Glass forms when melted solid materials are cooled quickly, so that they do not produce crystals. There are many types of glass, but the transparent glass that is most familiar to us is a very useful material. It is resistant to most corrosive substances, and it is a very good electrical insulator and heat insulator. Glass is stronger than most metals, but because it is brittle it shatters easily.
The most common forms of glass are made from a melted mixture of silica (sand), sodium oxide and calcium oxide. Lead crystal is a form of glass in which the calcium oxide is replaced by lead oxide. The most heat-resistant glass can be made from pure silica.
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