GLASS, ITS TYPESLECTURE # 6

Glass and its brief history Glass is a blend of metallic oxides, primarily Silica.

Silica does not crystallize when cooled from liquid to solid and is non crystalline and amorphous. That gives rise to the transparency of glass.

Early glass was made from Sand, Soda Ash, & Lime that was used to glaze stone beads 5000 years ago by Egyptians. Later it was used to make glass beads in 2500 BC and then into long hollow vessels by 1500 BC.

For many centuries it has been worked out by drawing molten material from furnace and than rolled out pressed into appropriate moulds which were later fashioned by cutting and grinding.

In Medieval Era Potash was added instead of soda ash with increase quantity of Lime that give birth to the less durable glass that is the cause of deterioration of some church glass in that period.

The various colors in glass were obtained by adding different metal compound to the melt. Cobalt was added to give blue color. Copper produced Blue or Red. Chromium produced green.

In fifteenth century white opaque glass was produced by addition of tin or arsenic.

Clear Glass could only be obtained by adding manganese or antimony as a decolorizer to remove green colorization due to presence of iron impurities within the sand

With the advent of Fully glazed facades by the late 20th century ; the construction industry has become major consumer of glass and acted as a proactive force in the development of new products.

COMPOSITION: Modern Glass is manufactured from Sand (Silica) , Soda Ash (Sodium Carbonate) & Lime stone (Calcium Carbonate) along with other additives..

Manufacture of Float GlassFLOAT GLASS: The term "float" refers both to a type of glass and to the process by which it is made.

Float glass is the basic glass from which almost all the flat glass products are derived. It may be clear or colored, and is produced in large sheets that are normally processed into secondary products.

The float process involves literally floating the molten glass on a bath of liquid tin, producing a perfectly flat surface on both sides.

The raw materials for float glass are 73% sand (silicon dioxide), 15% soda (sodium carbonate), 10% limestone (calcium carbonate) and 2% various additives.

MANUFACTURING PROCESS

A furnace produces a continuous supply of molten glass at approximately 1100 C.

It flows across the surface of a large shallow bath of molten tin in presence of hydrogen and nitrogen which prevents oxidation of molten metal.

The glass ribbon moves across the molten metal initially at high temperature for irregularities on both surfaces to become evened out leaving a flat and parallel ribbon of glass. It solidifies at 600C.

Any further residual stresses are removed as glass passes through the annealing lehr leaving a fire polished material

The glass is washed then.

Thickness of glass is controlled by the speed it it is drawn from the molten bath

The float glass made for construction industry is made within the thickness range of 2-25mm.

Non-Sheet ProductsNON SHEET PRODUCTS

Non Sheet Products are those products that are made directly without the formation of sheet. It includes the following products.

CAST GLASS: Glass may be cast and pressed in to shapes for glass blocks and extrude sections

Glass Blocks: Glass blocks for non-load bearing and partition walls are manufactured by casting two half blocks at 1050C And joining them together at 800 C and followed by annealing. The Standard blocks are 115, 190, 240 and 300 mm Square with 80 and 100 mm thickness. These blocks can be joined with mortar that gives fire resistance of 60 mins

PROFILED TROUGH SECTIONS:

Profiled trough sections are available in different sizes that can be used horizontally or vertically with single or double glazing

Silicon sealant is used to conceal the joints

Sheet Products

The standard thickness for float glass are 3, 4, 5, 6, 8, 10, 12, 15, 19 and 25 mm And the sheet sizes available in Pakistan are 4’-0” X 8’-0” and 5’-0”X 10’-0”

Although the maximum sheet size available is 3 X 12 m in other European countries

Annealed glass: Annealed glass is the basic flat glass product that is the first result of the float process. It is common glass that tends to break into large, jagged shards. It is used in some end products and often in double-glazed windows. It is also the starting material used to produce more advanced products through further processing such as laminating, toughening, coating,

Toughened glass/Tempered Glass: Toughened glass is treated to be far more resistant to breakage than simple annealed glass and to break in a more predictable way when it does break, thus providing a major safety advantage in almost all of its applications.

Toughened glass is made from annealed glass treated with a thermal tempering process. A sheet of annealed glass is heated to above its "annealing point" of 600°C; its surfaces are then rapidly cooled while the inner portion of the glass remains hotter. The different cooling rates between the surface and the inside of the glass produces different physical properties, resulting in compressive stresses in the surface balanced by tensile stresses in the body of the glass.These counteracting stresses give toughened glass its increased mechanical resistance to breakage, and are also, when it does break, what cause it to produce small, regular, typically square fragments rather than long, dangerous shards that are far more likely to lead to injuries. Toughened glass also has an increased resistance to breakage as a result of stresses caused by different temperatures within a pane.

Toughened glass has extremely broad applications in products for both buildings and, automobiles and transport, as well as in other areas. Car windshields and windows, glass portions of building facades, glass sliding doors and partitions in houses and offices, glass furniture such as table tops, and many other products typically use toughened glass. Products made from toughened glass often also incorporate other technologies, especially in the building and automotive and transport sectors.

Laminated glass: Laminated glass is made of two or more layers of glass with one or more "interlayers" of polymeric material bonded between the glass layers.

Laminated glass is produced using one of two methods:

1. Poly Vinyl Butyl (PVB) laminated glass is produced using heat and pressure to sandwich a thin layer of PVB between layers of glass. On occasion, other polymers such as Ethyl Vinyl Acetate (EVA) or Polyurethane (PU) are used. This is the most common method.

2. For special applications, Cast in Place (CIP) laminated glass is made by pouring a resin into the space between two sheets of glass that are held parallel and very close to each other.

Laminated glass offers many advantages. Safety and security are the best known of these, so rather than shattering on impact, laminated glass is held together by the interlayer. This reduces the safety hazard associated with shattered glass fragments, as well as, to some degree, the security risks associated with easy penetration. But the interlayer also provides a way to apply several other technologies and benefits, such as coloring, sound dampening, resistance to fire, ultraviolet filtering and other technologies that can be embedded in or with the interlayer.

Mirrored glass: To produce mirrored glass, a metal coating is applied to one side of the glass. The coating is generally made of silver, aluminum, gold or chrome. For simple mirrored glass, a fully reflective metal coating is applied and then sealed with a protective layer. To produce "one-way" mirrors, a much thinner metal coating is used, with no additional sealing or otherwise opaque layer.Mirrored glass is gaining a more prominent place in architecture, for important functional reasons as well as for the aesthetic effect.

Patterned Glass: Patterned glass is flat glass whose surfaces display a regular pattern. The most common method for producing patterned glass is to pass heated glass (usually just after it exits the furnace where it is made) between rollers whose surfaces contain the negative relief of the desired pattern(s).Patterned glass is mostly used in internal decoration and internal architecture. Today, it is typically used for functional reasons, where light but not transparency is desired, and the patterns are accordingly subtle. However, it has also at times been fashionable as a design feature in itself, in such cases often displaying more prominent patterns.

Decorative Etched/ Sand Blasted Glass: Acid Etched glass is available in all thicknesses of glass these glasses have low obscuration factor and should not be used in area of high humidity, as condensation or water causes temporary loss of the pattern. Etched glass should be handled with care at site as grease, oil and finger marks are difficult to remove completely. Etched glass may be toughened and laminated.

Similar visual effects can be achieved by sand blasting techniques although the surface finish is less smooth. Pattern may be clear on a frosted background or the reverse depending on the aesthetic effect and level of privacy required

Extra clear glass: Standard float glass is slightly green due to the presence of iron oxide impurities within the key raw material sand. However, clear white glass can be produced, at a greater cost by using purified ingredients. The light transmittance of clear white glass is 2% greater than standard float glass. Clear White glass is colorless at edges.

Structural Glass

Glass columns are frequently used as fins to restrain excessive deflection caused by wind and other lateral loads to glass facades.

The fixing between the glazing and columns is done through SS Clamps bolted through performed holes before applying any coating or toughening, although silicone adhesive can also be used. Silicon adhesive is only structural adhesive that is suitable for glazing units.

Typically a storey height fins are used that are connect with the floor through aluminum/ SS shoe.

Toughened glass is strong in compression therefore an appropriate material for load bearing columns and walls

Cla

mp

s fo

r h

old

ing

gla

ss