What is Heat-Resistant Glass?
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.
I have heard a lot of stories recently where heat resistant cookware shattered. Most of them seem to be the fault of the user. For example if they put an oven hot bowl into cold water, or if they try to use a Pyrex glass bowl which has a crack or chip in it. And the glass does just disintegrate into a bunch of small pieces, rather than exploding.
But I still find it a bit unnerving and I am very careful about using this kind of cookware in my oven.
@PBJ - I agree it would be better if most glass could be tempered, particularly around children. I can remember my sister running into a glass door as well when we were younger. We also managed to send plenty of balls into windows over the years which sent sharp shards of glass all over the carpet.
I don't think you should be overly worried about your children though, as long as you make them obey common sense rules like not running in the house. Tempered glass is heavy, so I think if it is used for larger panes of glass it is more likely to shatter. It's also more expensive, otherwise it would be more common I think.
Tempered glass is great stuff. Those little pebbles of glass you get when a car window breaks are completely harmless, as opposed to the long, sharp, shards you get when regular glass breaks.
My brother got cut up pretty bad when we were little. He was playing tag with a friend. When the friend ran out the door, a glass storm door, and slammed it behind him, my brother ran right through it. It was just one of those things that can happen to a child.
Now I have children of my own, and I see all the glass they could break, and it worries me. Not just windows, but drinking glasses, the glass in picture frames, glass is everywhere. And I can't be with my children all the time. Wouldn't it be better if all the household glass was tempered? Am I being over protective?
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