Any noncrystalline solid; i.e. a solid in which the atoms are random and have no long-range ordered pattern. Glasses are often regarded as supercooled liquids. Characteristically they have no definite melting point, but soften over a range of temperatures.

The common glass used in windows, bottles, etc., is soda glass, which is made by heating a mixture of lime (calcium oxide), soda (sodium carbonate), and sand (silicon(IV) oxide). It is a form of calcium silicate. Borosilicate glasses (e.g. Pyrex) are made by incorporating some boron oxide, so that silicon atoms are replaced by boron atoms. They are tougher than soda glass and more resistant to temperature changes, hence their use in cooking utensils and laboratory apparatus. Glasses for special purposes (e.g. optical glass) have other elements added (e.g. barium, lead). It has proved very difficult to get a good quantitative theoretical description of glasses. One reason for this is that glasses are associated with broken ergodicity. Although glass does not have long-range order, it does have a considerable amount of short-range order due to chemical bonding. In so-called metallic glasses, atoms are close packed but (unlike crystals) with a certain amount of randomness. In glass with covalent bonds there are networks of atoms that are not densely packed. Glass has been investigated using various diffraction techniques and computer simulations. Zachariasen’s rules and topology have been used to discuss the structure of glass. See also spin glass.