13. GLASS AND GLASS PRODUCTS

13. GLASS AND GLASS PRODUCTS

Glass products are enormously varied. They include windows, bottles, mirrors, optical instruments, chemical apparatus, building blocks, pipes, doors, cloth, thermal and electrical insulation, electrical condensers, boats, automobile and aircraft bodies, and filters.

Chemically, most glasses are silicates. Silica by itself makes a good glass (fused silica), but its high melting point (1,723 ®C) (degrees Centigrade) and its high viscosity in the liquid state make it difficult to melt and work. Fused silica products are used only when their special properties (low thermal expansion, light transmission) are essential.

Lime, CaO, is added to the glass, usually in the form of limestone, CaCO₃, to form the basic soda-lime-silica glass composition which is used for the bulk of common glass articles, such as bottles and window glass.