The major ingredient of glass is silica, in the form of sand. Lime, and soda ash are also added, the mixture is heated until the sand melts to a liquid form at around 1700°C. Impurities such as traces of iron produce a greenish glass, other minerals are added to produce different colours – gold for a red colour and cobalt for a blue.
Warm glass or kiln-formed glass is the working of glass by heating it in a kiln. The processes used depend on the temperature reached and range from fusing and slumping to casting.
At its most basic fused glass is where individual pieces glass are cut, placed in the kiln and fired to melt the glass together. The kiln is fired in carefully controlled stages up to a top temperature of around 850º C. It is held at this temperature to allow the glass to melt together before the temperature is reduced, again in a controlled manner, to avoid problems such as devitrification and making sure the stresses are removed from the glass. Different processes require different timings and temperatures.
Full fuse – layers of glass are melted together to become a single smooth piece.
Slump – allows glass to relax into a former to produce dishes
Drape – allows glass to relax over a former to produce pieces
Tack – creates semi rounded textured piece.