8.2 - Composition of Glass and Its Uses

Formation and Basic Properties of Glass

Different types of glass are produced by heating silica, $Si O_{2}$ , with selected chemicals.
All glass is generally:
- hard but brittle;
- chemically inert;
- transparent and waterproof;
- an electrical insulator and a heat insulator.

Type and composition	Properties and uses
Fused silica glass: silica (silicon dioxide), $Si O_{2}$ , only	Properties: Silica melts at about $1800^{\circ} C$ , so the glass has a high melting point; expands and contracts very little during large temperature changes. Uses: Telescope lenses.
Soda-lime glass: silica, $Si O_{2}$ ; soda (sodium carbonate), $N a_{2} C O_{3}$ ; limestone (calcium carbonate), $CaC O_{3}$	Properties: Soda lowers the melting point of silica to about $1000^{\circ} C$ ; easy to mould; cannot withstand high temperatures and cracks easily under sudden temperature change. Uses: Bottles, jugs and other glass containers.
Borosilicate glass: silica, $Si O_{2}$ ; soda (sodium carbonate), $N a_{2} C O_{3}$ ; limestone (calcium carbonate), $CaC O_{3}$ ; boron oxide, $B_{2} O_{3}$ ; aluminium oxide, $A l_{2} O_{3}$	Properties: Heat-resistant; low expansion coefficient; does not crack easily under thermal stress and can be heated immediately after removal from a refrigerator. Uses: Laboratory beakers and flasks.
Lead crystal glass: silica, $Si O_{2}$ ; soda (sodium carbonate), $N a_{2} C O_{3}$ ; lead(II) oxide, $PbO$	Properties: Lead replaces calcium, making the glass softer, denser and heavier; high refractive index. Uses: Prisms.