The Definition of Tempered Glass
Tempered glass differs from normal glass in the way it is made. The process strengthens the glass and changes the way in which it breaks. The resulting properties make the glass safer upon breaking, and therefore ideal for car windows or any glass object that needs to break in a safe manner. Glass can be tempered in different ways, and to varying strengths. It's important for anyone working with the material to know the difference.
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Construction
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In the process of annealing glass, glass is heated to a high temperature and allowed to cool slowly and evenly. This makes for a slightly stronger piece of glass because internal strain is minimized. Tempering, on the other hand, takes the annealing process one step further and deliberately creates internal strain. The heated annealed glass is instead cooled quickly under forced air. This shrinks the outside surface of the glass causing it to compress on itself, and at the same time causes stretching in the core of the glass where it cooled more slowly. Alternatively, molten salts can be used to chemically temper glass.
Advantages
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Tempered glass bend slightly further than annealed glass or non-annealed glass before breaking. It also tends to break into small pieces rather than large shards because of the differences is stress between the skin of the glass and its core. This makes it an ideal material for any glass that may break in a dangerous way such as car windows or storefront doors. Some cookware is make of tempered glass both because of its increased strength, and because of the way it breaks.
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Disadvantages
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The difficult part about working with tempered glass is it's fragility. Although it may be stronger overall, it is more brittle. This means that attempts to drill, grind, scratch, or bend the glass may cause the entire piece to shatter. Even a sharp rap from anything sufficiently hard or pointy can cause it to shatter. Because of this, all fabrication work needs to be down before the piece is tempered.
Uses
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Apart from stated uses such as automobiles and doors, tempered glass is frequently used in chemistry lab-ware. It is more resistant to heat than annealed glass, and it is scratch resistant. Both of these qualities make it a good choice for lab glass. Some touch displays, such as those on cell phones, use tempered glass for the same scratch resistance. Additionally, some restaurants use tempered or partially tempered glass.
History
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Although people have been tempering glass for quite some time (perhaps hundreds of years), the mechanism behind the tempering was not discovered until much more recently. Craftsmen were able to make tempered glass using a variety of techniques, but since the underlying mechanism was not understood, the effects were unpredictable.
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References
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- Photo Credit broken glass, image by Greg Pickens from Fotolia.com
