electro plating for the amateur 1976

8/2/2019 Electro Plating for the Amateur 1976

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of Volunteers in Asia

L. Warburtonublished by:Model and Allied PublicationsArgus Books LimitedP.O. Box 35, Wolsey HouseWolsey Rd., Hemel HempsteadHertfordshire HP2 4SS EnglandPaper copies are $ 3.50.Available from:META PublicationsP.O. Box 128Marblemount, WA 98267 USAReproduced by permission of Model and AlliedPublications.

Reproduction of this microfiche document in anyform is subject to the same restrictions as thoseof the original documenl-,.


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1


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MODEL & ALLIED PUBLlCATlONSARGUS BOOKS LIMITED, 14 ST. JAMES RD., WATFORD,

HERTFORDSHIRE, ENGLAND


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ISBN 0 85344 056 5

PRINTEDN GREAT RIT*N YHEADLEYROTHERS.TD109 KINGSWAYOXDON ca 6 ~ ANDASHPORDENT

Model and Allied PublicationsArgus Books Ltd.,14 St. James Road,

Watford. Hens.. England

c Argus Books Limited. 1976All rights reserved

First published 1950Second Edition 1955

Third Edition 1 963SecondImpression 1969

Thirdlmpression 1970Fourth Impression 1972

Fifth Impression 1973Sixth impression 1975

Seventh Impression 1976


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PREFACE

CHAPTER !. Mrcduction and

,I

3,

>>

I,

>>

General Considerations2. The Sup ply of Cur rent . . .3. The Plating Tank . . . .4. Prep aration of the Base . . .5. Electrolytes . . . . . .

6. Chrom ium Plating . . .

.

.

. . .

. . .

. . .

. . .

7. The Plating of Non -Condu ctors andElectra-Forming . .

8. An Example . . . . .9. Anod ising . . . . . . . .

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294456

74

8694

103


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PREFACE

N presenting this book, I have only two objects in mind .Firstly, to try and provide some really u seful informationabou t small-scale electro-plating and secondly, whilstavoid ing technicalities, to give emp hasis to those essentialdetails w hich mu st be carefully attended to if successfulwork is to be carried out.

The method s described are not just the result of adaptingcomm ercial processes to home-workshop cond itions, but arerather the reverse, in that they represent the outcome ofseveral years of experiment, starting more or less fromscratch, du ring which time a reliable method has beenevolved in respect of each process described.The subject of chromium plating has pu rposely beenleft un til near the end of this book, and I hope that those ofmy readers wh o are novices will not attempt chrome un tilthe processes in copper and n ickel have been mastered.The final chap ter is devoted to a description of two simplemethod s of anod ising, which is a comparatively recentdevelopmen t in the preservation and decoration of alu-minium and which, in spite of its simplicity, is highlysatisfactory; more so, in my opinion, than the actualelectro-plating of aluminium and its alloys.Regarding the various pieces of equipm ent wh ich I havedescribed, I do not suggest that they ar e ideal, but I do feelthat they have been reduced to their simplest form withoutserious loss of efficiency. I sincerely hop e that, having hadthe patience to equip himself with a small plant of the typedescribed, the reader will derive as mu ch pleasure andprofit from this fascinating pu rsu it as I myself have enjoyedover the last few years. L.W.

vi


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IiVTRODIICTION AN D GENERALCOASIDERATIOKS

HE art? or science, of electro-p lating has reached a veryhigh degree of perfection in present-day indu stry andhas innumerable commercial app lications, q,uite apar tirorn merely decorative or protective pu rposes, andindeed the electro-dep osition of various meta.ls in somebranches of indu stry, notably in pr inting, has beendeveloped into very specialised channels.This book is not intend ed to deal w ith the speciallyapp lied branches of the subject, and the various typ es ofprocess to be described are set forth only with a v&v toprov iding a decorative and a protective coatin,g of metal.In add ition, notes on electro-forming are given at the end

of Chap ter 7. The following chapters a.re intended to providethe amateu r engineer with w hat is hoped will be sufficientdata, not only to carry out successful electro-plating in thesmall workshop, but also to provide himself w ith theessential tools of the trad e, i.e., the electrical equipm ent andplating tanks.When one wishes to set up a small workshop to carry outmod el engineering, one can quite easily go forth an d pu r-chase a complete outfit of tools and equipment and get onwith the job. Not so, how ever, in the case of the prospectiveelectro-plater, because so far as the auth or is awarethe only p !ating plant obtainable is on a far bigger scalethan anything required by even the most enthusiasticamateu r. Consequently it has been thought fit to devote

1


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ELECTRO-PLATTSO FOR THE A8lATEVR

a q.ery considerable portion of this book to a detailed dis-cussion of a suitable size of plant. together with d etails asto how such a plant can be assrmbled in :he small workshop .For obvious reasons it has been assumed that the readeris one of the band of very p ractical peop le kn own collec-tively as mod el engineers, and bearing this in mind , it hasalso been assumed that he is quite able to look after theIJurely mechanical processes involved in setting up asuitable plant. Those wh o are not in that position, andare yet interested in the present subject can no dou btobtain the necessary assistance in the mechanical details.In setting out to equip oneself with a small plating plant,the primary consideration will be the size of job to beund ertaken because u pon this depend all the other mattersto be taken into account. In dealing with electro-dep osition, the size of the job m eans, first and last, thearea of metal in contact with the liquid , and on wh ich aplate is to be formed. For example, a circular rod & in.in diameter and 13 in. long, has roughly the same totalsurface area as a pen ny and is, electrically speaking, thesame size. It is obvious how ever .that it will need a verymu ch larger tank to accomm odate it. It will be seen,therefore, that for the pu rposes of this book some limitmu st be set both to surface area an d to longest d imensionof the work to be plated, and for the sake of simplicity ithas, been assum ed that the following will represent themaximu m measurements of any work und ertaken :

a Longest d imension-10 in.b Sur face area not to exceed-160 sq. in.

These measu rements correspond with, say, a 10 in. disc,both sides being treated at once, or with a 5 in. cube.The tanks described, as also the electrical equipm ent,are designed with these limits in mind , and if larger objectsrequ ire to be plated, the necessary all-round increase insize of equipm ent will have to be mad e.2


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BASE METALSThe terms base m etal, or simp ly base will be usedfrequently throu ghou t the text, and it should be explainedthat these mrn ms refer to the actual article to be plated,wh ich shall be our n ext consideration.Differcrrt bases require widely differing treatmen ts toprepare t11cm for receiving a dep osit, and there is no dou bttilat the initial preparation of the base is by far the mostimpor tant process in the series of operations. The auth orhas ilever experienced so strong a feeling of disapp ointmentas wh en an otherwise perfect plate has blistered and peeledoff du ring the final p olishing. For this reason, the readers

attention is invited most particularly to Chap ter 4, wh ichdeals with the initial prepara tion of the ba.se, and m eticulousattention to detail at this stage cannot be too stronglyrecommended.The following types of bases are d ealt w ith in this book,and will probably cover most of the field, so far as theamateu r is concerned.1. Copper.2. Cast iron.3. Wrought iron and steel.4. 3rass and tin-bronze.5. Lead and pew ter (tin-lead).6. Lead-tin-zinc-antimon y alloys (tilled Eritann iametal).7. Z inc and galvanised ware.8. Tinp late (on iron).9. Aluminium.10. Non-conductors such as glass and plastics.

In view of the adm itted difbculties experienced, even byexperts, in satisfactorily plat ing the light alloys, they havebeen comp letely left out of this book. The details givenfor plating pure sheet aiuminium are practically uselesswh en app lied to most of the alloys of this metal.

iNTRODUCTION AND OENEKAL. CONSro ER4TIONS


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ELECTRC-PLATING PCR THE AMATEUR

SOURCE OF SUPPLY OF CURRENT

tVith the exception of the next few paragrap hs, and abrief reference to generation in a later chap ter, it has beenassum ed throughout this book that the reader has a supplyof current laid on from the pu blic main, and the conversionof such cur rent, both D.C. and AC., is discussed at somelerif$~ in Chap rer~ 2.-the use of batteries is expensive and often somewh atdisapp ointing, but against this it can be said that theauth or has accomp lished some excellent silver-plating on aset of tea-spoons with the aid of nothing more than a 6 voltmotor-cycle battery, an amm eter and a length of ironwire for a resistance. If there is no alternative but to usebatteries, it is recomm ended that a heavy d uty starter-battery of at least 45 amp ere-hours be employed, and inthis case all that w ill be required by way of add itionalelectrical equipm ent is an amm eter in the negative lead,and a variable resistance in the positive, w ith some meansof tap ping off a less voltage if requ i.red. The resistancedescribed in the next chapter is quite suitable for use witha battery.Before leaving the subject of batter ies as a source ofcurrent: a point well wor th noting is that economy ofcurrent can be effected by plating at a somewh at lowerrate than those given in the tables. In all electrolyticprocesses there tends to be some decomposition of thewater in v;hich the various salts are dissolved. This pro-ceeds at a higher rate with higher current densities, and is atotal !oss of current so far as deposition of metal goes. Itfollows that using lower current densities will ensure that agreater percentage of the total cur rent used will be usefullyemp loyed in dep ositing metal on the cathode, but of coursea proportionately longer time will be required to dep ositthe same weight of plate. It is therefore suggested thatbattery currents be kept down to two-thirds of the valueslaid down in the tables, the time of plating being increasedby one-half.4


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liUTRODl,CTIO N AND CENERAL CONSIDER ATIONS

It mu st be emphasised that none of the foregoing remarkscan app ly in the case of chromium plating. Tn this thecurrent density is very critical and mu st be as stated;any considerable dep artu re from the given values will ruinthe plate. Tn any case, ow ing to the very high currentsrequired for chroming, only the smallest bases cansuccessfully be treated by battery pow er.

TYPES OF PLATE TO BE CONSID ERED

The scope of this little book is necessarily limited, and ithas been thou ght adv isable to give detai!ed informationabout a limited nu mber of the comm onest and most usefulplates,, rather than a sketchy outline of a great m any.Plating in the following meta ls only will, therefore, bediscussed :

a Copper.b \ Yhite brass (known also as Hard brass).c Zinc.d Nickel.P Silver.f Cup ro-nickel ( German silver ).g Chromium.h Gold.

Such p lates as cadmium , indium, rhod ium and platinum .etc.. are ignored, as being qu ite outside the usual require-ments of the amateur.DETERMINATIO N OF SURFACE AREA

The am oun t of current wh ich is to.be passed throu gh anyelectrolyte for the pu rpose of dep ositing metal is determinedby two main factors. Firstly by the nature of the solutioncontaining the metal, an d secondly by the area of surface incontact with the solution, and which is to receive the plate.

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ELECTRO-PLATING FOR THE AMA.TER

It must be adm itted at the oul,set that the accuratedetermination of sur facg; area can, in some instances, provevery d itEcult indeed, and in the case of very in tricatesurfaces may be frankly imp ossible. Fortunately, with theexception of chrome, the matter is not very critical, anderrors of from 20 to 40 per cent. are u sua lly well tolerated .The app earance of the work du ring actual plating is qu ite agood ind ication of wh ether the current density is app roxi-rrmtely correct, and notes on this are given in the details ofelectrclytes in Chap ter 5.

bX hTriangle area : I_ 2b X h X number of sidesPolygon area : - 2

Cylinder (hcflow ) out side area : f~ X d X LCylinder (solid) area : r X i (d X L) f 2 1Parall elogram area : b X h


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INTRODlJ CTlON AND GENERAL C*NS*DERATlONS

For convenience, the calculation of sur face a rea of a fewsimple geometrical figures is given in Fig. 1, and it issqgested that for irregular shapes, a fairly close estimatecan usu ally be made. An instance of this is given in Fig. 2,wh ere a pew ter sugarbowl is averaged out. Similart;;eatment will often suffice for objects of d ifferent shap e.It is assumed that the reader can calculate the surfacea~;-ea of cubes, rectangular blocks, etc., bear ing in mindthat the required figure is the sum of all the surfaces. InC---l1 I4 3 !! I

FIG. 2.

II

Approxima te area is obtained by treating t he sugar-bowlas a cylinder, w hose dimensions are :-

D (diam . of cylinder) : d, + d,and area in therefore : (7 X D X h) -k (x2 X d 8 *)This gives external area only . Int ernal are; is roughlyequal for thin buses.this connection, it should be borne in mind that in the caseof hollow articles, w here the p lating solution has access to

7


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ELECTRO-PLATINO EOll THE AMATER

the inside, the area of the inner surface mu st also be takeninto account. Where the material of the base is thin, theinside area \ vi!! be nearly equal to the ou tside.In cases where blocking-out is don e. th e area blocked-ou tmu st be ded ucted from the total area, as the solution hasno electrical contact yvith the metal at this point. Theuses and method s $of such blocking-ou t processes arediscussed in Chap ter 4.There remains one final point to be mad e regard ing thecalculation of surface area. It often hap pens that one sideonly of a base is requ ired to be plated, for examp le, a head -lamp reflector. Rather than go to the trouble of blocking-ou t the blind side, qu ite a useful p lan is to emp loy onlyon e anod e, this facing the working side of the base.Und er these circumstances, a certain amou nt of metal willbe dep osited on the blind side, but very mu ch less than onthe working side, and , although the correct calculation ofwh at current to allow d epends on innum erable factors, theauthor has fcu,nd by experience that a happ y m edium canbe found , in most cases, by reckoning one full area of thewor king side, plus half the area of the blind side. Theonly time w hen this system gives trou ble is in chromium -plating, when two anodes at least must be used, and thefull area taken into account.

If the reflector referred to above is being silvered, thetime spen t in thorou ghly blocking-out the blind side willbe well repaid by the saving in silver. It is an expensivemetal.

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THE SUPPLY OF CURR ENT

ETALS can be electro-dep osited only by a directcurrent. Alternating current wills not, under anycircum stances, produce an electrical plate, hence it isnecessary to arran ge for a sup ply of direct cur rent ofsmall voltage an d relatively large amp erage.The pu blic su pp ly mains in this country are almostentirely alternating, and it is with this type of sup ply thatthe present chapter is chiefly concerned. A brief noteregard ing D.C. mains is included at the end of the chapter.In the case of A.C. m ains, the supply voltage is im-material, as a transformer is invariably used to convertthe current to the correct voltage. The only effect whichthe sup ply voltage will have is to determ ine the. ratiobetween the num ber of turns in the primary and secondarywind ings. It being outside the scope of this book to givedetail of wind ings, etc., it will perhap s suffice to stat.e tha ta transformer will be required with primary to suit themains, and with secondary outp ut as follows :

a One m ain continuous wind ing, tap ped at 4. 6, 8,10 and 12 volts, capable of giving a susta inedcurrent of 10 amp eres for several hours.b If required , a subsidiary separate wind ing of 4 voltsat + amp . to light a pilot bu lb.c A third winding of correct voltage an d current tosup ply the small motor to be used for agitating thecontents of the plating tank (see Chap ter 3).


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ELECTRO-PLATING FOR THE AMATEUR

.4t th e time of writing there: is an am ple sup ply ofex-Governm ent transformers at a reasonable pr ice, wh ichmeet the requ irements nicely, and when pu rchasing, it isa Food fault to err on over-size ,rather th an un der-size.I he transformer second ary switch is similar in construc-tion to the resistance switch d escribed later in this chapter?except that on ly five contact stud s are requ ired. A blindstud must, however, be placed between each live one, toprevent the arm shorting the transformer secondary tap-pings by making contact with two live stud s at once.Connections are show n in Fig. 3. Under no circumstances

FIG. 3.

I

The tappings for m otor, as show n, should be of a volt ageto suit the motor fitted.10


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THE SUPPLY OF CURRENT

should an auto-wound transformer be used, as thiswou id render the wh ole plating plant alive.OTARY TRANSFORMERS

These consist of an electric motor, ru n direct from themains, coupled to a low-voltage direct cur rent generator.Whilst such a converter, as they are called, can be mad eto ru n on either D.C. or AC. .mains, its use is confinedalmost entirely to D.C., as A.C. can be stepp ed dow n andrectified mu ch more cheaply and conveniently by meansof a static tran sformer and rectifier.Accord ingly, the rotary transformer is dealt with at theend of this chapter, un der D.C. m ains section.RECTIFICATION OF CURRENT

The next most important item is the rectifier, and forthe small plant u nd er consideration, either a copper or aselenium rectifier will be suitable. The one chosen shouldbe capable of rectifying 12 volts, and of passing 12 amp s.without un du e heating, and should for preference be ofthe double type for bridge connection. This gives full-wave rectification of the current, and is techn ically adv an-tageous. For plating pu rposes, no smoothing app aratus isrequ ired, except in the case of chromium ; see chapter 6.If a bridge typ e rectifier is not obtainable, the alternativesare either to use a single rectifier for half-wav e rectification,or better, two such rectifiers for full wav e. Conn ectionsfor these various arran gements are detailed in Fig. 4.

THE VARIABLE RESISTANCEThe next item of electrical equipmen t to be consideredis the variable resistance, which controls the current passingthrough the plating tank.This is a very important instrum ent, and if not carefullyselected, or exceptionally well mad e, can lead to end lesstrouble, either throu gh over-heating or through faulty

contacts. The au thor has tried man y d ifferent types and11


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ELECTRO-PLATI? GO FOR THE AMAIEUR

patterns, and has eventually come to the conclusion thatthe step type, with rad ial switch, is the most satisfactory.It is also fairly easy to make, and its only d isadvantage liesin the fact th at cur rents cannot be controlled to a very finelimit. For instance, it may happ en that the requiredcurren t is calcuiatcd to be, say, 6.5 amps. On tryingvarious positions of the radial switch, quite possibly theoperator is forced to choose between , say, 6 and 6.8 amp s.12


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previously m entioned, how ever, the current values arecritkal, and the operator wou ld ch se the nearer value,i.e. 6.8 amps. n the assumption t the reader hassome skill in simple engineering, the following notes onconstruction of a suitable resistance shou ld be qu ite ade-quate to ensure that G satisfactory instrum ent is prod uced.

/

-i-

>

e general lay-out for single-panel control is show n in6 and d etails of the resistance pan el separately inFig. 5. The d imensions given, although by no merigid, are qu ite satisfactory, and they shou ld not be reduby any large amoun t, or d ifficulty may be experienced inaccommodating the coils of the resistance.The por tion of pan el carrying the resistance switch iseither of slate or metal. In the latter case, the rotor spind leali contact-stud s mu st be thorou ghly insulatedorcelain. The remaind er of the pan el can

as a very large nu mber of studs,bu t in practice, ten will be found to give adequate control.,


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Combined panel, w ith all the controls.esnecially as further control is obtainable bv selections olvarious transformer secondary voltages. The constructionof the swia el is qu ite straig ou lpTwktt no ties.14


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s are brass cheese-bea ed machine screws,Whit. or equivalent dia eter in finer thread.cureiy iixed in their respective holes in theanel a washer and nut at the back, and each carries70 b&s washers and a further nu t for secur ing theresistance wires. The slots in the beads should be arran gedto point towards the rotor sp indle. This

pp er or brass strip of 3 in. width andckness, and is carried on a brass spindlea brass or steel bush , wh ich is in turnbe fixed in positionexternal thread andnge bolted to the p

The rotor spindle is fitted carefully gh this bush,so that xvhen the fixing nu t and lock-nut are screwed up ,re of the arm on the studs, whichby the springiness of the arm itself.ay, should be of s cient wid th to restce. Contact to rotor throu gh thebush is not reliable wh en dealing with large currents, ana flexible c~~~ect~r is recomm end ed. This consists of ay mains flex, and is ured to the rears washers andto a terminalte amoun t ofslack to be taken up r no circumstae resistmce coils.the values of the resistances toconnected between each stud, an Table 1 shows th erims resistance wires their characteristics.om ehis mble can the length of anywire for any required resistance.here on the ~~~ ~QvisatiQ~ of resistances from a

mains heats elementelemersts of 240 volts%ree%v btainabEe, anSect&S as shown in15


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ELECTRO-PLATISC P"R THE AMATEURjust over 4 amps. at a but this tempera-ture is of ccsurse not p e apparatus und erIf the current flowing throu gh the wire is

0 I.$ amp s. the temperatu re rise will not be too

Off

iagmm o/ values for Resistalzce Section.

great? and a single Wanb F t h e s e c t i o n s m

in paraM will suparaliel for sectionlength of wire rmir.d that the

e wire can therefore be usedFig. 7. Two strandsnd three strands in

resistance of 55.2 ohms.section l, which is to be 4 ohms, will need to be4 1--

55.2 which is rou ghly 14 total length .

will have to be 28 al the total len


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THE SUPPLY OP CURRENT

%CFiOF? ough only half the resistance of 6, iscomp osed 0; two wires in parallel, and each of these,therefore, wilP have to be as long as 6, i.e., l/ 28 of totallength. ~~rn iiarly~ section E is to be & ohm, but in thiscase three strands are used , each of wh ich mu st be 1Q ohms,OF l/ 37 of the total length.The remaining sections, A, B, C and D, cannot be builtup conveniently from a mains heater element, owing tothe high current-carrying capacity required , an d the auth orh3s found that iron wire such as is used for light fencingpu rp oses is qu ite suitable for these sections. This wire ISrdrnost ~~va~iab~y galvanised, and the zinc should beremoved by careful filing, .and sand-papering, for ifallowed to remain, it will app reciably alter the resistance.The final diameter of the wire after removing zinccoating should be ascertained with a micrometer, a thes&seance determined by reference to Table 1. From thisthe appropriate lengfh for each section can be calculated .The resistance of this wire being very low, these sectionswi!l be quite bulky, and the reader should not make anyattempt to economise in space by choosing a wire wh ich,wh ilst thinner, wou id be un able to carry the necessarycurrent. The value of current wh ich each size of wire cancomfortably carry is shown in the table, an d m ust not beexceeded or otherw ise over-heating will occur .Having selected and cut to size all the sections, they willhave t 5 be prepared for fitting to the stud s asp follows.The ends of all the wires are thorou ghly cleaned byapiy and each ction is then formed up to the shape5x3 1x2Fig. 8: wit the exception of sections A, B, C andwhich ~31 requi longer legs to carry them furtherback from the pan el, on accoun t of their larger bu lk. Tofox-m. the wires are und around a suitable man drel, theactual diame ch will of course be determined bythe length of e wires, and may not be the samefm each. 7-h reduce the wires to a convenientshap e for fit etails of this are best left to theingemiry of There are only two snags to

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ELECTRO -PLATINO FOR THE AMATEUR

WZi out for : firstly, no two ad jacent turns of any coilmu e allowed to come in contact and , second ly, no twotouch each other, except at the point where theyby the stud nuts./;/-r+.f ,:!,I)~,fl\~Iwi L

j;

L ijic&z

The num ber of turns in the coils w illdepend upon the length of wire in each.

Disposition of triple-coils for SectionE. Double roils for Section F are arranged sim ilarly .

The loops at the end of each section mu st be mad e a goodfit on the stud shanks, and in the case of sections Eand IF, which have oreattention must be pa than one coi! each, particuiarto the dan ger of squeezing out oneor more loops wh en tightening the clamp ing nu t. Thetwin and triple coils of these two sections should be formedseparaeely, and spread out fan-wise as shown in semi-diagrammatic form in Fig. 9.This is a much better plan than twisting the wirestogether before forming the coil, as any heat prod uced ismuch more readily dissipated.

from Fig. 7, the beginn ing of coil Abut directly to the terminal marked


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TH E SUPPLY OF CURRENT

OUT in ig. 6, thu s ensur ing that at least Q ohm is alwaysin circuit.A useful tip, wh ich helps to guard against overloadingof any sections of the resistance, is to mark clearly over eachstud the maximum current wh ich that par ticular sectionis intended to pass. These values ar e shown in Fig. 7, andshould not be exceeded. In other word s, when the rotorarm is In contact with any particular stud , the wh olecurrent in the circuit mu st not be allowed to exceed thefigure wh ich is marked above that stud . If this shouldappear to be necessary at any time during actual use, it isan indication that too high a voltage is being used , and th etransformer secondary tapp ing should be reduced

according!y. In pr actice, it will rarely be necessary touse more than 6 volts, except wh en chroming, bu t theactual voltage requ ired will dep end largely up on theintern al resistance of the rectifier.To retu rn to the resistance, all that is now required is tofit a suitable knob or hand le to the rotor arm, and twostops, one at each end of the arc of stud s, to prevent thearm from slipping off. The tenth stud , it wi!! be seen, is ablind one, and serves to switch off the plating current,wh ilse making any adjustment in the tank. The a rmshould always be switched over to this off positionbefore switching the mains current on or off.

METERS

e only meter actually necessary is an amm eter,gh the add ition of a voltmeter is considered bysome workers to be an advantage. Personally, althoughthe author ha s a voltmeter includ ed in his plant, it mu stbe man y month s since an y n otice was taken of it. Afterah, it is the current passing in the circuit wh ich matters,and , provid ed all connections are good, the voltage acrossthe electrodes mu st be proportional to the current in anyiven solution, u nd er normal working cond itions.

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TABLE I Res is tances o f va r ious wi r e s s u i t ab le f o r the p r e s en t ca s e----,.

Wire S.W.G. Diameter in inches Feet per Ohm Ohms per Foot Max. Current forPresent Case--- ___-- --__--

IRON . . . 0.072 0.012 17 amps.Ii 0.064 :; 0.015 12 .0.056 49 0.021Ii 0.048 0.028 I: ::19 0.040 :; 0.036 3 9.___, .__-NlCKEL . . . 0.072 0.018 IO amps.Ii 0.064 ii 0.02217 0.056 0.029 : ::0.048 ;Ii 0.036it* 0.040 22 0.045 : ::20 0.032 18 0.056 3 .,--- .-@, NI 0.064 - 0.084 8 amps.17 0.056 9 0.107NICKEL- 18 0.048 76 0.136 G 1:SILVER 0.040 0.168lo 0.032 it 0.222 :i ::-- --VI-CHROME 0.072 - 0.208 8 amps.Ii 0.064 : 0.2590.056 0.333Ii r? ::0.048 :I 0.421 4 7,2f 0.5201: Eii 4 0.659 I-


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Front of Ammeter Panel.The am meter itself is requ ired to give accurate read ingsover a long scale, say , from +J amp. to 12 amps., and this. isun usu al in a single dial meter. The solution to this isquite simp le, and consists in emp loying an amm eterread ing from 0 to 3 amp s., combined with suitable sh un t

wires, and the setting u p of such an instrum ent could betackled as follows (Figs. 10 and 11).The am meter is moun ted on the instrum ent panel in anyconvenient way , and a thick copper wire, of about 14 s.w.g.

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ELECTRO-PLATINQ FOR THE AMATEUR

is firmly connected to one terminal, and run to the terminalmarked R.pos. on the Pan el (Fig. 6), where it is securelyfixed by a really sound job of soft-soldering. Thisis repeated at the other terminal of the meter, runn ing thewire, in this case, to the terminal marked RT on the

c&=====To&w.tf?dor

i0RtA+FIG. II.

Rear of A.mm eter Panel. Joint s mark ed IS are soldered.

resistance switch (Figs. 6 and 11). On each side of themeter is mounted a knife switch, of the common pattern ,with self-cleaning contacts. These switches must have allmetal par ts made of copper, and be of amp le size; theycannot, theoretically, be too large, althou gh for practicalpu rposes, those with a blade + in. wide and 1$ in. thickwou ld serve. The contact mu st be really close and positive.The nearest terminal of each switch is then firmly con-nected to the respective conductors on each side of themeter, again with 14 s.w.g. copp er wire, and again soft-

soldered thorou ghly. Over the left-hand switch, the panelis then marked X2, and over the right-hand one, X4.En the centre, over the meter, the panel is marked BothSwitches X5.22


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There remains now, only the calibration of the meter,by the fitting of two shun t wires, and this is done in thefollowing way. The transformer, rectifier, resistance andmeter are all connected up , as in Fig. 6, and the variableresistance set at maximu m resistance. The transform eroutp ut is set at 4 volts, and both knife switches are closed.The outp ut terminals on the panel, marked f and -,are then shorted by a piece of 14 s.w.g. copp er wire, andthe resistance is ad justed to give a 3 amp. deflection on theme,ter. If this cannot be done accura tely at 4 volts, the6 volt tapp ing should be tried, adjusting the resistanceagain. When a 3 amp . current is obtained, switch off themains, leaving the controls as they are. To the freeterminal of the left-han d switch ( X2 ), a length of 22 or24 s.w.g. copper wire is secured , and the other end ispressed tightly against the cond uctor connected from theterminal marked R.pos. to the meter. This contactshould be mad e as perfect as possible, by squeezing thewires together with pliers. By trial and error, the correctlength of wire is found which, wh en cur rent is switched on,will give a read ing of exactly l& amp. on the meter .Withou t cutting the excess off the shun t w ire, it is soft-soldered to the cond uctor, and the reading again checked.It may be a little low, owing to the imp rovement in theconnection when soldered? and if so, it 1s unsoldered , anda little more length allowed . When the correct length isfound , the shu nt is coiled into a spira l and finally solderedinto place, and the knife-switch opened . If, du ring th isfitting, it has been found that the required shunt is un -reasonably long, it should be replaced by a thinner wire,and vice versa. To the free terminal of the right-handknife switch, another shu nt w ire, this time of 18 or 20s.w.g., is fixed, an d th e whole of the above process repeated ,this time, of course, run ning the free end of the shun t wireto the conductor joining the meter to the RT terminalon the resistance pan el, so that the reading on the meteris redu ced to $ amp . when the right-hand switch is closed,and the other one opened.

23


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ELECTRO -PLATINO FOR THE AMATEUR

The pan el is now comp lete, and should be tested oncemore as follows.With the X2 switch closed, and the other open, thecurrent is adjusted to read 3 amp s. The X4 switch isclosed, and the current should read 14 amp s. Leaving the X4 switch closed, the X2 switch is open ed, and theread ing should then increase to l+ amp s.Opera tion is quite simple; for currents up to 3 amp s.both switches are open. For currents between 3 and 6amm ps.: he X2 switch is closed; betw een 6 and 12 amps.,the X2 switch is open and the X4 closed, and forcurrents between 12 and 15 amp s., both sw itches are closed.TQ find rh e actual current, the meter read ing is mu ltiplied

by either 2) 4 or 5, accord ing to the p osition of the twoswitches.

D.C. SUPPLY MAINSIf sup ply mains of, say, 12 to 20 volts D.C. were avail-able, all problems connected with voltage redu ction andrectification wou ld disapp ear. Unfortun ately this is neverthe case, an d mains sup ply at less than 100 volts is indeed

rare. By far the most common voltages are those aroun dthe 200 to 250 mark, and this fact introdu ces problems.It is quite p ossible, from a theoretical standpoint at least,to emp loy DC. current direct from a 250 volt main forelectro-plating, simply by inserting a suitable resistance.The matter, from a practical standpoint, how ever, is notso simple. Firstly, un der those circum stances, all theapp aratu s, includ ing the plating tank, w ould be alive,and u nder most conditions of supp ly it would be dangerousto hand le any par t of the app aratu s. Insulation, too,wou ld be extremely difficult to arrange, par ticularly so inview of the presence of liquid , and moist fum es. Perhapsthe greatest drawback, how ever, to direct use of DC.mains in this way, is the cost.24


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TXE SUPPLY OF CURRENT

mmeter A, shou ld be fitt ed. 10 and II;.27


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E PLATING TANK

HE tank or vat in which the actual plating is done is,theoretically, only a mean s to an end . That is, ittakes no actual part in the process except to hold theelectrolyte, and allow immersion of the base to be platedand the anodes, and as most so!:ltions are used at anelevated temp erature, it mu st he able to withstand heat.Unfortunately in practice most of the solutions to beused are of a corrosive natu re, and consequently thematerial of wh ich the tank is composed mu st be chosenwith this fact in mind ,. A brief consideration of the varioustypes of electrolyte, as the plat ing solutions are called, willgive an indication of the most suitable materials.

Broad ly speaking, there are two main classes of electro-lyte : the acid c!ass and the alkaline class. They can betabulated as shown in Table 2, wh ich indicates thematerials wh ich the various solutions attack, and wh ichtherefore mu st not be allowed to come in contact withthem.A study of this table will show that there are only threematerials su itable for hold ing all typ es of electrolyte, i.e.,glass, glazed earthenware, and stoved enamel. Glass isnot very satisfactory, owing to the fact, as mentioned above,that most electrolytes are used at higher temperatures thanthe room, and heating arrangements for a glass tank wouldnu merous difficulties, because of its fragile natu re.earthenw are, in order to be sufficiently robust,29


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THE PLATING TANK

woul need to be very heavy, and here again heatingdifficulties are met. Both of these types of tank could beheated only by an immersion heater, and the temperaturewou ld have to be raised very slowly, in order to avoid thedan ger of breaking. Nevertheless, in spite of these draw-backs, glass and earthenware have a very strong recom-mend ation in the fact that th ey are the only materialscompletely un affected by all types of electrolyte, and if alow-power immersion heater can be installed and adequateagitation provided , a glass or glazed earthenw are tank ishighly satisfactory.A further glance at Table 2 will show that the only typeof tank bea ring an asterisk in all cases is the stove-enamelled one, and by this is meant an iron tank with avitreous finish, applied at a red heat. This finish, beingin reality a kind of glass, is almost completely un affectedby all electrolytes.Such tanks can easily be adap ted from large domesticbreadbins, and here again, these are precisely w hat th eauthor used to make his tanks. A complete description ofhow one of these tanks was fitted (they are a ll exactly alike)will no dou bt furn ish the beginner with a ll the informationneeded in order to make a really first class tank.When pu rchasing a breadbin, it should be examinedcarefully for cracks an d chips in the internal glazing, and ,if electric heating is to be fitted, should have a flat, not aribbed bottom. Only one such tank w ill actually berequired for plating, but it should be borne in mind that ifthe various solutions are to be stored when not in use, thena further two will be required . This is more fully explainedin Chap ter 5, wh ere the actual solutions are discussed, bu tit can be mentioned here that if oil drum s are to be usedfor storing the solutions, they are not satisfactory either forthe chromium or silver, and it is suggested that these twosolutions be kept always in an enamell~ed breadbin. Theremaind er of the solutions can be kept in tinned iron-ware,such as cleaned oil dru ms, but only wh en cold. The actionof the various solutions when cold is sufficiently small to be

31


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ELECTRO-PLATIN G FOR THE A51A TER

but tinned iron is q te un suitable for platingn the electrolytes areliminary considerations regard ing a plating tank,m the material of wh ich it is constructed, areconcerned wit : 1. Size and shape of base article.2. Position, dis nce and num ber of anodes. 3. Fitting ofanod e- and cathode-bars. 4. eatmg arrangements.5. Agitation. 6. Furire clearance These items will beeait wi ) now, in the above order.

I I t emS and 2 are concerned only with e size and shapeof the tank, and it is quite possible to obtain a breadbin ofapproximately 11 in x 13 in. x 12 in. deep , wh ich is anexcellent size to accomm odate a base of the maximumdimensions, id dow n in Chap ter 1. Such a tank willhoki, wherl to within half an inch of the rim, abou tsix gallons, he quantities of chemicals to be dissolvedin this amount of wa ter can easily be calculated from theform&c given in Chap ter 5, as these quantities are allexpressed as ounces per gailcn (oz./ gall.) and require onlyto be ~~~t~~~~edby six.If the tank obtaine differs in dimensions ram theabove, its cubic capacity in gallons can be calculated fromthe formula

ensions of tank in inches, and GIft might be noted here that for nickel and acid copperel-ctrolytes: which are used cold, an excellent tank can hemade from an acp ium tank. Those constructed of steelangle-iron and glaz Itith non-oily pu tty are best, andcan be had in all sizes. They are not very suitable for rme&ectrali;tes, as the alternative heat ing and cooling xtsthe putty.

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THE PLATINO TANK

ELECTRODE BARSThe fitting of a n o d e and cathode bars can be discussedirements for small-scale work are qu ite

mist of two cond uctors from which thended, and one conductor from which theded . These bars are preferably of copper,

FIG. 14.End of tank bar, show ing meansof sonnecting conductors.

locks to support and insulate the tank bars.but brass is quite an e cient substitute, and is mu ch easiert o work. The author anks are all fitted with brass bars,and the only attention needed is an occasional cleaning

The bars are of Q in. round rod, carriedich rest on the sides of the tank, andas in Figs. 13 and 14.he bars are cut 4 in. 1 shorter dimensionole in each end isfrom the end, aly and tapp ed 4 in. for a set-

screw to bind the connecting wire in the hole. The woodenblocks. cut to the shape shown , are then dr illed a tightfit over the brass rod s, and dr iven on. Their posi-jImted ro thar the grooves cut in the bottomn icdy over the rim of the tank, one over eachside. Three such bars and supp orts are mad e and arelitted v,?t:h movab le contact bosses, as show n in Fig. 15,

cur horn 2 in. s e e t brass. .4lternatively, wood en runners33


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as

.+--------ae tank with bars in


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Y ehe bes t an

e r e r e m a i n s t h e6 s c a n b e d o n e

pared co wa i f . a H o n g

n o t a c t u a l l y i n u s e .

r e a d e r w h o h a s t h e n e c e s s a


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suggested that the first exercise in copper plating could veryeasily be the plating of these pa.rts. They are scru bbedwith a nail brush, a little p um ice pow der and a good d eal

FIG. 17.Approximate shape of iron clamping-plate for the heatingelement.

of soap, and the tank is then par tly filled with No. 2 coppersolution. The tank itself is connected to the cathode barvia the earthing terminal un dern eath, and a smallcopper anode is immersed in the solution. A very minutecurrent is passed, say, as mu ch as a 12 volt 3 watt side-lampbulb will pass on the 4 volt tap ping, and plating is carriedon for ten minu tes. The No. 2 copper is then pou red out,the tank rinsed with water, and refilled with copper solutionNo. 1, plating being resum ed for a further period of aboutforty Iminutes. This will give a robust copp er plate, notonly to the nuts, etc., but also to any parts of the iron tankleft exposed by accidental chipp ing of the enam el.A shallow wooden frame should now be mad e, on whichthe tank can stand, thus preventing any dam age to, orshorting of the connectio..,nc undern eath. A small portionof this will have to be cut aw ay to allow the cable to passthrou gh, and it is essential that this latter be of first qualityrubber-covered, to guard against access of any spilledsolution to the actual wires.38


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THE PLATING TANK

AGITATIONThe final attachment to the tank itself concerns agitation,or stirring of the contents, and perhap s a word in explana-tion of the need for this will not be out of place.When a current is passed th rou gh an electrolyte contain-ing salts of a meta l, the metal itself comes ou t of the solu -tion, and adheres to the cathod e. At the same time, ifanod es of the same metal be used , a rou ghly equivalentamou nt of metal d issolves from the anod es and passes intosolution in the form of salts. If the solution is allowed toremain perfectly at rest, it will be seen that there is a ten-dency for the electrolyte to become deficient in metal in

the region su rroun ding the cathode, and to become abnor-mally enriched in the vicinity of the anod es, the nett resultbeing that dep osition of metal is retarded, and gassingencouraged . There are also man y other important effectson the quality of the plate, wh ich need not be detailedhere, bu t it can quite readily be un derstood that if theelectrolyte is constan tly in a state of motion, it will remainat the same uniform strength through out, and electro-plating will proceed at a un iform and satisfactory rate foran indefinite period .The auth or has tried various method s of agitation, such

as blowing air un der slight p ressure into the lower part ofthe electrolyte (the common commercial pr actice), and byexternal combined circulating and heating coils, bu t hasfound that, for small tanks, none is qu ite so satisfactory asa propeller, dr iven by a tiny electric motor.The m otors used are of the min iature 24-volt ex-R.A.F.

type, man y of wh ich are available at a few shillings each,at the time of writing. These m otors, althou gh intendedfor working on D.C., are almost all provided with laminatedfields, and work very w ell on 16 to 24 volts A.C., d irectfrcm the second ary of the transformer. Those installedby the author have, at times, run continu ously on 18 voltsA.C. for over twenty-four hours, and have not shown signsof heating up . As they are, moreover, totally enclosed,

39


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ELECTRO-PLATI NG FOR THE AMATEUR

they are reasonably immu ne to attack by acid or alkalinespray, a little of wh ich is always p resent near the tank,when in use. In the authors plant, a separate wind ing

FIG. 18.Method of setting up the agitato r-mot or. The propellershould just clear the bottom and the side of the tank.

is provided on the transformer to sup ply these motors, andthe amou nt of current taken by three working at once isquite negligible.Fig. 18 shows how su ch an agitator can be fitted up ,and is almost self-explanatory. The propeller-shaft, in theauthor s case, is a length of stainless steel, which h appen edto be handy at the time, bu t it is suggested that a length ofcopper tubing, of fairly stout gauge, w ould serve equallywell. The dr iving end of the motor spind le is usu ally4 mm ., and a piece of tubing wh ich is a very tight fit onthis should not be hard to find . This end of the tube issplit for abou t 4 in. and a boss -with grub-screw fitted over.The grub-screw should be sunk, to minimise centrifugalforce.40


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FIG. 19.The propeller.

FIG. 20.Diagram of propeller aridshaft. Show ing one method ofconnecting to motor.

trial and error. The object is to arran ge the speed of themotor to be between, say, 600 and 1,000 revs./ min., andthe pitch of the propeller is adjusted so that a reasonablecirculation of the electrolyte is obtained . The speed ofcirculation should be about 3 or 4 seconds per circuitof the tank, as ascertained by allowing a few scraps ofpap er to dr ift in it. The speed of the motor is best keptwithin the above limits, as no !ower bearing is provid edfor the propeller shaft, and high sp eeds would pick up any slight w obble in the shaft, and throw it completelyout of line.

Figs. 19 and 20 show sh ape of propeller.To those readers w ho have some experience of electro-plating, the foregoing descriptions of the various items ofequipm ent, and the method s of fitting u p, will dou btless41

THE PLATING TANK

At the other end of the shaft, a very small prop elleris soldered or screwed on, and this can be mad e quitesimply out of a scrap of 24 or 26 gauge copper sheet. Themaximum diameter of the propeller need not be more thanabout an inch, and the pitch of the blades is found by


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ELECCRO-PLATI NG FOR THE AMA TEUR

be of use as suggestions, w hich are quite capable of mod ifi-cation. On tie other hand , th e beginner is advised toconsider them as definite instructions, wh ich, if faithfullycarried out, will prod uce, if not an ideal plating plant, atleast one which will do the job in a very satisfactorymanner.

To all readers, how ever, wh ether experienced or not,the author would emphasise most strongly the dangerarising from fumes.

Jn the case of chromium plating, very unpleasant fumesare given off by the electrolyte, du ring passage of current,and , wh ilst these fumes are dan gerous, in that they cancause permanent dam age to the skin and mu cous mem-branes of the mou th, throat and lungs, they cannot be saidto be lethal. Their presence, moreover, is distinctlynoticeable long before they reach a harm ful concentration.

In the various electrolytes containing cyanide, h owever,we meet w ith a vastly d ifferent state of affairs. Theseelectrolytes give off at all times, whether in actual use ornot, gases wh ich are very insidious, and infinitely moredan gerous than those proceeding from the chromium tank.These gases consist mainly of hyd rogen cyanide, wh ich isrecognised by its characteristic odour . It is not easy todescribe this odour, bu t it is exactly the sam e as is noticedwh en taking the stopper out of a bottle of sodium cyanide.This gas possesses the qua lity of par tially paralysing thesense of smell, and it is qu ite possible to remain in a roomwh ere the concentration of this gas is rising, and yet beunder the imp ression that it is becoming less.Und er actual w orking cond itions, du e to the fact thatcyanide baths are all worked warm, the evolution of gasis accelerated, and in a closed room can qu ickly reachdan gerous prop ortions. Hyd rogen cyanide is one of themost powerful poisons known, and in lethal concentration

it is so rapid lv fatal that there is no time for counter-measures to & taken.All this might appear, perhaps, to be rather dramatic,e author wishes to emphasise, as strongly as possible,

42


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THE PLATING TANK

the need for exceptionally free ventilation. If the work-shop is in an out-building, all wind ows and doors mu st bewide open during the whole of the time that the solutionsare exposed, and the operator should n ot remain in theroom longer than is necessary.If the workshop hap pens to be in the cellars of a hou se,as in the authors case, it mu st be remem bered that h ydro-gen cyanide is a light gas, wh ich rises in air, and if allowedfree passage, will concentrate in the bedroom s, etc., at thetop of the house.In such cases, where the plating mu st necessarily becarried ou t in a confined space, it is essential to providefor the positive withd raw al of all poisonou s fumes. Theauth or solved this problem by fitting a tinplate cowl overthe tank, and leading the fumes out throu gh a hole mad ein a chimney breast, by means of a tinplate shaft, solderedon to the cowl. The cowl and shaft are fixtures, an d arearranged in such a position that th e tanks can be slid un derwh en about to be used. Ow ing to the lightness of the gas,and the heat rising from the warm tank, the fumes pass upthrough the shaft without any actual suction by fan beingnecessary.For those wh o prefer simp licity, and do not mind havingto depend on the weather, there is always an alternativein carrying on the whole p rocess ou t of doors, and theonly p oint to note, in this case, is that the cable carryingthe current from the panel to the tank shou ld be as shortand as heavy as is reasonably possible, to avoid anyconsiderab le voltage dr op.

43


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PREPARATION OF THE BASE

It is not the p urpose of this book to enter into a discussionof the machining of castings, etc., it being assumed thatwh atever articles the reader might w ish to plate will be ina more or less finished condition. Any castings whichhave not been machined over the entire su rface shouldbe mad e r easonably smooth, either by filing or grinding,before attemp ting to prep are for plating.The actual preparation is carried out in some, or all ofthe following stages :

I. Smoothing, i.e., grind ing, smooth-filing, sand -papering.2. Partial polishing , i.e., buffing w ith very fine emeryand grease, or with coarse polish+g comp o.3. Chemical cleaning, by immersion in either acid oralkaline baths.4. e-greasing.5. Electrolytic cleaning, in wh ich the base is im-

mersed in suitable solutions, and a current passed.6. Etching, to provide a very fine matt su rface.7. In the case of chromium plat ing only, a very high

polish of the under-coating, prior to final plating.8. For silver-p late on copper or brass, the qu ick-silvering process.

require differing treatments, it isof clearness, to describe the variousprocesses ou tlined above, in detail, and then to tabulatethe variou s bases (in Table 3), to show which processesthey re+re, and in wh at ord er they are to be carried out.Procefs I (Smoothing)This is pu rely a matter of removing all indentations andexcrescences from the base, and redu cing it to a reasonablysmooth surface. The file, grindstone, and emery or sand-11 have th eir u ses, and , with a little pa tience, qu itesurface can be obtamed. At this stage, the base

45


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ELBCTRO-PLATINO FOR THE AMATEUR

should pr esent a clean app earance with none but the verylightest scratches visible.Process 2 (Partial polishing)A light polishing with grade o emery, bonded withgrease, and u sed on a calico wheel. The au thor find s thususeful for cast iron and hard steels, wh ere sand pap ering istoo tedious. It is not requ ired for softer bases.Process 3a (Alkaline dip)

The base is immersed in the following solution, at roomtemp erature, for five minu tes :

Pure Cau stic Soda . . . 6 ozs.Sodium CarbonateSodium Cyan ide, 130% 1: 1Water to . . . . . . . . .Process 3b (Acid dip)

The base is immersed in the following solution, at roomtemperature, for three minutes :

Sulphu ric Acid , pu re . . . 5: ozs. (fluid )Hydr ochloric Acid, pu re.. . ,, ,,Nitric Acid, pure . . . 3 ,, ),Water . . . . . . . . . 60 ,, ,,warningIn prep aring this solution, the following routine mu stbe observed, w ithout deviation :

Mix the water, nitric acid and hydrochloric acid. Ad done Qu arter of the sulphu ric acid, and cool thorou ghly.When cold , add a further quarter of the sulph ur ic acid,and again cool. When qu ite cold , add the remaind er ofthe acid, and cool. The solution shou ld be kep t in glasscontainers with a glass stopp er.Process 4 (Degreasing)

The base is carefully swabbed over with a small piece ofclean rag, saturated with carbon tetrachloride, changingthe rag at least three times. This is followed by scrubbing46


54/114


with hot water and hou sehold detergent, using a cleannail-brush.Process 5 (Electrolytic cleaning)

The base is connected to the cathode bar and immersedin the following solution, a t 70 Centigrade, and a currentof 2 amp s. Per square decimetre is passed for five minu tes,using a steel plate for anode :

Sodium Cyanide . . . . . . 4 02s.Sodium Carbonate . . .Sodium Tribasic Phosphate g 7,Caustic Soda . . . . . . 1 0;.ater to . . . . . . . . 1 gallonProcess 6 (Etching)6a For non-ferrous metals

The base is mad e ANODE, and immersed in the follow-ing solution, at 70 Centigrade, and a current of 2 amp s.per square decimeter is passed for three minu tes, u singsteel or copper cathodes :Citric Acid . . . . . . 8 ozs.Water to make . . . . . . + gallon-issolve, and add strong ammonia solution, un til thed just smells of amm onia, constant stirring beingnecessary whiist neutralising.Then add :Gitric Acid . . . . . . 3 ozs.Water to make . . . . . . 1 gallon

6b For ccrst and zevought iron and st eelThe process is identical with the above, except that thefollowing solution is substituted :

Sulphuric Acid, pure . . . 5 ozs.Hyd rochloric Acid, pu re.. . 5Water to make . . . . . . 1 gZlonThe cathodes should be copp er, and current should befor five minutes.

I 47


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ELECTRO-PLATING FOR THE AMA TEUR

Process 7The hi ~1. polish requ ired pr ior to chrom ium-plating

cannot be pl.odu ced on cast or wrou ght iron, and thesemetals are therefore plated with nickel before chroming.The degree of polish obtained is reprodu ced exactly by thefinal chrom ium, and therefore mu st be as perfect as possible.The author has foun d that th e red polishing soap as itis common ly called is the best med ium for producing amirror surface, either on copp er, brass or nickel, and it is agood plan, after p olishing thu s, to give a very light ru bover with a little precipitated chalk an d w ater, on a pieceof cotton wool.Process 8 (Quick-silvering)

This process is used solely to prepare copper, brass andnickel bases for silver plating, and its function is to pr eventthe silver from lifting. For some un know n reason, silverplated direct on to these metals d oes not adhere, and athin film of mercury interposed ensures perfect adh esion.The process consists in swabbing the base with smallpieces of very clean rag or cotton wool, libera lly soaked inthe following solution, rinsing very rap idly in clean w ater,and then immediately proceeding with the silver plating.The d eiay shou ld not be more than ten minutes, and d uringthis time th e prepared base mu st be kept u nd er cleanwater.Quick-silvering Solution

Perchlorid e of Mercur y . . . + oz.Sod ium Cyan ide, 130% . . . 2 ozs.Water to make . . . .,. * gallon

This solu tion is used cold , and a good pressure is app liedwith the rag, to distribute the mercury well over the sur-face. Half the above quan tity will cover many squ are feet,and the solution will keep ind efinitely in glass, bu t mustnot be allowed in contact with metal du ring storage.48


56/114


IIL Ii Y ww LLci d u u u u u uuG d

wci

Sqaqd wn!woJy3 uo sadoq3 aqlu

m nw m m mu u

cl m(3 m m U

m m aaaaaaa a a a a

:gJz i i ii i i is Q 3fL1 7%. =. Ip) g : : : : :. .5 5 Q) i i* * .z I= $s -3 2 azj; $:

520.0 ::5 -0 3 2a. 55 *$5

5 k .E b big $2.i! cm 2 g A o h8 8 $7 .!5 .- 5 5 =a

2 7 E3.L .- ~i k k.d2 0$ -* 23 j :E:: E gj 2 Bh g- gtjgUg&32 5F FJ a


57/114

ELECTRO-PLATI NO FOR THE AMAT EUR

Process 9 (Iron dip )This is used only on alum inium bases, and serves to give

a coating of iron to the base, which can then be copp eredin the No. 2 copper bath. There mu st be no delay betweencarrying out this iron dip, and the initial coppering.The process consists of boiling the base in the followingsolution for five minu tes, and shou ld be carried out in anenamel pan.

Solution of Ferric Ch loride,BP. . . . . . . . 8 ozs.Hydrochloric Acid . . . 2 ,,Water to make . . . . . . 1 gallon

PROCEDURES WITk VARIOUS BASESTable 3 gives the correct sequence of operationsfor preparing various bases for various plates, and to

read it, take the appropr iate base in the left-handcolum n, and read off the sequence of operations alphabeti-cally. The processes are referred to in the table by thenu mbers wh ich they have been given in the foregoingparagraphs.BLOCKING-OUT

The final process of preparation of the base to bedescribed is a means of preventing deposition of plate oncertain parts of the base, where this is desirable. Blocking-ou t is carried ou t for several reasons, chief of which arefirstly, for decorative pu rposes, wh ere contrasting platesare required , and second ly, for reasons of econom y whendep ositing expensive metals such as gold and silver. Forexample, the reverse side of a reflector need not be silvered,and blocking-out will prevent wasteful plating of thismeta l, wrhich is gu i:e cxpeasive.

The process is qu ite simple, an d is carried out imm e-d iately after completion of other prepara tory processes,so that the work may be introdu ced to the plating tankwithout further h and ling. Various varnishes are in use as50


58/114

PREPARATION OF THE EASE

bl~ck~~~ agents, ut cellulose lacquer a pears to be theThte clear kind is perh aps best, and one brushing quality rather than oneapp lication, as the lattermeta l su rfaces. A singlebrush, and of course taking extremeallowed to come in contact with partsrequ iring plating, After the plating is complete, thelacquer may be removed with cellulose thinners on a rag..n hours drying-time should be allowed afterefore plating is commenced.

C~~N~C-~~N G THE ASE TO CATHODE BARbe wr itten about this subject, as it is such ar, but it is surp rising to find it completelyneglected in so man y text-books.There are four commonly-used method s, and the choicedepend s on e base to be plated.

1. Soft-soldering.2. Nut and3. Spring clip.

Of these, soft-soldered connections, wh ere possible, aree most satisfactory, and should be adopted m allre a little space can be found which need not bewhich will not be seen in the finished object.reflector (close to rim);under extreme tip of leg of

The nut and bolt is a very good means of making con-nection, if it can conveniently be fitted. Its use dem and sthe existence of suitable holes in the base, and examplesof its app&cati.o~ are the centre holes in flywheels, loco-mcmve wheels: etc., existing constru ction holes in pa rts of

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side or face of the base an equal chance, by turning roun doccasionally du ring dep osition. For example, a cube, asshown in Fig. 22, wh ich requ ired thirty minu tes plating,wou ld best be treated by allowing ten minutes in each of thethree positions shown. The pr inciple app lies to all work,and to a ll electrolytes, but most par ticularly to the acides, i.e., No. 1 copp er, No. 1 nickel, and chromium .xatever the actual shap e of base, the operator shoulduse his initiative in ensur ing that all sur faces each havetheir turn to be exposed to the anodes.

FIG. 21.W ire Slings, various types.

The au thor has found the plating of small spoons to beone of the most awkw ard problems in respect of connectionto the cathode bar.The ideai position for a spoon is on its edge, that is,suspended horizontally, with th e blade in a vertical plane,as show n in Fig. 21, with the fron t (concave) side tw ice asfar from its facing anod e as the back (convex) side. The53


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PLECTRO-PLATING FOR THE AMAT EUR

reason for this is the need to have a greater weight of metaldep osited on the back, as this side is subject to mu ch morewear than the front. The reader will find it a good exer-cise in man ipulation to arran ge a wire sling, of the patternsketched , wh ich, w hilst holding the spoon in the correctposition, will yet allow sufficient play for the spoon to bemoved about, thus prod ucing a uniform plate. As a matterof fact, wh en the beginner has advanced so far as to be ableto produ ce a fine, even plate of silver on half a dozenteaspoons, he can rest assured that he has progressed wellbeyond the ia novice stage.

PO&n Posibl ftdon1 2 3FIG. 22.

Position of base, show ing t he shift ing around necessary toproduce an even plat e.

One final point, before p roceeding to a consideration ofthe variou s electrolytes. This concerns the high pr ice ofsilver, and the fact that a considerable quantity of thismetal is dep osited on t-he sling-wires and connectors11generauy.Rather than allow th is silver to be wasted , the followingprocess of recovery is carried out. By recovery, is meant,of course, the pu tting back of this silver into the solution,wh ere it is available for dep ositing again. The variouswires are bunched together, and suspended from the anodebar. One of the usu al anodes is taken from the normal54


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position and connected to the cathode bar, the other anodebeing removed from the tank altogether. A small current,equivalent to 1/ 30th amp . per inch length of the wires, ispassed, un til all the silver is dissolved off them . A littlecopper will also pass into soiution, bu t th is does notmatter, as it will not be dep osited under working conditionsof the bath.During this process, an amount of silver almost equ al tothat dissolved will be dep osited on the anode (now cathod e),but this will again p ass into solution wh en the anod eresumes its norm al role. The wh ole of the silver from thesling-wires is thus recovered.

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ELECTROLYTES

AWING. Many of the following solutions containsodium cyanide. This is a most dangerou s poison, and

mu st at all times be hand led with extreme care. Underno circumstances should acid be allowed . in contact withit, or with solutions containing it, and it mu st be storedout of reach of children and any persons not familiar withits dan gerous natu re. All solutions of cyanide give off,when exposed to the air, prussic acid gas. On no accountshould this be inhaled as it is highly poisonous, even invery small quantities. This evolution of gas takes placemu ch m ore rapidly wh en the solutions are warm , andwh en they are being used electrolytically to dep osit metals.Whenever electro-plating is being carried on with cyanide-containing solutions, either the fumes must be carried awayby means of a suitable cowl and du ct, or else the wholeoperation mu st be cond ucted either out in the open or ina well-ventilated shed. In the latter case, the operatormu st not remain in the shed for longer than is absolutelynecessary to make required connections and adjustmentsto the app aratu s. The reader is referred to previous noteson this subject, in the latter par t of Chap ter 3.We arrive, now, at the prepara tion of the variouselectrolytes, which are the solutions of meta llic salts fromwh ich the required plates are dep osited by the passage ofcurrent.There are one or two points to be strongly emphasised,chief amongst them being the fact that it is quite useless to56


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ELECTROLYTES

prepare any of the following solutions f:rom impu rematerials. All chemicals used m ust be of the h,ighest grad e,and exactly as specified. For example, if the formu la givenstipulates Sodium cyanide, GO%, this salt only mu st beused ; it wou ld be quite futile to emp loy p otassium cyanide,wh ich might vary in cyanide content between 20% and980/ ,.Similarly, althou gh the exact comp osition of some of theelectrolytes is not critical, in others it is extremely so, andthe reader wh o has little or no experience shouid tlnereforeaim at absolu te accuracy in all of them . Pure chemicalsare available throu gh chemical trad e channels, but theseare not readiiy open to the average person, and the authorhas always found mu ch satisfaction in the high standardof chemicals obtained throu gh the retail ph armaceuticaltrad e. Pew pharmacists have a stock of all the chemicalsrequired by the amateu r electro-plater, but most of themwill willingly obtain wliiat is requ iied at a few days notice.Cyanides, and the salts of mercury, are controlled by thePharm acy and Poisons Acts, and, in add ition to thepu rchaser having to sign the poison book, he mu st either beknown to the vendor, or else be introduced by a mu tualacquaintance. Such is the law, an d it app lies in so far asthe subject of this book is concerned, only to cyanides, andmercuric chloride. It should not be assumed, h owever,that these are the only poisonous substances used , and itis a good p lan to regard all chemicals as poisonous, andtreat them with du e care. Any solutions wh ich are spilledshould be immediately mopp ed up , and und er no circum-stances should any of the various cyanide solutions beallowed in contact with any liquid containing acid, as thisgives rise to copious evolution of the intensely poisonousprussic acid gas.

To proceed, then , to the actual formu lae for electrolytes,it will be noticed that these are given titles, such asNo. 1 copper, and so on. These tides are referredto in the tables of procedu re later in this chapter. Whereno individu al instructions for making up are given, the

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c. Excessive green pow der on anodes, or suspendedin the electrolyte indicates insu ffrcrent cyanide inthe solution.Solution may be stored in any suitable container, exceptgalvanised or other zinc ware.

NO . 1 NICKELoz./ gal.

Nickel Sulp hate, crystals 12Epsom Salts . . . . . . 4Boric Acid . . . . . . l*Ammonium Chloride . . . 1Glucose . . . . . . . . . 1Method: Boil one-eighth of wa ter with the boric acid, inenamelled pan , till dissolved; pou r into three-quartersremaind er of water, at about 80 Centigrad e. Dissolvenickel, epsom and amm onium salts, and finally the glucose.Allow to become quite cold, and d ecant, or strain th roughfine linen.The glucose is an interesting ingredient? in that it playsno par t in the chemical ch,anges wh ich occur du ringplating, but for some obscure reason, it considerablybrightens the dep osit, w ith consequent saving in the finalpolishing. It is know n as an add ition agent.Plating Temperature : Cold (15 to 20Centigrad e).Current Density : 4 amp. per square decimetre.Anodes : Nickel bars, three-quarter area ofcathodes.Time : To dep osit 0.0001 in., thir ty minutes.Solution may be stored in any convenient container, wh ichmust be quite cleanCorrect Appearance of Cathode during Plating: Silvery,tend ing to become du ller after th irty minu tes.Faults during Plating :

A. Dark streaks, usually vertical, indicate insu &ientagitation.B. Rough, grey patches mean excess current.

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ELECTROLYTES

c. Failure to deposit ind icates excess acid. CorrectionOf is is d ifficu lt, bu t try adding successive smallquantities of pu re borax, say 4 oz. at a time,dissolved in 5 ozs. wa ter. Stir well, and try a testpiece of plating after each add ition.

Amongst the numerou s electrolytes used in plating, thisis the only one wh ich has absolutely no cleaning prop erties,and , therefore, the need for scrupu lous cleanliness of thecathode, prior to immersion, is impera tive.NO. 2 NICKEL

(&pro-Nickel, or German Silver)oz./ gal.Nickel Sulph ate Crystal 4

Copp er Sulph ate Crystal 12So&urn Cyanide, 1300/ , 18Sodium Carbonate . . . 6

W AR .hlNG. During the preparation of all solutionsconta ining copper sulpha te and cyanide, poisonousCyanogen gas is evolv ed. This is a very dangerous gas, andall such preparations MUST be done outdoors or in anexceptionally w ell-ventilat ed place. O n no account shouldthese fumes be inhaled.Method : issolve the nickel and copp er salts in half thewater. Dissolve the cyanide in one-quarter of the water,and pour the former into the latter stirring constantly.Dissolve the sodium carbonate in the solution, and makeup to volume with w ater.Plating Temperature : Variable betw een 40 and 90Centigrade (see below).Current Density : 4 amp . per square decimetre.Anodes : Three copper, one nickel.Time : To deposit 0.0001 in., twenty-five minutes.Solution may be stored in any container, except zinc orgalvanised ware.

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ELECTRO-PLATINO FOR THE AMAT EUR

B. Powdery d eposits on anod es ind icate lack ofcyanide in solution.Temperature o/o Zinc o/o Copper Final Plate(Polished)

20C. 80 20 Blue-white30C. 70 30 White40C. 60 Pale Yellow50C. 48 E? Brass60C. 35 65 BronzeThe plate laid dow n at 20 Centigrad e looks almostidentical with chrome, bu t is, of course, softer, and tar-nishes more easily. All the above plates will take a high

polish very easily, and are excellent for protecting ironand steel, in which cases they may be plated direct.SILVER

(Pure Silver Plate)oz. / gal.Silver Nitrate . . . . . 4Sodium Cyan ide, 130%Sodium Carbonate . . tCaustic Soda .., . . . 1

Method : Dissolve the silver salt in one-quarter of thewater, and the cyanide in one-half. Pour the silver solu-tion into the cyanide solution w ith constant stirring. Dis-solve the sodium carbonate and the caustic soda, and makeup to volume with water.Plating Temperature : 30 to 35 Centigrade.Current Density : 4 amp. per square decimetre.Anodes : Silver sheet or foil, same area as

cathod es, or steel.Time : To d eposit 0.0001 in., fifteen minu tes.Correct Appearance of Cathode during Plating : Pure whiteor slightly yellowish, flat surface, resembling white velvet.64


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ELECTROLYTES

Fault s during Plat ing :A. Black or brow n patches on cathod e ind icate excess

current and / or insufficient agitation.B. Failure to deposit ind icates exhau stion of silverfrom electrolyte (this oniy occurs when u sing steelanodes).

It shou ld be explained at this stage, that the use of steelanod es causes the silver content of the solution to diminishdu ring p lating, as opp osed to the use of silver anodes inwhich case tire silver content of the electrolyte remainsconstant. The only reason that steel electrodes are sug-gested is that silver sheet or foil is not so readily obtainableas silver nitra te. If steel anodes are used , it will be neces-sary to replace the silver in the solution, from time to time,and this is don e by dissolving 3 ozs. of silver nitrate inabou t 10 ozs. water, and add ing to the solution with con-stant stirring. This shou ld be followed by the add ition of14 ozs. sodium cyanide, dissolved in a small amou nt ofwater. These additions to the solution can be mad e aboutfive times, after which the solution, on final exhau stionof silver, should be discarded and a fresh solution mad e.Solution should be stored in non-metallic container, orstove-enam elled bin, to avoid loss of silver by depositionon metal surface.

NO. 2 SILVERFormer editions of this book have given d etails of acombined silver and copper electrolyte un der the abovetitle, which w ill prod uce an effect var iable betw een silverand a rich golden colour.It has been deleted from the present ed ition because it

has been foun d th at, for amateur use, the conditions ofplating, together with solution comp osition, have proved tobe too critical. The au thor has carried ou t some excellentwor k with this bath, and little d ifficulty was experienced.How ever, it app ears from reports mad e from time to time

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ELECTRO-PLATIN G FOR THE AMAT RUR

by wou ld-be users of this electrolyte that it is somewh aterratic. The reasons are probably complex, bu t are mostlikely concerned with p ur ity of ingredients and watersup ply, together w ith the fact that electrolytically pu recopper mu st be used for anod es. For these reasons, there-fore, it has been d ecided to delete it, bu t if any read erwishes to experiment, he should consult either of the twoprevious editions.Chromium

This is dealt with as a separate subject in a later chap ter.Gold

It may h app en that the reader is interested in the gildingor regilding of small articles such as jewellery, watch-cases,etc., and in tha t case, the following description of a simplemethod of ca.*rying out su ch plating may be of value.For small articles, there are two comm only used method sof gold-plating. Firstly, there is the norm al m ethod wh erethe base is connected to the cathod e bar in a small tank,and p lating carried out in the usu al way, using an electro-lyte containing gold salts in solution, and either a gold orcarbon electrode at the anod e bar. The second m ethodconsists in making a cell , which is a typ e of batteryin wh ich the base to be plated constitutes one of the polesof the cell, and generation of current is automatic, andindep endent of outside supply. This latter method isprobably the simp ler, bu t as an electrode of fine gold(24 carat) is requ ired, it will pr obably be outside the scopeof the average amateur. For this reason, the present rem arkswill be confined to the first method.Preparation of the base

Such articles as are normally required to be gold platedare usua lly made of brass or bron ze, or occasionally, silver.These bases require no prep aration other th an an excep-tionally good polishing, followed by a thorou gh de-greasingas described in Chap ter 4. They are then slung on a suitable66


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ELECTROLYTES

wire and introdu ced to the plating tank. In the event ofsteel or iron articles being involved , it is stron gly recom-mend ed that th ey be first plated with either copper ornickel, as gold has very poor ad hesive properties wh enapp lied direct on ferrous bases. In all cases, the mostimpor tant thing is to obtain a highly p olished surfacecompletely free from grease, and , as stressed previously inthis book, bases prepared for the tank shou ld never befingered.The Tank

As only small articles are envisaged, the most suitabletank is an enamelled iron pan, and one holding a pintwill most likely be qu.ite big enough. Und er no circum-stances shou ld a pan of bare metal be used , as gold willdep osit on practically all metals ur ithout the passage ofcurrent, and the solution would thus be depleted of itsgold content. The reason for not making use of this auto-matic dep osition of gold in the actual plating is that it isnot sufficiently adh esive wh en so app lied, and , secondly,such a plate is limited to an extreme thinness, furtherdep osit of gold being prevented wh en once the article iscovered by a very thin layer.The ElectrolyteThe following is the formu la for one pint of solution.Dissolve 30 grains (apothecaries weight) of Gold Chloridein half a pint of lukewarm water. Dissolve one drachm(apoth .) of sodium cynanide (130%) in one ounce of water.Mix the two solutions. Dissolve half a drachm of sodiumcyanide and half a drachm of pu re caustic soda in oneounce of water and add to the gold solution. Make up tovolume with cold water. Store in glass container, in thedark.AnodeEither stainless steel, say equal to base area, or carbon.The latter is recomm ended , and may take the form of a

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ELECTRO-PLATING POR THE AMATEUR

rod of grap hite removed from the middle of a small torchcell. This shou ld be soaked in several changes of waterfor a few days to remove the Sal-amm oniac and zinc saltswith wh ich it will be saturated . The brass cap should notbe allowed in contact with the gold solution.Plating temperature

About 50 to 60 deg. Centigrade gives good results, butthe colour of the gold plate dep ends to some extent on thetemperature. Lower temperatures give a redd er colour,and h igher ones a yellowish tinge.Cumxt Density

This is in the region of + amp . per sq. dm . but as thebase will be considerably smaller than one sq. dm . thecorrect current is best found by trial. It is suggested thatsomewhere near th e correct current will be passed if thetransformer tapp ing is set at 4 volts. and the whole ofthe resistance is in circuit.Method

Having prep ared the base, and slung it from a thincopper wire, this is connected to the negative sup plyterminal. The anod e is connected to the positive, and thesolution placed in the pan and heated to the correcttemperature. Next, th e anode is lowered into the bath,followed by the base, wh ich is kept moving by hand forabout one minute. This will usu ally give a thick enoughplate, and th ickness can be estimated by ru bbing with arag soaked in liquid metal polish. If the gold plate resiststhis treatment for twen ty m5.nutes, one might consider thatit is thick enough to withstand a fair am oun t of ordinarywear and tear. Those rea d ers who are equipped with adelicate balance could, of course, obtain an accurate figureby weighing before and after p lating. On small articles,however, the weight of goid dep osited will be found to beso small as to be almost impossible to weigh .68


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ELECTROLYTESLECTROLYTES

When using an insoluble anode, as above, the gold ishen using an insoluble anode, as above, the gold isgrad ually rem oved from solution, and eventually no dep ositrad ually rem oved from solution, and eventually no dep ositwill be obtained. When this occurs, the old solution shouldill be obtained . When this occurs, the old solution shou ldbe discarded, and a fresh one prepared .e discarded, and a fresh one prepared .The following tables show the sequence of operationshe following tables show the sequence of operationsfor dep ositing the various plates on various bases, and it isor dep ositing the various plates on various bases, and it isassumed that the preparation of each base has been carriedssumed that the preparation of each base has been carriedout, as detailed in Table 3, Chapter 4.ut , as detailed in Table 3, Chapter 4.

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~~AT~N ~ ON WR~U~~F IRON AN D SIEIX, AND ON A~W~~~N IUM, ~~I~I.~~WINIRON DIP (Process 9, Chapter 4)

Final BlareRequired copper-Process I . . . As for Cast

Ii-00I ---Process 2 . . . I~~~

Process 3rocess 3 . . .. .

Process 4rocess 4 . . .. .


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TABLE fiI~~A~iN~ ON COPPER, BRASS, ~R~N~~ AND NICKEL

Final PlateRequired -_^I___PfQcess I . Final plate direce,required thickness to required thick-

Proeess 2 . . . Rinse, dry and---

Process 3 . . ._____~

Process 4 . . .

Process 5 . . .I


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ELECTRO-PLATI NG FOR THE AMAT EUR

to check up , by means of an ammeter, that the batteryitself is not taking an y large curren t. Charge rate, as shownby the ammeter, should not exceed one or two amps. Thecurrent in the battery circuit should again be checked wh enthe plating current is passing, to ensure th at the battery isnot discharging heavily. If it is, it should be tapped off ata lower voltage.

Accuracy is dem and ed at all stages of the process,including :A. Original compu tation of surface area.IL Making-up of the electrolyte.c. Temperatu re of the electrolyte du ring plating.D. Current-density.E. Preparation, size, and d isposition of the anod es.

SIZE OF JOB TO BE UNDERTAKENThe app aratu s described in the earlier chapters of this

book, has been d esigned with a view to prov iding sufficientpow er to plate bases up to 160 sq. in. total su rface area.This ap plies to the electrolytes so far described, but , owingto the peculiarly high currents demand ed by the chromebath, the size of base to be chromed, using the sameapp aratu s, will be limited to abou t 16 sq. in.FIG. 23a,

r- w POE.,-* NEG. Arrangement of smoothing con-

densers for Chromium plating.Connections are : Pos. to out -

PECT4.put terminal positive, Neg. toJh output terminal negative, Rect.+z -+

I-.!

100 + to positiv e output lead fromm d d r rectifier (R + on panel , seec&2*


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ELECTRO-PLAT ING FOR THE AMA TEUR

This solution i: corrosive and should n ot remain incontact with the skin. Cold ru nn ing water w ill remove it.In view of the very nar row margin of error allowed inmaking up this solution, the greatest care mu st be t,aken toensure accuracy. There is no par ticular d ifficulty in d is-

solving th e chrom ic acid, it being extremely soluble, andgoes qu ite easily in cold water. The sulph ur ic acid is add edlast, and the whole well stirred. The correct propor tionof sulphuric acid to chrom ic acid is very critical, andshou ld be exactly 1 : 100. In such small quantities, theexact weighing of the sulphu ric acid wou ld be d ifficult,and , therefore, the recipe for the solution has been w rittento show 10% acid being used. This acid is obtainablefrom any ph armacy, accur ately standard ised to 10% con-tent, and is know n as Dilute Sulphu ric Acid , B.P. Of th is,ten times more will be required than of the pu re con-centrated acid, and it is conveniently measured by volum e,instead of weight. If the r eader does not possess a measu ringglass, an ord inary med icine bottle m ay be used , each table-spoon m arking being half a fluid ounce. These bottle-grad uations are norm ally sufficiently accurate, bu t to makesure the amoun t measured should be checked in a secondmedicine bottle.

PREPARATION OF THE BASECopp er, brass, nickel an d iron can all be plated d irectin the chromium bath, and of these, the three formerusually are.As, how ever, chromium gives protection only againsttarnish, and not against rusting or corrosion, iron is, orshould be, provided with a good u nd ercoating of eithercopper or nickel before chr

electro plating for the amateur 1976

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