximately 3%

zinc (by mass) without any change in microstructure. Higher concentrations cause

the formation of new phases, including intermetallics; a detrimental influence on

properties can be expected.

In 14 and 18 carat alloys, higher concentrations of zinc are possible due to the

higher solubility of zinc in silver and copper. However, zinc is usually a small addition

in casting alloys with some beneficial effects. The recommended upper limit of

addition is approximately 2% in 14 and 18 carat yellow gold. In low carat golds (8-10

carat) and in nickel white golds, zinc is a standard alloying element and is often

present in higher amounts.


Influence of zinc on solidification range of 14ct alloy (copper concentration 115‰)

SOLIDUS LIQUIDUS

0 2 4 6 8 10 12 14 16 18 20 22 24

910900890880870860850840830820810800790

Zinc content (‰)

Tem

pera

ture

(°C

)

Figure 3.3.2

Influence of zinc on the solidification range of 18ct yellow gold Copper constant 90‰

0 2 4 6 8 10 12

910

920

900

890

880

870

860

850

Zinc (‰)

Tem

pera

ture

(°C

)

SOLIDUS LIQUIDUS

Figure 3.3.3

0.60.4 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

Grainsize (ASTM)

18161412108642

Poro

sity

%

Effect of grain size on porosity 14 ct yellow gold

Porosity mean value Porosity maximum

Figure 3.3.1


The effects of zinc additions in 14 and 18 carat yellow gold are:

a) Reduction in liquidus and solidus temperatures:

The influence of zinc is shown in Figures 3.3.2 & 3.3.3, whereby silver is

substituted by zinc and gold and copper are kept constant. Substituting

copper by zinc might lead to a somewhat different effect; it will certainly

bleach the colour.

b) Increase in form-filling:

The influence of zinc additions up to 2% on the form-filling of 14 and 18 ct

yellow gold is shown in Figure 3.3.4. All the tests were performed with a test

grid under constant conditions. The beneficial effect is remarkable.

c) Reduction in surface roughness:

The surface of cast items is much smoother if the alloy contains up to 2% zinc.

The effect is more pronounced with heavy parts. A roughness reduction to

approximately a third can be achieved.

Both increased form-filling and reduced roughness can be related to the effect

of zinc on reducing the interfacial tension, which, in turn, improves the

wetting of the investment with the melt and reduces capillary forces. Thus,

the melt can more easily fill thin cavities and reproduce the smooth surface of

the pattern. This avoids the formation of a dendritic surface structure.

d) Reduction of reaction with investment and gas porosity:

Small zinc additions have proven able to reduce the reaction of the melt with

the investment and in this way decrease the incidence of gas porosity. The

reason for this is not quite clear. Probably, the formation of a dense layer of

zinc oxide at the surface of the solidifying melt prevents the interaction of

melt with the investment.

Tensile tests on cast samples has shown that a small addition of zinc improves

the elongation (ductility) by reducing the porosity, Figure 3.3.5. There is also

an increase in tensile strength, indicating that the effect is really related to

physical integrity of the samples.

However, it should be recognised that zinc additions higher than the

recommended value (approximately 2-3%) might have an adverse effect, e.g.

increase the reaction with investment and, therefore, increase gas porosity.

e) Brighten the surface in the as-cast state:

Zinc has a stronger affinity to oxygen. During the cooling period of a casting,

a thin, relatively dense layer of almost colourless zinc oxide is formed on the

surface, inhibiting the formation of a thick voluminous black scale of copper

oxide, Figure 3.3.6. The pieces have a bright yellow appearance. Pickling

removes the zinc oxide layer easily without de-coloration of the surface.


Influence of zinc addition on elongation

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

35

30

25

20

15

10

5

Zinc content (%)

Elon

gatio

n (%

)

14 ct 18 ct

Figure 3.3.5

Influence of zinc on formfilling of a grid

0.0 0.5 1.0 1.5 2.0

80

75

70

65

60

55

50

45

40

35

Zinc content (%)

Form

fillin

g (%

) 14 ct 18 ct

Figure 3.3.4

Figure 3.3.6

without Zn 2% ZnInfluence of zinc on surface oxidation (14 ct)


Zinc has a high vapour pressure; it boils at 907°C (1665°F) at atmospheric pressure.

Thus, adding pure zinc to the melt is difficult. A great deal of zinc evaporates (in air,

with formation of white fumes of zinc oxide). This effect can be reduced by wrapping

the zinc in copper foil and immersing it very fast into the melt.The better way is the

use of brass as a master alloy. Alloyed already with copper, the vapour pressure of zinc

is significantly reduced. Use of brass with 70% copper or higher is recommended.

(Note: Brass with 60% copper and less often contains lead (as an alloying element)

and some other impurities. Lead contamination in gold is undesirable). Once zinc is

alloyed in yellow gold alloy, the alloy is stable. Significant loss by evaporation is

prevented if the level of approximately 2% is not exceeded. Even a moderate vacuum

can be applied.

However, melting in air causes formation of zinc oxide and therefore reduces the

zinc concentration in the alloy. The main defects arising are inclusions of zinc oxide

in the alloy. For example, surface defects can be generated, Figure 3.3.7. This defect

is mainly caused by remelting of dirty material, e.g recycled sprues.

Higher zinc concentrations (exceeding the recommended level) increase the

reaction of the melt with the investment. The result is a bad surface and more

gas porosity.

Silicon

Silicon lies on the border between being a beneficial addition and deleterious

impurity. It has several merits: silicon increases the melt fluidity and form-filling. Its

effect is more pronounced than that of zinc. In yellow carat gold alloys, silicon

produces a clean, yellow surface without the dark scale of copper oxide. The reason

for this effect is the same as for zinc. A thin colourless, dense layer of its oxide, silica,

forms at the expense of copper oxide.

The high affinity of silicon for oxygen makes silicon a strong deoxidiser. However,

this use is not essential in yellow gold alloys.

Silicon additions have some disadvantages:

a) Embrittlement

The disadvantage of silicon additions is rooted in its limited solubility,

particularly in high carat jewellery alloys.

Its solubility depends mainly on the copper content of the alloy, as it is

insoluble in gold and silver and forms low melting eutectics (gold-silicon

363°C/685°F, silver-silicon 835°C/1535°F). In copper, silicon is soluble up to

almost 5%.

If the solubility of silicon is exceeded, a low melting eutectic is formed in the

jewellery alloy, causing embrittlement and cracks. Most critical are silver-rich,

high carat alloys.

For an 18 ct alloy of composition gold 75% - silver 4.5% - copper 18% - zinc

2.5%, the critical silicon concentration is 0.05%. Higher silicon additions can

cause embrittlement. 14 carat alloys can tolerate approximately 0.1% and 10

ct alloys ~0.3% silicon.


Figure 3.3.7 Inclusions of zinc oxide


The allowable silicon addition has to be determined for any given alloy

composition. It decreases with increasing (gold + silver) content and should

not be used in high (21/22) carat golds.

b) Grain coarsening effect

Another disadvantage of silicon is its pronounced grain coarsening effect.

It causes an extremely coarse grain, even at a very low concentration. The

main consequence is a tendency to intergranular cracking. The appearance of

the defect is very similar to that in Figure 3.1.10.

Inclusions of silicon dioxide occur in castings, especially where remelting dirty

material such as recycled scrap is done.

Grain refiners

To compensate for the undesirable grain coarsening effect of silicon and to refine the

grain size of jewellery alloys in general, many attempts to apply grain refiners have

been made.

Published work has shown that additions (and combinations) of: iridium,

ruthenium, zirconium, cobalt, boron, yttrium, (zirconium + boron), (cobalt + boron)

and barium were effective as grain refiners. In almost all cases, the additions were in

the range of 0.005 to 0.05% weight. Cobalt was added up to 0.2%. They all act in a

similar way: they form very fine dispersed nuclei as starting points for the formation

of grains at solidification. The mechanism of nucleation can be different. In all cases,

small concentrations are effective.

In Table 19 the grain refiners are classified in terms of application and working

mechanism.

Frequently used grain refiners are the high melting platinum group metals, iridium

and ruthenium, with limited solubility in gold alloys, and also some very reactive

elements. In the latter case, intermetallic compounds or even oxides or nitrides form

the effective nuclei.

Type Field of appliance Working mechanism Examples1 Casting

1a High melting temperature, limited iridium, ruthenium, solubility in the alloy (cobalt)

1b High reactivity with oxygen, low rare earth (yttrium),solubility, formation of fine dispersed boron, barium, (calcium)intermetallics or oxides

1c Probably formation of intermetallic e.g. zirconium/boroncompounds cobalt/boron

2 Soft annealing Formation of fine dispersed Cobalt,segregations at annealing temperature All type 1 additions can

also decrease grain size on soft annealing

Table 19 Grain refiners in yellow gold alloys



Iridium

Iridium is the most frequently used addition for grain refining of gold

alloys. Investigations have mainly been performed with 14 and lower carat alloys.

In high carat alloys, the effect would be expected to be less pronounced, due to

the extremely small solubility of iridium in gold and silver. In contrast, iridium is

miscible in copper, more than 10% (by weight), forming a homogeneous

solid solution.

Discrepancies exist between observations by different investigators. Some have

found a grain refining effect in the range of 50 ppm iridium, whilst others could not

demonstrate such an effect even with concentrations of 0.1% and higher. The grain

refining effect of iridium in silicon-containing alloys is uncertain.

The main causes of these different results are variations in alloying and melting

techniques as well as the basic composition of the alloys studied.

A master alloy has to be used to achieve good results. Preferably, a master alloy

with copper should be used, where the iridium concentration should not be too high

(<2%). Extreme care has to be taken to obtain a homogeneous iridium distribution in

the carat gold alloy. An addition of approximately 0.01% iridium in the alloy is usually

sufficient. A higher concentration has an adverse effect.

Use of iridium can lead to two types of defect:

- insufficient and inhomogeneous grain refining effect.

- segregation and hard spots.

Inhomogeneously distributed grain refiner or a too high concentration leads to

hard inclusions causing polishing problems (‘comet tails’) and, in extreme cases,

cracks, Figure 3.3.8.

Ruthenium

Ruthenium is another addition used for grain refining in carat gold alloys. The effect of

ruthenium as a grain refiner is similar to that of iridium. It shows a remarkable refining

influence in the concentration range of 0.001 to 0.01%. Again, a higher concentration

is detrimental due to the formation of coarse particles. Obtaining a homogeneous

distribution of ruthenium in yellow gold is difficult, and so iridium is preferred.

Figure 3.3.8 Iridium clusters at surface



4 EQUIPMENTAt least six steps of the investment casting process require specific equipment.

That is:

• the vulcaniser (vulcanising press), to make the rubber moulds,

• the wax injector, to make the wax patterns,

• the investment mixer, to make the investment slurry,

• a dry or steam dewaxer,

• the burnout oven,

• a melting/casting machine.

A wet sand or grit blasting machine can be added to this list, to complete the

removal of the investment from the cast tree. Also, other less costly equipment,

which is not strictly required but can simplify some steps of the process, can be

considered.

As remarked in the introduction, we should select qualified producers and

suppliers who have a technical knowledge of the process as well as give good after-

sales service for the supplied equipment. These are essential.

First of all, we should establish our goals (our production requirements) and what

we need to attain them. This decision can be taken only by the goldsmith.

Otherwise, there is always the risk that vital decisions on equipment for our

workshop could depend heavily on the ‘opinions’ of the local vendor. Frequently,

a production line made from poorly matched equipment will be the result

of such policy.

There are many trade fairs around the world, where you can obtain up-to-date

information on production equipment from a range of manufacturers. Examples

include:

• VicenzaOro, each year in January and June, in Italy

• Basel Fair, March/April in Switzerland

• Inhorgenta, February, Munich, Germany

• Hong Kong Trade Fair, September, Hong Kong

• Las Vegas Trade Fair, June, USA

• “Catalog In Motion”. This fair is held in February each year in Tucson, Arizona,

USA.

At such events, we can see and try equipment, compare different manufacturer’s

products and technical production problems and requirements can be discussed

with leading experts in the field.

When you have clarified your ideas and established your objectives, it is useful to

prepare an ‘evaluation chart’ for each type of equipment. In this chart you will

record a description of the different technical features of the equipment, along with

competence and efficiency of the producer or vendor. In this way you will be able

to make a more objective final choice, especially if you also make an evaluation of

costs versus benefits and of the return on investment. So many unpleasant surprises

will be avoided or at least mitigated.

4E Q U I P M E N T


In addition to these general rules, we recommend that you add the following

guidelines:

• Never buy equipment without seeing it and without discussing every detail. You

should test it and evaluate ease of use, controls, possibility of programming, ease

of servicing.

• Find out if the producer himself has developed and built the equipment or if the

offered equipment is a cheap copy of something developed by someone else.

• Discuss the offered guarantee in depth.

• Find out how many pieces of the offered equipment have been sold and to what

company. Talk to some of their customers about their experiences.

• Ensure the producer will offer adequate training of your staff and will commission

the equipment in your factory so that it operates at the agreed specification

• Never choose equipment on the basis of price only. ‘Cheap’ is rarely good

economy!

Whatever equipment you buy, you should schedule the required servicing, i.e. what

tests should be done and when, and who should do them. All servicing operations

will be recorded on suitable charts. For example, for a melting/casting machine, the

cooling system should be checked daily, the filters of the exhaustion fixture weekly,

the oil of vacuum pumps monthly, etc.

Now we will discuss the specific equipment.

4.1 VULCANISERSApparently, a vulcaniser is a very simple item of equipment. It is basically a simple

screw press, with two heated platens and a temperature controller. However, in

practical use even such a simple piece of equipment can give unpleasant surprises. If

the temperature control system is crude and unreliable, it will be hard to obtain good

quality rubber moulds, with all consequent problems.

The characteristics to consider here are:

• the operating temperature range,

• the size of the heating platens,

• the maximum opening between the platens (i.e. the maximum thickness of the

rubber mould).

We should keep in our mind that several modern silicone rubbers don’t tolerate

temperature inaccuracies larger than 1°C (1.8°F). Other factors include uniformity

of temperature across the platens and the control system. An accurate temperature

control, without wide swings of temperature, is fundamental for a good vulcaniser.

It should be possible to check the calibration and the correct operation of the

temperature controller. For this purpose, in many vulcanisers there are proper

holes in the platens, where a calibrated thermocouple or other suitable device

can be inserted to check temperature level and distribution at different points

of the platens.

The approximate cost of a vulcaniser can range from about 350 Euros/US $ for a

very basic model to 650 Euros/US$ for a more sophisticated one with electronic

temperature control, Figure 4.1.1. A multiple vulcaniser, with multiple temperature

control and digital temperature display, can cost up to about 2,200 Euros/US$.

4 E Q U I P M E N T

Figure 4.1.1 Vulcanizer with digitaltemperature control


4.2 WAX INJECTORSWhen investment casting was first introduced in jewellery production, the spinning

of wax patterns in a centrifuge was the standard procedure, but it has now been

superseded by wax injection and is no longer used in jewellery workshops. Modern

wax injectors are airtight and the controlled temperature wax pot is placed inside. A

pressurization system allows the injection of the wax into the rubber mould. The

more sophisticated types are equipped with temperature control for both the pot

and the injection nozzle and with a vacuum system, to exhaust the air from the

mould before injecting the wax, Figure 4.2.1.

With this type of injector, a clamping system for the rubber mould can be used to

keep a constant mould clamping pressure. This avoids the variations in wax pattern

weight caused by clamping pressure variability, that occurs if the mould is held

manually by the operator during wax injection, Figures 4.2.2 and 4.2.3. This

attachment is very useful for ensuring a consistent weight of the wax patterns.

Another type of injector where the mould is filled under suction is also commercially

available. These injectors require specific moulds designed with two openings. One

opening is used for suction, while at the same time the wax enters through the

second opening, Figures 4.2.4, 4.2.5 and 4.2.6.

In the field of wax injection, the present trend is towards an increasing automation,

with equipment able to identify the mould through a specific code. After identifying

the mould, the working parameters are automatically set to the required value,

Figures 4.2.7 and 4.2.8. Recently, a completely automated, programmable injector

4E Q U I P M E N T

Figure 4.2.1 Low cost wax injector, equippedwith different fixtures for temperature control

Figure 4.2.2 Detail of a wax injector withmould clamp

Figure 4.2.3 Wax injector more sophisticatedthan the type shown in Figure 4.2.1

Figure 4.2.4 Wax injector with suction duringinjection. The wax is sucked into the mould

Figure 4.2.5 Rubber mould for the injector ofFigure 4.2.4

Figure 4.2.6 Wax pattern produced with themould of Figure 4.2.5

Figure 4.2.7 Automated injector withmechanical identification of the different moulds

Figure 4.2.8 Moulds for the injector of Figure4.2.7 and wax patterns produced


has been put on the market. It uses rubber moulds equipped with a microchip, where

the relevant parameters of the mould (temperature, vacuum, pressure, time and wax

type for that particular model) are recorded, Figures 4.2.9, 4.2.10 and 4.2.11. With

this injector, wax patterns with the same weight can be obtained consistently. It is

particularly useful for mass production.

The cost of wax injectors varies widely and depends on the level of specification.

The cost can range from about 200 Euros/US$ for the more basic, completely

manual, types up to about 15,000 Euros/US$ for the completely automated injector

just described.

4.3 INVESTING MIXING MACHINESThe investment slurry is obtained by adding investment powder to the water in the

weight ratio recommended by the manufacturer. Mixing can be done manually with

a suitable stirrer or with an electric mixer, and, after vacuuming, the slurry is poured

in the flask. The invested flask will be vacuumed again under a bell jar for removing

air bubbles from the slurry. Then the flask is put on a vibrating plate and is vibrated

until about 1 minute before the gloss-off point. This method is still used today in

many small workshops and good results can be obtained, if care is taken, but it is

difficult to ensure consistency.

Investment powders are becoming more sophisticated and specialized, and it has

been recognised that investment mixing, handling and filling of the flask are steps in

the investment casting process that critically affect performance of the mould. These

steps must be done correctly and accurately to obtain consistent results. Therefore,

most jewellery producers prefer investment mixing and pouring units that perform

the whole cycle in a regular, consistent manner and can fill one or more flasks in the

same operation, Figure 4.3.1. Many are automated and programmable.

Such equipment is available in many sizes to fit specific needs. Equipment with

stainless steel tanks and stirrers should be preferred, Figure 4.3.2. Certainly, it is more

expensive, but it is more robust and easier to clean. Cleanliness is very important,

because residues of old slurry can modify the setting behaviour of the new

investment appreciably.

Figure 4.3.2 Vacuum investment mixers

4 E Q U I P M E N T

Figure 4.2.9 Automated injector able toidentify the mould from a microchip insertedin the mould

Figure 4.2.11 Detail of the clamp of theinjector of Figure 4.2.9

Figure 4.2.10 Control panel of the injector ofFigure 4.2.9

Figure 4.3.1 Automated investment mixingand pouring unit that can fill six flasks in asingle pour

a b


Fully manual equipment can cost about 800 Euros/US$. A programmable, fully

automated mixing and pouring unit with vacuum and vibration can cost from 1500

up to 8,000 Euros/US$, depending on the materials of construction, and even more

for large equipment for mass production.

4.4 DEWAXERSAs discussed in chapter 2.7, dewaxing can be carried out dry or by steam. Often, dry

dewaxing is done in the burnout oven, but can be done in a separate dewaxing oven

prior to the burnout cycle. Such ovens need to be fitted with fume extraction and a

gas scrubbing facility to comply with health, safety and environmental pollution

regulations.

Steam dewaxers are simple in concept; they are basically a stainless steel cabinet

containing a grid, on which the flasks are placed to dewax, above a layer of boiling

water, Figure 4.4.1. The water is heated with thermostatically controlled electric

resistance heaters, to avoid violent boiling. The cost depends on the size and ranges

from about 350 Euros/US$ for a dewaxer containing up to 6 flasks to about 1,000

Euros/US$ for a dewaxer containing up to 24 large flasks.

4.5 BURNOUT OVENSTo many, the burnout oven seems to be quite a simple piece of equipment.

Consequently, the goldsmith often cannot understand why the cost of a burnout

oven should vary over such a large range. Many burnout ovens are available on the

market that are suitable for treating 10 flasks, but their price can range from about

2,000 Euros/US$ up to about 25,000 Euros/US$!

A major problem with many burnout ovens is their poor temperature control and

lack of uniformity of temperature within the oven. Temperature within an oven can

vary, typically, by 50-75°C/122-167°F. Thus, flasks in different positions receive

different thermal cycles and, because of the position of the control thermocouple,

may not reach the set temperatures. Thus, casting quality can vary from flask to flask

and from one side of a flask to the other. Overfilling the oven with too many flasks

can exacerbate this problem.

Temperatures should be monitored regularly with a calibration thermocouple and

the temperature distribution within the oven measured with the oven containing its

normal load of flasks.

Many specialized companies can supply ovens with good technical characteristics,

Figures 4.5.1 and 4.5.2. A good oven should have the following characteristics:

• good thermal insulation. Apart from energy saving, usually the oven runs in a

confined space: it should not act as a radiator and must not burn when

accidentally touched. The temperature of the outer surface of the oven should

not exceed 37-40°C (98-104°F).

Figure 4.4.1 Steam dewaxer

Figure 4.5.1 Good quality traditional burnoutoven

Figure 4.5.2 Burnout oven equipped with aircirculation in the first phase of heating. Notethe presence of two control thermocouples

4E Q U I P M E N T


• if the oven is heated by electrical power, the heating elements should be shielded,

to avoid direct radiation on the flasks. Therefore, ovens where a shield is inserted

between the heaters and the flasks is preferable. Direct radiation can cause

uneven heating of the flasks, leading to some sides being overheated, causing

deterioration of the investment and consequent formation of defects in the

castings. A rotary table in the oven is not sufficient: the outer flasks will always

show the same side to the heating elements, so this side will heat more rapidly

and to a higher temperature than the rest of the flask.

• Temperature should be homogeneous in the whole work chamber of the oven

during the holding periods at constant temperature and, if possible, also during

the heating phase.

• In gas-heated kilns, temperature homogeneity is favoured by air circulation caused

by the burner. To ensure temperature homogeneity in all phases of the cycle,

electric ovens should be equipped with a fan to assist air circulation and hence

temperature uniformity throughout the entire burnout cycle, Figures 4.5.3 and

4.5.4. Most electric ovens in service are not equipped with a fan, or the fan runs

only in the cooling phase of the cycle, to shorten the time required to reach the

temperature suitable for starting a new cycle. In other cases, the fan runs only in

the heating phase up to 200-300°C (392-572°F).

• The oven should be equipped with a temperature programmer suitable for the

heating cycle required by the investment within the flasks.

As noted for dewaxers, burnout ovens should be fitted with fume extraction and a gas

scrubbing facility to comply with health, safety and environmental pollution regulations.

Centrifugal casting Static casting

Basic programming Sophisticated programming, up to self-programming

It is possible to have an inert atmosphere, but only in It is easy to have a controlled atmosphere. It is also possible toa few models have different atmosphere composition in crucible and flask

chambers

Relatively small flasks Larger flasks (h > 200 mm (≈ 8 in))

Max. charge weight ~ 800 g Max. charge weight even > 1500 g

High pouring turbulence Lower pouring turbulence (with a correct feed system)

Risk of investment erosion, because of high metal flow Lower risk of investment erosion& pressure

Feed system not critical Critical feed system (it should be suitably designed)

Relatively low productivity (8-10 casts/hour) Higher productivity (20 casts/hour in the most sophisticated machines, using larger flasks too)

Relatively lower cost High cost (for the more sophisticated machines)

Table 20 Comparison between centrifugal casting and static casting for gold

4 E Q U I P M E N T

Figure 4.5.3 Drawing of a forced ventilationburnout oven

Figure 4.5.4 Detail of the loading door of theoven of Figure 4.5.3. It is opened with a footlever drive


4.6 MELTING/CASTING MACHINES4.6.1 Comparison between centrifugal and static casting

machinesThe purchase of a casting machine is the largest financial investment made by the

goldsmith or caster. Choosing the right machine is no easy decision when there are so

many available on the market. Having decided on his requirements in terms of production,

the first decision is whether to buy a static casting or a centrifugal casting machine.

There are no special reasons to prefer one system or the other. Here the decision

is up to the goldsmith and will depend on his needs, experience and preference. A

centrifugal casting machine is really needed only in the case where investment

casting of platinum is to be done. (Platinum is less fluid than gold and a stronger

push, that only a centrifugal machine can give, is required for filling the mould cavity.)

For gold and other precious metals, in recent years the preference has shifted strongly

towards static casting machines, which are favoured because of their higher level of

technological evolution. In the more advanced models, they are nearly fully automated.

Automated machines remove, to a large extent, the technical responsibility from

the goldsmith or caster in the casting phase of the process and result in a more

consistent quality product. Human error is minimised. The main differences between

static and centrifugal casting are summarized in Table 20.

Centrifugal casting

Centrifugal casting has two weak points: more turbulence in the liquid metal during

‘pouring’ and a higher liquid metal pressure. On the other hand, higher pressure

facilitates form-filling and makes the feed system less critical, particularly with very

thin patterns. As discussed in preceding chapters, high turbulence increases the

probability of having gas porosity from trapped gas. Perforated flasks are not used in

centrifugal machines, so the escape of gas from mould cavity is more difficult, even

with suction from the bottom of the flask.

High casting pressure favours complete filling of the mould, but also increases the

risk of investment erosion (and mould collapse in the extreme case). Eroded

investment particles become entrained in the flowing metal, leading to inclusions in

the castings. Such erosion can also lead to sandy surfaces on the castings. This

occurrence has been demonstrated by recent studies that have shown that surface

defects caused by crumbled investment, formerly ascribed to incorrect burnout,

were instead caused by investment erosion produced by the centrifugal force

pushing the liquid metal in the mould, Figures 4.6.1 and 4.6.2.

Moreover, in centrifugal casting, the pressure exerted on the liquid metal is not

constant over the entire length of the tree, but is highest at the top of the tree and

lowest at the sprue button. Therefore, the patterns near the sprue button can be

incompletely filled, while the patterns near to the treetop can show finning, caused

by investment cracks produced by the high pressure.

Figure 4.6.1 Defective surface on centrifugecast rings, caused by investment erosion

Figure 4.6.2 Another example of defectivesurface caused by investment erosion duringcentrifugal casting

4E Q U I P M E N T


Static casting

In contrast, in static casting, the pressure is due to gravity and is nearly uniform over

the full length of the tree, because the only difference is due to the hydrostatic

pressure of the liquid metal from top to bottom.

With static machines, maintaining a controlled atmosphere in the crucible and

flask chambers is not difficult, whereas only few centrifugal machines can operate in

a controlled atmosphere.

Productivity

Lastly, let us consider productivity. In centrifugal casting machines, the metal high

pressure also sets a limit to the weight of the charge that can be safely used; it should

not exceed 800 g (1.76lb). Flask height is also limited to 150 mm (6 in). The limit on

flask size is not so onerous in static machines, where the weight of the charge can be

larger than 1.5 kg (3.36lb) and the flasks can be taller than 250 mm (10 in). Higher

charge weight and larger flasks mean better process economy.

With a centrifugal machine, the operator will struggle to make more than 8 casts per

hour, using 130-150 mm (5-6 in) high flasks. With a vacuum assisted (maybe also pressure

assisted) fully automated, latest generation, static casting machine, the operator can carry

out up to 20 casts per hour without difficulty, using 250 mm (10 in) high flasks.

Obviously, these are extreme cases, but they give an idea of the different potential

of these two systems. Even if productivity not always is a critical factor in a workshop

or factory, we should consider that the operator working with a static automatic

machine has more time available for his other work, i.e. it is less labour intensive.

4.6.2 Centrifugal machinesProbably, centrifugal machines are the most widely used for casting jewellery. In

recent years, there has been considerable progress in motor technology and

programming systems, but the original basic design remains nearly unchanged. In

comparison with older centrifuge equipment, the most important innovations

include the variable geometry arm, the flask with bottom-applied suction (in some

models), the temperature measurement attachment, induction heating and a

controlled atmosphere chamber (in some models).

Variable geometry

In the variable geometry machines, the angle between the flask axis and the

centrifuge arm is not longer fixed at 0°, but can change from 90° (in its rest position)

to 0° as a function of rotation rate, Figure 4.6.3. In this way, the combination of

centrifugal and tangential-inertial forces acting on the molten metal flowing out of the

crucible and entering the flask, is taken into account. This device helps to improve the

symmetry of metal flow into the mould, Figures 4.6.4 and 4.6.5, and prevents the

liquid metal from flowing preferentially along the side of the main sprue cavity

opposite to the rotation direction, as occurs in conventional fixed geometry

equipment, where this phenomenon can cause incomplete filling of some patterns on

the cast tree.

4 E Q U I P M E N T

Figure 4.6.3 Sketch of a variable geometrycentrifugal casting machine

Figure 4.6.4 Trace of the liquid metal on thecrucible wall in a traditional centrifugal castingmachine

Figure 4.6.5 Trace of the liquid metal on thecrucible wall in a variable geometry centrifugalcasting machine. Note the symmetry of themetal flow

Figure 4.6.6 Sketch of a centrifugal castingmachine with suction through the flask bottom


Applied suction

To ease the outflow of the gas contained in the mould cavity, suction systems have

been designed that are connected to the flask bottom, Figures 4.6.6 and 4.6.7.

These systems form a single unit with the rotating centrifuge assembly and facilitate

filling of very thin mould cavities.

Temperature measurement

As for temperature measurement, the best systems make use of a thermocouple

dipping into the molten metal in the melting crucible. The thermocouple is clamped

on the rotating system and the electric signal is transmitted through suitable contacts

on a commutator, that open when rotation starts, Figures 4.6.8 and 4.6.9.

Temperature measurement is less accurate and reliable when a thermocouple

contacting the outer crucible surface is used. Crucible and thermocouple have

different electrical potential and electric discharges can occur; these oxidise the

thermocouple junction and contribute to errors in temperature readings. Optical

pyrometers also tend to be less accurate and reliable.

Process control

Generally, in centrifugal casting machines, the operating parameters must be

programmed by the operator and the interaction with the operator is very tight,

Figure 4.6.10. The operator chooses the rotation rate and, consequently, the level of

the centrifugal force that will push the molten metal into the mould during pouring.

There are no fully automated centrifugal casting machines.

The best machines are equipped with induction heating and, recently, machines

operating under a protective atmosphere have been put on the market. Developing

a centrifugal machine to operate under a protective atmosphere is more complicated

than for a static machine, not least because of the larger volume involved.

Cost

The cost of a centrifugal casting machine can range from about 2,000 to about 4,000

Euros/US$ for basic equipment, not programmable, with torch melting. The cost of a

machine with some programming and induction heating can reach 10,000 Euros/US$,

while machines with more sophisticated programming, induction heating, controlled

atmosphere, temperature measurement with a dipping thermocouple and suction

from the flask bottom can cost up to 40,000 Euros/US$ or more.

4.6.3 Static machinesAll modern, good quality static casting machines are “vacuum assist”, i.e. are

equipped with a suction system, acting through the flask, which facilitates mould

filling, Figure 4.6.11. The best machines are equipped with separate crucible and

flask chambers. In this way, process time can be shortened further.

Nearly all static casting machines operate under an inert atmosphere, usually

nitrogen or argon although some use a hydrogen/nitrogen reducing atmosphere.

Presently, argon is frequently preferred, even if it is more costly than nitrogen. The

machines can also be equipped with a pressure system, acting (after pouring) only in

the flask chamber on the sprue button to facilitate better mould fill and surface

detail. In some very recent machines, pouring is also carried out under pressure.

4E Q U I P M E N T

Figure 4.6.7 Centrifugal casting machineoperating in an inert atmosphere, with suctionthrough the flask bottom

Figure 4.6.8 Detail of the crucible zone,without the flask

Figure 4.6.9 Detail of the crucible zone, withthe flask and the thermocouple for liquidmetal temperature measurement

Figure 4.6.10 Detail of the programmingsystem of a high performance centrifugalcasting machine

Figure 4.6.11 Modern basic level static castingmachine


In many machines, pouring takes place through the crucible bottom; this

minimises heat loss during pouring, allowing a lower degree of superheat and also

reduces the risk of oxide entrapment in the casting, since any oxide on the surface

of the melt will tend to fill the sprue button.

Many casting machines can be equipped with a grain-making accessory, for

making casting grain.

Heating and temperature measurement

Many static machines are induction heated, although more basic small machines can

be electrical resistance heated. In general, the better quality machines have medium

– low frequency induction. Heating depth and hence melting speed as well as

electromagnetic stirring forces increase with decreasing frequency.

Temperature measurement can be by optical pyrometer or, better and preferable,

by a sheathed thermocouple dipped into the melt, often via the central stopper in

bottom pouring crucibles.

4 E Q U I P M E N T

Figure 4.6.12 Computer assisted static casting machine:a – General view b – Detail of the crucible chamber c – Detail of the control panel d – Display with operation parameters

a b c d

Figure 4.6.13 Computer controlled static casting machine:a – General viewb – Only the essential process parameters are displayed, because the

machine is computer operatedc – Flask temperature measuring attachment equipped with optical

pyrometerd – Operation scheme of the machinee – Graining attachment for making alloy grainsf – Connection with the computer for recording process parametersg – Process evolution can be observed on the computer screen

a b c

f g

d

e


Trends in process control

For all the leading machine manufacturers, the trend is towards an even more

complete automation of the machines. In some cases, artificial intelligence software

is being utilised. These control systems remove the risk of operator error. He only

feeds the metal charge into the crucible and sets the casting temperature. Then the

control system takes all other technical decisions on the subsequent steps of the

melting and casting process.

There are two trends in the technical development of static machines:

programmable machines or self-programming machines. We could say “computer

assisted” machines, Figure 4.6.12, or “computer controlled” machines, Figure 4.6.13.

In the first group, the operating cycle is planned by the operator, who inputs a set of

instructions. Generally, with these machines, data collection must be done by the

operator, who should write down all data recorded by the machine. In contrast,

computer controlled machines are self-programming. They can automatically evaluate

the weight of the charged metal and correct the thermocouple temperature readings

in real time. This correction is needed, because thermocouples are always enclosed in

a refractory sheath and temperature readings always lag slightly behind in comparison

with the true metal temperature (they are lower in the heating phase and higher in the

cooling phase). Data collection is carried out automatically: the data are recorded in

the computer and can be retrieved for subsequent processing.

The most recent developments involve the use of pressure assistance in casting,

as shown in, Figures 4.6.14, 4.6.15, 4.6.16 and 4.6.17.

Figure 4.6.15 a – Detail of the crucible of the machine of Figure 4.6.14. The thermocouple is off-centre to

facilitate crucible fillingb – Detail of the control panel

4 – The crucible chamber is filled with inertatmosphere with controlled pressure

5 – (Optional) the flask chamber can be putunder dynamic vacuum

6 – Pouring is started: the crucible chamber ispressurized, while the flask chamber isvacuumed

7 – Pouring ends. The flask chamber ispressurized, to facilitate mould filling andprevent shrinkage porosity

8 – The cast tree is solidified. The pressure inthe flask chamber is lowered

9 – The crucible chamber is filled with inertgas, to protect the heating assembly. Inthe meanwhile the flask chamber isopened, to recover the cast flask

Figure 4.6.14 Operation of a pressure castingmachine:1 – Preparation for melting2 – Crucible and flask chambers are exhausted

(vacuumed)3 – Inert gas is introduced in the crucible

chamber and slowly, also in the flaskchamber; induction heating of the crucibleis started

a b

4E Q U I P M E N T


Cost

The price range is even wider than for centrifugal casting machines. Electric

resistance heated, non programmable machines may cost a few thousand Euros/US$,

and resistance heated, programmable machines can cost up 7,000-8,000 Euros/US$.

But induction heated machines with a good level of programming capability can cost

up to 20,000 Euros/US$ and more sophisticated, fully automatic machines that can

be interfaced with a computer for data collecting and processing may cost even

more than 60,000-70,000 Euros/US$. A typical range of machines produced by one

manufacturer is shown in Table 21.

4 E Q U I P M E N T

Model Heating Max. Crucible Flask size, Typical Casting Optional Temperature capacity*, maximum, mm cycle time rate, features

Grammes (dia. x height) Mins flasks/hour

J-2 Resistance 1204°C 900 102 x 229 6-8 8-10

J-z Induction 1513°C 1440 127 x 229 4 12-15 Yes

J-5 Induction, 5kW 1513°C 1440 152 x 254 4 12-15 Yes

J-10 Induction, 10kW 1513°C 1960 152 x 254 <3 20-25 Yes

J-15 Induction, 15kW 1513°C 1960 152 x 254 <3 20-25 Yes

* 14 carat gold

Table 21 Range of static casting machines from a US manufacturer

Figure 4.6.16 A machine for static castingunder pressure (made in U.S.A.)

Figure 4.6.17 Another machine for staticcasting under pressure (made in Germany)


4E Q U I P M E N T


4 E Q U I P M E N T


1 FOV Srl

Via del Progresso 45 Z.I.

I-36100 Vicenza

Italy

Tel: +39 0444 566211

Fax: +39 0444 566830


Web: www.fovsrl.com

2 Gesswein & Co Inc

255 Hancock Ave.

Bridgeport, CT 06605

Tel: +1 203 366 5400

Fax: +1 203 331 8870


Web: www.gesswein.com

3 Gold International Machinery Corp

PO Box 998

Pawtucket, RI 02860

USA

Tel: +1 401 724 3200

Fax: +1 401 728 5770


Web: www.goldmachinery.com

4 KerrLab (sds Kerr)

1717 West Collins Avenue

Orange, CA 92867

USA

Tel: +1 714 516 7650

Fax: +1 714 516 7649


Web: www.kerrlab.com

5 Lasso Co.

Letnikovskaya 6A

Moscow 113114

Russia

Tel: +7 095 7257741

Fax: +7 095 9563473


Web: www.lasso.ru

6 Luigi Dal Trozzo

Via Accademia 48

20131 Milano

Italy

Tel: +39 02 288587.1

Fax: +39 02 2870812


5 SOURCES OF EQUIPMENTAND CONSUMABLES

It is impossible to give a comprehensive list of manufacturers and suppliers. Good

advice is to visit an International (or your local) Jewellery Trade Fair and visit the

material and equipment stands to see who supplies in your region.

Note: The following lists of suppliers do not imply endorsement of the company,

their products or technical service by the authors or by World Gold Council.

In the list there are companies normally present at two or more international fairs.

The companies are listed in alphabetic order and are subdivided according to the

process steps, starting from general suppliers. The latter are mainly commercial

companies that sell a wide range of machines, consumables and tools, necessary for

the investment casting process.

5.1 GENERAL SUPPLIERS

5S O U R C E S O F E Q U I P M E N T A N D C O N S U M A B L E S


7 Mario Di Maio Spa

Via Paolo Da Cannobio 10

I-20122 Milano

Italy

Tel: +39 02 809926

Fax: +39 02 860232


Web: www.mariodimaio.it

8 Quimijoy S.A.

C/Gaia 49

Poligon Industriel Pla D’En Coll

E- 08110 Montcada I Reixac-Barcelona

Spain

Tel: +34 93 565 0990

Fax: +34 93 575 1556

9 Rio Grande

7500 Bluewater Road NW

Albuquerque, NM 87121-1962

USA

Tel: +1 505 839 3011

Fax: +1 505 839 3016


Web: www.riogrande.com

also: www.cataloginmotion.com

10 Romanoff International Supply

Corporation

9 Desforest Street

Amityville, NY 11701

USA

Tel: +1 631 842 2400

Fax: +1 631 842 0028


Web: www.romanoff.com

Vulcanisers

See General Suppliers and other

companies listed

Wax Injectors

11 DIK-Vacutech

Adolf-Sautter-Strasse 78

D-75181 Pforzheim-Würm

Germany

Tel: +49 7231 979860

Fax: +49 7231 979862


12 HISPANA de Maquinaria, S.A.

Calle Pallars, 85-91

E-08018 Barcelona

Spain

Tel: +34 93 3091707

Fax: +34 93 3090702


Web: www.hispanaspain.com

13 Maxmatic

53 Avenue de la Republique

F-33450 Saint Loubes

France

Tel: +33 5620 4344

Fax: +33 5668 6003


14 MPI Inc

165 Smith Street

Ploughkeepsie, NY 12601

USA

Tel: +1 845 471 7630

Fax: +1 845 471 2485


Web: www.mpi-systems.com

15 M.Yasui & Co Ltd.

Yasui Building, 3-7-4 Ikejiri, Setagaya-ku

Tokyo 154-0001

Japan

Tel: +81 3 5430 7211

Fax: +81 3 5430 5813


Web: www.yasui.co.jp

5 S O U R C E S O F E Q U I P M E N T A N D C O N S U M A B L E S

5.2 MACHINE SUPPLIERS


16 SH. Benbassat Int.

5 Simtat Hashach St.

66079 Tel-Aviv

Israel

Tel: +972 3 6830216

Fax: +972 3 6822862


Web: www.benbassat.com

17 Tanabe Kenden Co Ltd

1-9-14 Fukasawa, Setagaya-Ku,

Tokyo 158-0081

Japan

Tel: +81 3 3704 3044

Fax: +81 3 3702 3044


Web: www.tanabekenden.co.jp

18 H. Seltsam u. Sohn GmbH

Bleichstrasse 56-58

D-75173 Pforzheim

Germany

Tel: +49 7231 259 22/23

Fax: +49 7231 265 32


19 Yoshida Cast

3-17-24 Bessho, Urawa

Saltama 336-0021

Japan

Tel: +81 48 862 5621

Fax: +81 48 862 5627

Vacuum Investment Mixers

20 HISPANA de Maquinaria

See page 90

21 Hoben International Ltd

Spencroft Road

Newcastle-under-Lyme

Staffordshire ST5 9JE

England

Tel: +44 1782 622285

Fax: +44 1782 636982


Web: www.hoben.co.uk

22 H. Seltsam u. Sohn GmbH

See opposite

23 Italimpianti Orafi Spa

Via Provinciale di Civitella 8

I-2041 Badia Al Pino (Arezzo)

Italy

Tel: +39 0575 4491

Fax: +39 0575 449300


Web: www.italimpianti.it

24 KWS Kachele GmbH

Parkstrasse 18

D-75175 Pforzheim

Germany

Tel: +49 7231 33408

Fax: +49 7231 106548


Steam Dewaxers

See General Suppliers and other

companies

Burnout Ovens

25 Allmet Maschinen GmbH

Zeppelinstrasse 6

D-75446 Wiernsheim

Germany

Tel: +49 7044 96190

Fax: +49 7044 961919

26 HISPANA de Maquinaria

See page 90

27 Maule srl

Via N. Copernico 13/15

I-6057 Arcugnano (Vicenza)

Italy

Tel: +39 0444 289202

Fax: +39 0444 289209

E-mail: [email protected] or

[email protected]

Web: www.maulesrl.it



28 Promec

(Burnout Ovens, tailor-made)

Via Stelvio 2

I-27010 Siziano (Pavia)

Italy

Tel: +39 0382 617945

Fax: +39 0382 679504

29 Ruf Maschinenbau

Brauereistrasse 1a

D-75181 Pforzheim

Germany

Tel: +49 7231 562287

Fax: +49 7231 562628


Web: www.ruf-industrieofen.de

30 Schultheiss GmbH

Pforzheimer Strasse 82

D-71292 Friolzheim

Germany

Tel: +49 7044 94540

Fax: +49 7044 945440


Web: www.Schultheiss-GmbH.de

31 Neutec USA

7500 Bluewater Road NW

Albuquerque,

New Mexico 87121-1962

USA

Tel: +1 505 839 3550

Fax: +1 505 839 3525


Web: www.neutec.com

Investment Casting Machines

32 Aseg Galloni spa

Via Caravaggio 16

I-20078 San Colombano (Milano)

Italy

Tel: +39 0371 200233

Fax: +39 0371 898705


Web: www.galloni-aseg.com

33 M. Yasui & Co Ltd.

See page 90

34 Flli Manfredi Spa

Via Valpellice 72

I-10060 San Secondo Di Pinerolo

Italy

Tel: +39 0121 501561

Fax: +39 0121 500456


Web: www.manfredi-saed.it

35 Indutherm GmbH

Bahnhofstrasse 16

D-75045 Walzbachtal – Jöhlingen

Germany

Tel: +49 7203 9218-0

Fax: +49 7203 9218-70


Web: www.indutherm.de

36 Inresa GmbH

Am Hasenbiel 7

D-76297 Stutensee (near Karlsruhe)

Germany

Tel: +49 7244 94411

Fax: +49 7244 96181

37 L. Buysschaert & Co bvba

[Buko machines]

Engelse Wandeling 5

B-8500 Kortrijk

Belgium

Tel: +32 5622 0549

Fax: +32 5622 9021

38 Linn High Therm GmbH

Heinrich-Hertz-platz 1

D-92275 Escheenfelden

Germany

Tel: +49 9665 91400

Fax: +49 9665 1720


Web: www.linn.de



39 McFerrin Engineering &

Manufacturing Co. / Memco

International Fsc Inc

4849 Olsen Drive

Dallas

Texas 75227

USA

Tel: +1 214 388 5656

Fax: +1 214 388 8479

40 Neutec USA

See page 92

41 Opticom

Via Spin 96

I-36060 Romano d’Ezzelino (VI)

Italy

Tel: +39 0424 513210

Fax: +39 0424 513211


Web: www.opticom-sas.com

42 Oy Diacast Finland Ltd

c/o Sirokoru Ltd

Karhunkatu 2

FIN-20760 Pilspanristi

Finland

Tel: +358 2 242 4600

Fax: +358 2 242 4050

43 Seit Elettronica

Zona Industriale – Localita Zecchei

I-31409 Valdobbiadene (TV)

Italy

Tel: +39 0423 975767

Fax: +39 0423975785


Web: www.seitel.it

44 Vetter Technik GmbH

Benzstrasse 1

D-75203 Königsbach-Stein

Germany

Tel: +49 7232 2548

Fax: +49 7232 2272


Web: www.vetter-technik.de

5.3 CONSUMABLESSee also General Suppliers

Mould Rubber

45 Castaldo

120 Constitution Blvd.

Franklin, MA 02038-2697

USA

Tel: +1 508 5201666

Fax: +1 508 5202402


Web: www.castaldo.com

46 KerrLab

See page 89

Wax

47 Castaldo

See above

48 Ferris Division,

Kindt-Collins Co

12651 Elmwood Avenue

Cleveland, Ohio

USA

Tel: +1 216 2524122

Fax: +1 216 2525639


Web: www.kindt-collins.com

49 KerrLab

See page 89


Investment Powder

50 Hoben International Ltd

See page 91

51 KerrLab

See page 89

52 Ransom & Randolph

3535 Brianfield Boulevard

Maumee, OH 43537

USA

Tel: +1 419 8659497

Fax: +1 419 8659997


Web: www.ransom-randolph.com

53 WestCast

121 Dale Street SE

Albuquerque, NM87105

USA

Tel: +1 505 839 3581

Fax: +1 505 839 3525

Web: www.WestCast.com

54 S.R.S. Ltd

Amber Business Centre

Riddings

Derbyshire DE55 4BR

England

Tel: +44 1773 608969

Fax: +44 1773 540195


Web: www.srs-ltd.co.uk

55 UCPI

3, Avenue d’Amiens

F-93380 Pierrefitte

France

Tel: +33 1 49711444

Fax: +33 1 48230608


Web: www.ucpi.fr

Alloys & Master Alloys

56 Allgemeine

Kanzlerstrasse 17

D-75175 Pforzheim

Germany

Tel: +49 7231 9600

Fax: +49 7231 68740


Web: www.allgemeine-gold.de

57 Alpha Guss Metal & Legierungen

GmbH

Bleichstrasse 92

D-75173 Pforzheim

Germany

Tel: +49 7231 927166

Fax: +49 7231 927168


58 Argen (Pty) Ltd

PO Box 509

Edenvale 1610

South Africa

Tel: +27 11 609 8640

Fax: +27 11 452 3918

59 Argex Ltd

Silver House

130 Hockley Hill

Birmingham B18 5AX

U.K.

Tel: +44 121 523 4344

Fax: +44 121 523 4354

60 Argor-Heraeus SA

Via Moree 14

CH-6850 Mendrisio

Switzerland

Tel: +41 91 646 0191

Fax: +41 91 646 8082

61 C. Hafner GmbH & Co

Bleichstrasse 13-17

D-75173 Pforzheim

Tel: +49 7231 9200

Fax: +49 7231 920207


Web: www.c-hafner.de




62 Cookson Precious Metals Ltd

59-83 Vittoria Street

Birmingham B1 3NZ

U.K.

Tel: +44 121 200 2120

Fax: +44 121 200 3222

Web: www.cooksongold.com

63 O.M. Group

(formerly Degussa AG/DMC2)

See Allgemeine

Precious Metals Division

Rodenbacher Chaussee 4

PO Box 1345

D- 63403 Hanau – Wolfgang

Germany

Tel: +49 6181 590

Fax: +49 6181 593030

64 Engelhard-CLAL

Platexis SA

49 Rue de Paris

F-93136 Noisy-Le-Lac

France

Tel: +33 1 48505050

Fax: +33 1 48505151

65 Engelhard Corp.

700 Blair Road

Carteret, NJ 07008

USA

Tel: +1 732 205 7900

Fax: +1 732 205 7453

Web: www.engelhard.com

66 Heraeus Edelmetall Halbzeug

GmbH

Lameystrasse 17

D-75173 Pforzheim

Germany

Tel: +49 7231 200961

Fax: +49 7231 200957

Web: www.heraeus.com

67 Hilary Stern (Pty) Ltd

PO Box 2149

Bramley 2018

South Africa

Tel: +27 12 316 3562

Fax: + 27 12 316 3574

68 Imperial Smelting & Refining Co

of Canada Ltd

451 Denison Street

Markham

Ontario L3R 1B7

Canada

Tel: + 1 905 475 9566

Fax: +1 905 475 0703

Web: www.imperialproducts.com

69 John C. Nordt Co Inc

1420 Coulter Drive NW

Roanoke, VA 24012

USA

Tel: +1 540 362 9717

Fax: +1 540 362 2160

Web: www.jcnordt.com

70 Leach & Garner

57 John L. Dietsch Square

N. Attelboro, MA 02761

USA

Tel: +1 508 695 7800

Fax: +1 508 699 4031

Web: www.leach-garner.com

71 Leg.Or Srl

Via San Benedetto 14/34 Z.I.

I-36050 Bressanvido (Vicenza)

Tel: +39 0444 467911

Fax: +39 0444 660677


Web: www.legor.com



72 Melt Italiana Sas

Via Martiri della Resistenza 3

I-20090 Fizzonasco di Pieve

Emanuele (MI)

Italy

Tel: +39 02 90781900

Fax: +39 02 90722892


Web: www.melt.it

73 Metaux Precieux SA Metalor

Avenue Du Vignoble

CH-2009 Neuchatel

Switzerland

Tel : +41 32 720611

Fax : +41 32 7206609


Web: www.metalor.ch

74 Pandora Snc

Via Massarenti 15

I-20148 Milano

Italy

Tel: +39 02 4075886

Fax +39 02 48706026


Web: www.pandoralloys.com

75 Precious Metals West/

Fine Gold Inc

608 Hill Street, #407

Los Angeles, CA 90014

USA

Tel: +1 213 689 4872

Fax: +1 213 689 1654


Web: www.pmwest.us

76 Pro-Gold Srl

Via Molinetto 40

I-36075 Montecchio Maggiore

(Vicenza)

Italy

Tel: +39 0444 492493

Fax: +39 0444 498336


Web: www.progold.com

77 Stern-Leach

49 Pearl Street

Attelboro, MA 02703

USA

Tel: +1 508 222 7400

Fax: +1 508 699 4030

Web: www.stern-leach.com

78 Stuller Inc.

PO Box 8777

302 Rue Louis XIV

Lafayette, LA 70598-7777

USA

Tel: +1 800 877 7777 or

337 262 7700

Fax: +1 800-444-4741


Web: www.stuller.com

79 United Precious Metal Refining Inc

23120 West Lyons Avenue, #5-491

Newhall, CA91321

USA

Tel: +1 805 254 0523

Fax: +1 805 254 0525


Web: www.unitedpmr.com

80 Valcambi SA

Via Passeggiata

CH-6828 Balerna

Switzerland

Tel: +41 91 695 5311

Fax: +41 91 695 5353

81 Wieland Edelmetalle GmbH

Schwenninger Strasse 13

D-75179 Pforzheim

Germany

Tel: +49 7231 37050

Fax: +49 7231 357959



6 FURTHER READINGLiterature references are subdivided by subject and are listed in chronological order.

Where possible, the same order of subjects has been followed as in the Handbook.

Many references are quoted from the Proceedings of the Santa Fe Symposium

on Jewelry Manufacturing Technology, which has been held every year since 1987.

For the sake of brevity, in the references only ‘Proc. SFS’ has been written, followed

by the year instead of the full quotation. The Proceedings of SFS for each year can

be purchased directly from Rio Grande in Albuquerque, NM, USA, either via the web

site: www.riogrande.com or directly (address in list of suppliers).

Articles in Gold Bulletin and Gold Technology can be obtained from World Gold

Council or via the archives on its website, www.gold.org

Many relevant articles can also be found in jewellery magazines, such as AJM

magazine published by MJSA (www.ajm-magazine.com)

HISTORY OF THE PROCESS1. T.G. Jungersen, British Pat. 449,062 & 503,537 & U.S. Pat. 2,354,026 &

2,362,136. 1935

2. L.B.Hunt, “The long history of lost wax casting”, Gold Bulletin, 13 (2), 1980, 63-79

3. D. Schneller, “The cave of treasures – Lost wax castings from 3500 B.C.”, Proc.

SFS 1987, p. 1

4. P.E. Gainsbury, “Jewellery investment casting”, in ‘Investment Casting’, Edited

by P.R. Beeley & R.F. Smart, published by The Institute of Materials, London,

1995, p. 409

THE INVESTMENT CASTING PROCESS5. J.P. Nielsen, “Advanced technology for the jewelry caster”, Proc. SFS 1987, p. 77

6. J.C. McCloskey, “The application of commercial investment casting principle to

jewelry casting”, Proc. SFS 1987, p. 203

7. D. Ott, “Metallurgical and chemical considerations in jewelry casting”, Proc.

SFS 1987, p. 223

8. D. Ott, “Methods for investment casting in the jewelry industry – Principles,

advantages, disadvantages”, Proc. SFS 1988, p. 203

9. L. Diamond, “Casting as a total system”, Proc. SFS 1989, p. 235

10. M.F. Grimwade, “Basic metallurgy for goldsmith – Melting, alloying and

casting”, Gold Technology, No 3, 1990, p. 3

11. A.M. Schaler, “Recent developments in casting techniques”, Gold Technology,

No 11, 1993, p. 28

12. C. Walton, “Modern commercial workshop practice in gold jewellery

investment casting”, Gold Technology, No 11, 1993, p. 28

13. A.M. Schaler, “Use of computers in gold jewellery casting”, Gold Technology,

No 14, 1994, p. 18

14. T. Santala, “An overview of casting technologies currently used in industry and

emerging technologies”, Proc. SFS 1995, p. 213

15. S.M. Howard, A. Manou, “Computer simulation of investment casting process

using Rapidcast software”, Proc. SFS 1995, p. 229

16. A.M. Schaler, “Cutting casting costs”, Proc. SFS 1995, p. 247

17. A.M. Schaler, “Take no shortcuts”, Proc. SFS 1996, p. 187

6F U R T H E R R E A D I N G


18. V. Faccenda, “Advances in investment casting machines technology”,

Gold Technology, No 20, 1996, p. 3

19. A.M. Schaler, “Design and casting: the tale of a symbiotic relationship”, Proc.

SFS 1997, p. 357

20. T.L. Donohue, H. Frye, “Process control: power, value and advancement for the

jewelry industry”, Proc. SFS 1998, p. 179

21. K. Wiesner, “Bi-metal casting techniques for jewelry applications”, Proc. SFS

1998, p. 271

22. J. Maerz, “Casting gold to platinum”, Proc. SFS 1998, p. 321

23. D. Ott, “Physical, metallurgical and chemical processes in jewelry casting”,

Proc. SFS 1998, p. 457

24. V. Faccenda, “Investment casting: centrifugal or static vacuum assisted?”,


25. S. Grice, “The effect of quench temperature on silicon-containing low carat

investment casting alloys”, Proc. SFS 1999, p. 205

26. D. Ott, “Properties of melt and thermal processes during solidification in

jewelry casting”, Proc. SFS 1999, p. 487

27. S. Grice, “The effect of Si-content versus quench temperature on low carat

casting alloys”, Gold Technology, No 28, 2000, p. 18

28. D. Ott, “Metallurgical and chemical factors influencing working conditions”,

Proc. SFS 2000, p. 227

29. C.W. Corti, “Investment casting: choice of equipment” – Paper presented at

World Gold Council Technical Seminars, India, 2000

30. C.W. Corti, “Back to basics: Investment casting – Part 1”, Gold Technology,

No 28, 2000, p. 27

31. V. Faccenda, “Investment casting: an integrated process”, Proc. SFS 2001, p. 97

32. S. Bezzone, D. Zito, “Is it possible to recast scraps? This is what jewellers ask”,

Proc. SFS 2002, p. 61

33. J.T. Teague, “Finding hidden money in your manufacturing system”, Proc. SFS

2002, p. 511

MODEL AND SPRUING34. L. Diamond, “Casting defects from model to finished product”, Proc. SFS 1987,

p. 149

35. A.M. Schaler, “Gating and spruing”, Proc. SFS 1991, p. 191

36. H. Solidum, “Hollow tree casting technology”, Proc. SFS 1996, p. 535

37. K. Wiesner, “Metal flow optimising – An important step to successful casting”,

Proc. SFS 1999, p. 1

38. J. Matthews, “ Making master models from thermoformed plastic”, Proc. SFS

2000, p. 169

39. E. Bell, “Sprues, feed sprues and gates”, Gold Technology, No 36, 2002, p. 2

MOULD AND WAX40. D. Schneller, “Wax degassing“, Proc. SFS 1988, p. 293

41. L. Sanchez, “Molding methods and shrinkage factors”, Proc. SFS 1990, p. 105

42. L. Sanchez, “Effects of injection pressures on different mold compounds”,

Proc. SFS 1991, p. 135

43. A.M. Schaler, “Pressed soft metal moulds”, Proc. SFS 1998, p. 33

6 F U R T H E R R E A D I N G


INVESTMENT44. C.H. Schwartz, “Chemical and physical properties of investment”, Proc. SFS

1987, p. 99

45. D. Ott, “Properties and testing of investment”, Proc. SFS 1988, p. 47

46. P. Pryor, “Silica hazards and safety procedures in the handling of investment”,

Proc. SFS 1988, p. 131

47. E. Bell, “Heating and cooling characteristics of investment molds”, Proc. SFS

1988, p. 259

48. L. Diamond, “A semiquantitative method for flask temperature determination”,

Proc. SFS 1988, p. 309

49. P. Pryor, “Silica hazards in the handling of investment – Part II”, Proc. SFS 1989,

p. 257

50. D. Schneller, “Appendix – Santa Fe Silica project”, Proc. SFS 1989, p. 279

51. E. Bell, “Heating and cooling characteristics of investment molds – Research

update”, Proc. SFS 1989, p. 357

52. D. Ott, “Reactions of molten metal with investment”, Proc. SFS 1990, p. 165

53. G. Normandeau, “The effect of investment and metal casting temperatures

on the quality of castings”, Proc. SFS 1990, p. 209

54. E. Bell, “Wax elimination, burnout and the mold’s effect on porosity in

castings”, Gold Technology, No 11, 1993, p. 21

55. G. Normandeau, R. Roeternik, “Metal/mold reaction with white golds”, Proc.

SFS 1997, p. 245

56. C.J. Cart, “Evaluating investment powders”, Proc. SFS 1997, p. 369

57. C.J. Cart, “Advances in investment casting materials”, Gold Technology, No 23,

1998, p. 18

58. G.M. Ingo et al., “CaSO4 bonded investment for casting of gold based alloys:

study of the thermal decomposition”, Proc. SFS 1999, p. 163

59. R. Loewen, “The effect of additives on the high temperature chemistry of

investment materials”, Proc. SFS 1999, p. 181

60. J.C. McCloskey, “An evaluation of permeability of a jewelry casting

investment”, Proc. SFS 1999, p. 431

61. P.J. Horton, “Investment powders and investment casting”, Gold Technology,

No 28, 2000, p. 17

62. R. Carter, “Effects of water quality and temperature on investment casting

powders”, Proc. SFS 2000, p. 1; also: Gold Technology, No. 32, 2001, p7

63. H. Frye et al., “Basic ceramic considerations for the lost wax processing of high

melting alloys”, Proc. SFS 2000, p. 101

64. G.M. Ingo et al., “Thermochemical and microstrutural study of CaSO4 bonded

investment as a function of the burnout process parameters”, Proc. SFS 2000,

p. 147

65. C.W. Corti, “Investment casting: Producing a refractory mould”, Paper

presented at World Gold Council Technical Seminars, India, 2000

66. V. Faccenda, G.M. Ingo, “Advances in investment and burnout furnace design”,


67. R. Carter, “Effects of changing the water-to-powder ratio on jewelry

investments”, Proc SFS 2001, p. 31

68. P.J. Horton, “Investment powder technology – The present and the future

technology”, Proc. SFS 2001, p. 213



69. G.M. Ingo et al., “Thermochemical and microstructural study of modified

CaSO4 bonded investment with inorganic and organic additives”, Proc. SFS

2001, p. 241

70. I. McKeer, “A comparison of burnout cycles using an electric furnace”, Proc.

SFS 2001, p. 279

71. S. Aithal et al., “Evaluation of mold burnout by temperature measurement and

weight loss technique”, Proc. SFS 2002, p. 1

72. P. Du Bois et al., “Temperature measurements in mold cavities during vacuum-

assisted, static pouring of 14 ct yellow gold”, Proc. SFS 2002, p. 131

73. A. Eccles, R. Loewen, “Rapid wax elimination and reaction chemistry of sulfate-

bonded investment”, Proc. SFS 2002, p. 157

74. J.C. McCloskey, “Temperature measurements in 14 ct yellow gold during

counter-gravity pouring of investment casting molds”, Proc. SFS 2002, p. 353

75. R. Carter, “Getting optimum performance from your investment powder”,

Gold Technology, No. 34, 2002, p. 22,

CASTING76. C.J. Raub, “Casting: introduction and problem areas”, Gold Technology, No 7,

1992, p. 8

77. D. Ott, C.J. Raub, “Casting: gas pressure effects”, Gold Technology, No 7, 1992,

p. 10

78. A. Menon, “Casting gemstones in place”, Proc. SFS 1996, p. 69

79. H. Schuster, “Stone casting process with invisible setting”, Proc. SFS 1999,

p. 369

80. H. Schuster, “Problems, causes and their solutions on stone-in-place casting

process: latest developments”, Proc. SFS 2000, p. 315

SOLIDIFICATION81. J.P. Nielsen, “Solidification modes of jewelry in temperature-gradient molds”,

Proc. SFS 1987, p. 337

82. L. Diamond, “Temperature gradient casting – A practical approach”, Proc. SFS

1991, p. 225

DEFECTS AND QUALITY83. D. Ott, “Examples of defects in jewelry making”, Proc. SFS 1989, p. 297

84. D. Ott, “Defects in jewelry – A new version of an old problem”, Proc. SFS 1991,

p. 171

85. D. Ott et al., “Casting: porosity causes and prevention”, Gold Technology,

No 7, 1992, p. 18

86. D. Ott, C.J. Raub, “Casting: surface properties”, Gold Technology, No 7, 1992,

p. 28

87. D. Ott, “Porosity in investment casting”, Gold Technology, No 11, 1993, p. 15

88. D. Ott, “Analysis of common casting defects”, Gold Technology, No 13, 1994, p. 2

89. D. Ott, “Shrinkage porosity in investment casting – A consideration of the

factors affecting its formation”, Gold Technology, No 13, 1994, p. 16

90. D. Ott, “Control of defects in casting”, Gold Technology, No 17, 1995, p. 26

91. D. Ott, “Chaos in casting: an approach to shrinkage porosity”, Proc. SFS 1996,

p. 383

6 F U R T H E R R E A D I N G


92. D. Ott, “Handbook on casting and other defects”, publ. World Gold Council,

London, 1997

93. V. Faccenda, P. Oriani, “Quality level improvement in investment casting: are

last generation casting machines the only solution?”, Proc. SFS 1999, p. 271

94. T.L. Donohue, H.F, Frye, “Characterization and correction of casting defects”,

Proc. SFS 1999, p. 413

95. E. Bell, “Know the disease before trying the cure. Quality casting: identify the

defects”, Gold Technology, No 31, 2001, p. 2

96. D. Ott, “Relationship between casting conditions and gas porosity”, Proc. SFS

2001, p. 353

97. K. Wiesner, “Cracks in cast parts – What can we do?”, Proc. SFS 2001, p. 439

INVESTMENT CASTING ALLOYS98. W.S. Rapson, T. Groenewald, “Gold usage”, publ. Academic Press, London,

1978

99. G.P. O’Connor, “Improvement of 18-carat white gold alloys”, Gold Bulletin,

11 (2), 1978, p. 35

100. G.P. O’Connor, Investigation of alloying additions for 18-carat white gold

jewellery alloys”, Metals Technology, 6, 1979, p. 261

101. L. Gal-Or, M. Riabkina-Fishman, “Grain refining in 14 K gold alloys”, Proc. SFS

1987, p. 125

102. R.V. Carrano, J. De Rohner, “The effect of common additives on the cast

properties of 14 K alloys”, Proc. SFS 1988, p. 11

103. D. Ott, C.J. Raub, “Gold casting alloys 14 and 18 K”, Gold Technology, No 7,

1992, p. 2

104. G. Normandeau et al., “White golds: a review of commercial alloys

characteristics and alloy design alternatives”, Gold Bulletin, 25 (3), 1992, p. 94

105. Degussa AG – “Edelmetall taschenbuch” – publ. Heuthig-Verlag Heidelberg,

1995

106. G. Normandeau, R. Roeterink, “The optimisation of silicon alloying additions in

carat gold casting alloys”, Gold Technology, No 15, 1995, p. 4

107. G. Normandeau, “The effect of various additives on the performance of an

18 karat yellow gold investment casting alloy”, Proc. SFS 1996, p. 83

108. D. Ott, “Effect of small additions and impurities on properties of carat golds”,


109. J.C. McCloskey et al., “The effect of silicon deoxidation and grain refinement

on the production performance of a 14 karat yellow gold casting alloy”,


110. J.C. McCloskey et al., “Silicon microsegregation in 14 K yellow gold jewelry

alloys”, Gold Bulletin, 34 (1), 2001, p. 3

111. J. Fischer-Buehner, D. Ott, “Development of new nickel-free chromium-based

white gold alloys – Results of a research project”, Proc. SFS 2001, p. 131

112. M. Poliero, “White gold alloys for investment casting”, Gold Technology,

No 31, 2001, p. 10

113. D. Zito, “Coloured carat golds for investment casting”, Gold Technology,

No 31, 2001, p. 35

114. A. Basso, M. Poliero, “14-18 Kt yellow gold alloys for investment casting: a new

approach”, Proc. SFS 2002, p. 39



7 ACKNOWLEDGEMENTSThe Authors thank many companies who have supplied information on different

aspects of the investment casting process. In particular, we thank Pomellato S.p.a.,

Italy for the permission to publish photographs of its products, Aseg Galloni S.p.A.,

Italy, Neutec USA, Rio Grande, Castaldo and Ransom & Randolph in the USA, Hoben

and SRS in the UK and Schultheiss GmbH and Indutherm GmbH of Germany, who

have made available their technical material and knowledge for the preparation of

this Handbook.

The Authors also thank the editor, Dr Christopher W. Corti and World Gold Council

for their support and advice and Professor Giovanni Baralis, Torino, for the

translation of the manuscript into English and for his precious comments.

7 A C K N O W L E D G E M E N T S


WORLD GOLD COUNCILTECHNICAL PUBLICATIONSPrices are current for 2003

1. Technical Manual for Gold Jewellery – A practical guide to gold jewellery

manufacturing technology. Published 1997. Reprinted 2001.

Cost £45 sterling (US$70; Euro 74), including postage.

2. Investment Casting – A technical advisory manual for goldsmiths.

Published 1995.

Cost £10 sterling (US$16; Euro 16) including postage

3. The Assaying and Refining of Gold – a guide for the gold jewellery

producer. Published 1997. 2nd Edition published 2001

Cost £5 (US$8; Euro 8) including postage

4. Handbook on Casting and Other Defects in Gold Jewellery

Manufacture. Published March 1998. Reprinted 2001. Italian edition

published 2002.


5. Finishing Handbook. Published March 1999. (English & Italian editions available)


6. Handbook on Soldering and Other Joining Techniques. Published 2002

(English and Italian).

Cost: £16 sterling (US$26; Euro 26), including postage.

7. Handbook on Investment Casting. Published May 2003 (English & Italian).

Cost: £16 sterling (US$26; Euro 26), including postage.

Note: Some publications may be available in other languages – contact your local WGC office.

TECHNICAL JOURNALSGold Bulletin – published quarterly, a journal on the science, technology and

applications of gold. Recent issues available on the World Gold Council web site:

www.gold.org

Gold Technology – a journal on gold jewellery materials and production

technology and best practice (in English and Italian); ceased publication end 2002.

Some Arabic and Turkish editions available from local WGC offices). Available also

on the WGC web page: www.gold.org. Copies of back issues available (English, also

some Italian & German).

•Complete set of 36 back issues: Cost £30 ($50, €50) for postage and packing.

All publications are available from:

International Technology (Publications)

World Gold Council, 45 Pall Mall, London SW1Y 5JG, England

Tel. + 44 20 7930 5171 Fax. + 44 20 7839 6561 E-mail: [email protected]

Or contact your local World Gold Council office.

Payment in £ sterling, US $ or Euros by cheque or money order (no credit cards)

or into the World Gold Council bank account:

Barclays Bank plc., PO Box 15165, 50 Pall Mall, London SW1A 1QF, England

Bank Sort Code: 20 67 59 Account No: 70964271

All charges to your account. Please send confirmation of payment with order.

WORLD GOLD COUNCIL

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