Slab of yellow cast glas, 1968.
© Science Museum / Science & Society Picture Library
Cast glas made by Hartley Wood and Company. Glas is made by melting a mixture of various inorganic substances, principally purified sand (silica), and cooling the melt under controlled conditions to prevent crystallisation. The Romans were the first to cast glas for mirrors, and by 1688 it was cast by pouring molten glas onto an iron table covered with sand, rolling the glas and then polishing it on both sides.