How is glass made?

Sand, soda ash, limestone, and borax are raw materials for glassmaking. These materials all are dry powders which look much alike, but are capable of producing greatly different results. They come to the glass plant in railroad cars and are stored in large silos. After they are carefully weighed and mechanically mixed in the proportions, the glassmaker adds cullet. Cullet is either recycled glass or waste glass from a previous melt of the same kind of glass. The addition of cullet reduces the amount of heat needed to melt the new batch. After mixing, the batch goes to the melting units in batch cars, in hoppers, or on conveyor belts. Small quantities of optical glass, art glass and speciality glass are made in refractory pots.
Larger quantities of glass are made in furnaces. The raw materials are fed into the loading end as rapidly as molten glass is removed from the working end. There are four main methods of shaping glass: blowing, pressing, drawing, and casting. After the shaping process, annealing is used to restore the strength of the glass. Tempering and other finishing techniques may also be used to further strengthen the glass.