Most of the glass we see around us in our everyday lives in the form of bottles and jars, flat glass for windows or for drinking glasses is known as commercial glass or soda-lime glass, as soda ash is used in its manufacture.
The main constituent of practically all commercial glass is sand. Sand by itself can be fused to produce glass but the temperature at which this can be achieved is about 1700°C. Adding other minerals and chemicals to sand can considerably reduce the melting temperature.
The addition of sodium carbonate (Na2CO3), known as soda ash, to produce a mixture of 75% silica (SiO2) and 25% of sodium oxide (Na2O), will reduce the temperature of fusion to about 800°C. However, a glass of this composition is water-soluble and is known as water glass. In order to give the glass stability, other chemicals like calcium oxide (CaO) and magnesium oxide (MgO) are needed. These are obtained by adding limestone which results in a pure inert glass.
Commercial glass is normally colourless, allowing it to freely transmit light, which is what makes glass ideal for windows and many other uses. Additional chemicals have to be added to produce different colours of glass such as green, blue or brown glass.
Most commercial glasses have roughly similar chemical compositions of:
70% - 74% SiO2 (silica) 12% - 16% Na2O (sodium oxide) 5% - 11% CaO (calcium oxide) 1% - 3% MgO (magnesium oxide) 1% - 3% Al2O3 (aluminium oxide)
Flat glass is similar in composition to container glass except that it contains a higher proportion of magnesium oxide.
Within these limits the composition is varied to suit a particular product and production method. The raw materials are carefully weighed and thoroughly mixed, as consistency of composition is of utmost importance in making glass.
Nowadays recycled glass from bottle banks or kerbside collections, known as cullet, is used to make new glass. Using cullet has many environmental benefits, it preserves the countryside by reducing quarrying, and because cullet melts more easily, it saves energy and reduces emissions.
Almost any proportion of cullet can be added to the mix (known as batch), provided it is in the right condition, and green glass made from batch containing 85% to 90% of cullet is now common.
Although the recycled glass may come from manufacturers around the world, it can be used by any glassmaker, as container glass compositions are very similar. It is, however, important that glass colours are not mixed and that the cullet is free from impurities, especially metals and ceramics.
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