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Heat resistant glass is used in industrial, construction, and cooking applications to prevent thermal shock. Borosilicate glass has a lower coefficient of thermal expansion and is used for laboratory equipment, trays, and serving dishes. Tempered glass is stronger and used in cookware and fireplace screens. Glass ceramic is used for hobs, fireplace screens, and fire-resistant glass panels. Pyrex is a trademark of Corning, Inc.
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 could cause ordinary glass to shatter. Heat resistant glass is usually borosilicate glass, tempered soda lime glass or glass ceramic. Glassware is often referred to as Pyrex® by the general public, but this is properly a trademark of Corning®, Inc., a major manufacturer of glass and ceramic products.
When most materials are heated, they expand; once cooled, they contract. This property is known as thermal expansion, and the number that indicates how much a given material expands or contracts is called the coefficient of thermal expansion. Glass has a high coefficient of thermal expansion, which means 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 common glass is made by mixing silicon dioxide, lime, soda ash, and small amounts of other minerals. This is known as soda lime glass and is used for glass bottles and jars, window panes 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 coefficient of thermal expansion. It is used for heat resistant glass laboratory equipment, such as test tubes; for trays and serving dishes; and in industrial applications where high resistance to thermal shock is required, such as thermal insulation tiles. It is also used as a high-quality optical glass, for example in large telescopes, because it transmits light well and resists shape changes.
Corning® popularized the use of borosilicate glass in baked goods under its Pyrex® brand, but the majority of Pyrex® sold in the United States today is tempered soda-lime glass. Tempered glass has been heat treated during manufacturing, causing the inside of the glass to contract and increase surface tension, resulting in better stress balancing in the glass. Tempered glass is stronger than non-tempered glass and resists thermal shock better.
Heat resistant tempered glass is used in cookware and glass fireplace screens. The main disadvantage of using this type of glass is that its shape cannot be changed after it has cooled. Cutting or chipping tempered glass unbalances its internal stresses, making it break easily. When it breaks, it tends to break into small pieces rather than 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 wide range of possible coefficients of thermal expansion. Glass ceramic is mainly used for glass hobs and hobs, for fireplace screens and for fire resistant glass panels. In construction, a window or pane of glass described as “fire resistant” is usually soda-lime glass that has been glazed with glass ceramics or is made up of two panes and filled with a flame retardant material.
