Constituents of Glass

Glass

Glass is an amorphous substance having homogeneous texture. It is a hard, brittle, transparent or translucent material. It is the most common material glazed into frames for doors, windows and curtain walls. The most common types used in building construction are sheet, plate, laminated, insulating, tempered, wired and patterned glass. Most ordinary colourless glasses are alkali-lime silicate and alkali-lead silicate with tensile and compressive strengths of about 30-60 N/mm² and 700-1000N/mm², respectively and modulus of elasticity in the range 0.45 × 10 ⁵ to 0.8 × 10 ⁵ N/mm². The strength is very much afftected by internal defects, cords and foreign intrusions. The main shortcoming of glass is its brittleness which depends on a number of factors, the chief one being the ratio of the modulus of elasticity of the material to its tensile strength.

Constituents

The raw materials used in manufacturing glass are sand, lime (chalks) and soda or potash which are fused over 1000 o C. Oxides of iron, lead and borax are added to modify hardness, brilliance and colour. The functions of the various ingredients are as follows.

Silica is used in the form of pure quartz, crushed sandstone and pulverised flint; should be free from iron contents for best quality glass. Since it melts at very high temperatures (1710 o C) carbonates of sodium or potassium are added to lower down the fusing temperature to about 800 o C. These also make liquid silica more viscous and workable.

Lime is used in the form of limestone, chalk or pure marble and sometimes marl. The addition of lime makes the glass fluid and suitable for blowing, drawing, rolling, pressing or spinning. It also imparts durability and toughness to glass. Excess of lime makes the molten mass too thin for fabrication.

Soda  acts as an accelerator for the fusion of glass and an excess of it is harmful.

Potash  renders glass infusible and makes glass fire resistant.

Lead Oxide imparts colour, brightness and shine. When 15-30% of it added to substitute lime it lowers the melting point, imparts good workability, while its transparency is lost with the glass becoming brittle and crystalline.

Cullets are broken glasses added to act as a flux to prevent loss of alkali by volatisation during the process of forming glass and also to lower the fusion temperature. However, flux may reduce the resistance of glass to chemical attack, render it water-soluble or make it subject to partial or complete devitrification (crystallisation) on cooling. These crystalline areas are extremely weak and brittle. Stabilizers are added to overcome these defects.

Titanic acid, oxides of Nickel and Cobalt are used for chromatic neutralisation.

Note: Iron is not desirable as a constituent. However, when present it imparts a bottle

green colour to the glass. To overcome this manganese dioxide known as glass maker's soap is added which washes the liquid glass and removes the colour.