Definition
A hard transparent material made
by heating calcium oxide (lime), sodium
carbonate, and sand (silicon(IV) oxide).
This produces a calcium silicate – the normal
type of glass, called soda glass. Special
types of glass can be obtained by incorporating
boron oxide in the glass (borosilicate
glass, used for laboratory apparatus)
or by including metals other than calcium,
e.g. lead or barium.
Glass is an amorphous substance, in the
sense that there is no long-range ordering
of the atoms on a lattice. It can be regarded
as a supercooled liquid, which has not crystallized.
Solids with similar noncrystalline
structures are also called glasses.
Preparation
The majority of industrial glass is produced by continuous melting processes, while batchtype processes are restricted to customized formulations for special purposes.
Large-scale production of industrial glasses utilizes huge melting crucibles with a rectangular shape called glass tanks that are heated from the bottom and sidewalls by natural gas
or oil burners; sometimes auxiliary electric heaters immersed in the melt (i.e., booster electrodes) are used to provide additional heat. The temperature of the melt can be as high as
1660°C to ensure the complete melting of alumina-rich raw materials (Ca-feldspars); the specific energy consumption is about 2.8 kWh/kg of glass.
Commercial glass tanks can hold up to
1200 tonnes of molten glass. The thick bottom and sidewalls are built with refractory materials, usually mullite bricks, while electrofused alumina-silica-zirconia bricks are used for the
inner layer, which is in direct contact with the melt. The vault or cupola is usually made of
silica bricks.
