Glass sheets have a wide application; they are used in windows of houses, trains, motorcars, ships and aeroplanes. The list of its products is, in fact, endless and includes water tumblers, decoration articles, various kinds of bottles, sunglasses, spectacles, flasks, laboratory items, light bulbs, fluorescent tubes and a large number of other items used in our daily life. There is hardly any field in which glass is not used in some form or the other. Do you know how this useful material is made? The art of making glass is very old. A blue glass rod made in 2,600 B.C. was found in Babylonia. It is evident from these findings that man knew the art of making glass some time around 2,600 B.C. Sand (silica), soda (sodium carbonate) and lime (calcium carbonate) are the main constituents used in the manufacture of glass. These materials are finely ground, mixed together and melted in a big furnace. When the mixture becomes homogeneous after melting, it is moulded in the form of sheets or rods. The solid material so formed is glass. There are many kinds of glass. To make different kinds of glass, some additives are mixed to the aforesaid mixture before melting it. In general, magnesium oxide, boric oxide, aluminium oxide, zinc oxide, lead oxide are the main additives used in glass manufacturing. For making coloured glass, oxides of copper, iron, selenium, chromium, cobalt, etc. are used. Cobalt salt is mixed to make blue glass, chromium or copper compounds impart green colour, whereas selenium salts makes glass red. There is also fibreglass, which is being used in research work. Today, scientists have developed a kind of glass, which changes its colour when illuminated. A special kind of glass is now in use for making bricks used in building houses. By mixing special materials to glass before melting, transparent substance which is as strong as steel has been developed. This type of steel is used in space ships. Coating the surface of borosilicate glasses with a metallic oxide makes it a conductor of electricity. These kinds of glasses are used in panels for industrial and house heating.
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