HOW IS GLASS MADE?
Glass is made by mixing raw materials such as sand, limestone, soda, dolomite, felspar and saltcake then heating them together at a temperature of over 1500oC. When the materials are melted, they form a viscous liquid called the quiescent melt. As this melt cools, the viscosity increases until the glass becomes hard. If the liquid cools too quickly, the glass does not have time to release stresses. This process is done by heat treatment and is called annealing.
Silica sand is the main raw material in glass, but very high temperatures are needed to melt it. By adding soda ash, a fluxing agent, the glass can be melted at a lower temperature, but the result is water glass which is soluble in water. Limestone, a stabiliser, is thus added to the silica and soda ash. Cullet (scrap glass) is also added to assist the melting process.