Glass is formed when certain substances are cooled rapidly and do not crystallize—that is, their atoms do not arrange themselves in the repeating, orderly pattern characteristic of most solids. Instead, the atoms become fixed in a disorganized pattern characteristic of the atoms of a liquid. For this reason, scientists often refer to glass as a liquid—a liquid with a very high viscosity (resistance to flowing). The viscosity of glass is so great that it is a rigid material.

The most common substance that can be cooled from the molten state without forming crystals is silica (silicon dioxide), the chief constituent of sand and sandstone. However, pure silica is not used for commercial glass products for two reasons: (1) it must be heated to a high temperature in order to melt, and (2) when molten it has a very high viscosity and is difficult to form.

Most glass is soda-lime glass. It is made from silica combined with soda ash (sodium carbonate) and lime. The soda ash lowers the temperature at which the mixture melts and reduces the mixture's viscosity; the lime makes the glass insoluble. (Silica and soda ash alone yield a type of glass called water glass, or sodium silicate, which will dissolve in water.)

Two other important types of glass are lead glass and borosilicate glass. Lead glass is made chiefly from silica, lead oxide, and soda ash or potash (potassium carbonate); borosilicate glass, from silica, boric oxide, and soda ash or potash.

Small amounts of arsenic oxide, antimony oxide, or other compounds are usually added to glass as fining agents, substances that help eliminate the gas bubbles formed during the melting process. Many other ingredients may be added to these basic materials to give the glass specific colors or other properties. For example, iron oxides can be added to give glass a green color.