Glass is mainly made from the chemical silica (silicon dioxide) which comes from sand. A very high temperature is needed to melt silica, so soda (sodium carbonate) is added to lower the melting point. Silica and soda produce a glass which dissolves in water. This is not suitable for making windows, so limestone (calcium carbonate) is added to make normal, strong glass.
The ingredients can be varied to make special kinds of glass. Adding lead oxide instead of most of the limestone gives a heavy glass which is used to make wine glasses.
Making glass
The raw materials are mixed together in the right quantities and melted in a huge furnace. The size of the furnace depends on how much glass is to be made. A typical furnace for flat glass may hold 2,000 tonnes of molten glass. Usually some waste glass (cullet) of the same colour and type is added.
Coloured glass is made by adding different metal compounds. For example, copper oxide produces blue glass while chromium compounds give green or yellow glass.
The silica found in sand is the main ingredient used to make glass.
Limestone is added to silica and soda to make glass stronger.