Silica glass: It is produced by heating pure sand to its melting point. Because of absence of refining agents, it is extremely difficult to get rid of all air-bubbles. Moreover, owing to the high viscosity of this glass at its working temperature, shaping is rather difficult. The final product is translucent its softening temperature is about 16500C. Its thermal expansion is lowest. If silica glass is heated for long periods, above its melting points, it finally becomes transparent and is then known as "clear silica glass". Glass of considerable light transmission properties, e.g. 1m thick of this material allow no less than 93% of light to pass. It is used mainly for chemical plants, chemical laboratory wares, electrical insulating materials in electrical heaters, furnaces etc.