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Sheet of Glass
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If it were not for the rapid increase in viscosity, crystallisation would occur more and more quickly with fall in temperature. The increasing viscosity, however, makes it less and less easy for the constituents of the glass to assume their crystalline form. The result of these two opposing factors is that there is a defined temperature of maximum tendency to devitrify.
    Composition is an important factor in devitrification. In general the silica should not exceed 73.5 per cent or there is danger of silica crystallisation. The soda should not exceed 14.5 per cent or there is a danger of surface decomposition or weathering of the glass after manufacture; the lime may in some cases reach 15 per cent when the devitrification period is passed through rapidly, but in other cases has to be kept as low as 8.5 per cent by the introduction of a fourth ingredient, magnesia, which prevents the crystallisation of silicate of lime during the manufacturing process. Instances have occurred where sheets of glass, when unpacked at their destination, have been found stuck together in a solid mass, due to an abnormally high soda content.

R A W   M A T E R I A L S

Common to all different types of glass sheets there is the all important problem of the raw materials, which are checked daily by means of chemical and physical tests for their purity and suitability. The principal raw materials for these lime soda silicates are sand, soda ash and limestone. These materials, with the addition of perhaps arsenic and anthracite, alumina or magnesium carbonate, all in a finely divided state, are given a strongly intimate mixing, and are then ready for the melting process.