Heat-resistant glass is glass that is more resistant to thermal shock than ordinary glass. It is therefore suitable for industrial, construction, and cooking applications where changes in temperature would likely cause ordinary glass to shatter. Heat-resistant glass is usually borosilicate glass, tempered soda-lime glass, or glass-ceramic. Glass bakeware is often referred to by the general public as Pyrex®, but this is properly a trademark of Corning®, Inc., a major manufacturer of glass and ceramic products.
When most materials are heated, they expand; when cooled, they contract. This property is known as thermal expansion, and the number which designates how much a given material expands or contracts is called its thermal expansion coefficient. Glass has a high thermal expansion coefficient, which means that it expands very rapidly when heated. This rapid expansion can cause the glass to shatter, a consequence known as thermal shock. Glass can be made to resist thermal shock by changing its chemical composition, method of manufacture, or both.
Most ordinary glass is made by mixing silicon dioxide, lime, sodium carbonate, and small amounts of other minerals. This is known as soda-lime glass and is used for glass bottles and jars, windowpanes, and other common applications. In the late 19th century, German chemist Otto Schott invented borosilicate glass by adding boron to the mix.
Borosilicate glass is lighter and stronger than soda-lime glass, has a higher melting point, and has a much lower thermal expansion coefficient. It is used for heat-resistant glass laboratory equipment, such as test tubes; for bakeware and serving dishes; and in industrial applications where high resistance to thermal shock is necessary, such as thermal insulation tiles. It is also used as a high-quality optical glass — in large telescopes, for example — because it transmits light well and resists changing shape.
Corning® popularized the use of borosilicate glass in bakeware under its Pyrex® brand name, but most Pyrex® sold in the United States today is tempered soda-lime glass. Tempered glass has been heat-treated during manufacturing, causing the internal portion of the glass to contract and surface tension to increase, resulting in a better balancing of stresses in the glass. Tempered glass is stronger than untempered glass, and better resists thermal shock.
Heat-resistant tempered glass is used in cookware and in glass fireplace screens. The main disadvantage to using this type of glass is that its shape cannot be modified after cooling. Cutting or chipping tempered glass unbalances its internal stresses, making it break easily. When it does break, it tends to break into tiny pieces rather than into large, sharp fragments. For this reason it is often used in automobile windshields.
Glass-ceramic is glass that has been heat-treated until crystals begin to form in the glass. By varying the amount of crystal, it is possible to create heat-resistant glass with a broad range of possible thermal expansion coefficients. Glass-ceramic is mostly used for glass stove tops and cooking surfaces, for fireplace screens, and for fire-resistant glass panels. In construction, a window or glass panel that is described as "fire-resistant" is usually soda-lime glass that has been glazed with glass-ceramic, or is constructed of two panes and filled with a fire-retardant material.