Silicate glass

From ArticleWorld

Silicate glass is a complex mixture of silicates like sodium and calcium along with an excess of silica. The common glass which is used for windowpanes, bottles, dishes and the like is produced by heating together sand, sodium carbonate or sulphate and calcium carbonate.

Manufacture of silicate glass

Silicate glass is manufactured by heating a mixture of sand and carbonates of sodium and calcium. The following chemical reactions take place: Na2CO3 + SiO2 → Na¬2SiO3 + CO2 (g) Na2SO4 + SiO2 → Na¬2SiO3 + SO3 (g) Ca2CO3 + SiO2 → Ca¬SiO3 + CO2 (g) After the bubbles of carbon dioxide and sulphur trioxide have been allowed to escape from the mixture, a clear viscous melt is left behind. This molten material is then poured into moulds or shaped using dies to produce glassware of desired shapes. Articles such as bottles, flasks and beakers are formed after lumps of molten glass are taken on a hollow tube and inserted into a mould. Compressed air, when blown into the mould can bring about the desired shape in the glass. High quality plate glass, also called float glass, is made after drawing glass from a furnace, preparing broad strips and rolling it to the desired thickness and finally grinding it flat and polishing it from both sides. Another technique could involve pouring the molten glass on a layer of very pure molten tin. The glass hardens on the perfectly smooth tin surface. The glass too is smooth because it floats on the tin eliminating the need for grinding and polishing; hence the name ‘float glass’. Glassware can be ‘annealed’ by heating it for a certain time and then allowing it to cool.

Forms and uses of glass

As mentioned above, glassware for household and laboratory purposes is very useful. Pyrex glass, used in chemical laboratories, is very suitable when sudden changes of temperature are involved. It has some of the silicon atoms replaced by boron atoms. A glass called flint glass, with a component of lead present in it, has a very high refractive index and high density; hence it is used in making lenses. Automobile windshields use safety glass, which consists of a thin layer of plastic onto which two sheets of float glass are attached on either side. This prevents glass pieces from flying if a car’s windshield is shattered. Silicate glasses are commonly used as insulators between the layers of semiconductor structures.