Composition Soda lime silicate glass

Modern glass is manufactured from sand (silica), soda ash (sodium carbonate) and limestone (calcium carbonate), with small additions of salt-cake (calcium sulfate) and dolomite (magnesian limestone). This gives a final composition of typically 70-4% silica, 12-16% sodium oxide, 5-12% calcium oxide, 2-5% magnesium oxide with small quantities of aluminium, iron and potassium oxides. The addition of 25% broken glass or cullet to the furnace mix accelerates the melting process and recycles the production waste. Most raw materials are available within the UK, although some dolomite is imported. The production process is relatively energy intensive at 15 000 kWh/m3 (c.f. concrete - 625 kWh/m3), but the environmental pay-back arises from its appropriate use in energy-conscious design. Soda lime silicate glass may be chemically strengthened by an ion exchange process which replaces small surface ions by larger ones, thus putting the surfaces and edges into compression.

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