Clear glass starts out annealed and then can then be tempered to increase its strength and safety characteristics.
Etched glass at Govinda's Restauran...
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Etched glass at Govinda's Restaurant, TucsonGlass shelves for normal residential applications do not need to be tempered (but see exceptions below). In commercial applications (retail stores, restaurants, beauty salons, etc, tempering is highly recommended.
Most architectural glass is manufactured by melting silica sand, soda ash, dolomite, limestone, and sodium sulfate to 1480 degrees C, and then floating it on a pool of molten tin. (this is why it is referred to as “FLOAT” glass and not the old term “PLATE“). The glass passes, on a conveyor, through an annealing lehr – a furnace that controls the cooling process. The flat, cooled, solidified material at the end of the process is annealed glass. This is cut into sheets and shipped to other factories where secondary processes, like polishing, drilling, laminating and tempering, are applied.
How does annealed glass become tempered?After the glass is cut to size it is passed, on rollers, through a tempering oven which heats it to a temperature of 620 degrees C.
Note that this diagram above is a repeating animated gif – the glass only goes through the tempering process once!
In the tempering oven, the hot glass is quickly “quenched” by high-pressure air from an array of nozzles. Quenching cools the outer surfaces of the glass much more quickly than the center. As the center of the glass cools, it tries to pull back from the rigid outer surfaces. As a result, the center remains in tension, and the outer surfaces go into compression, giving tempered glass its strength.
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Comparison of key features of annealed and tempered glassSurface compression about 2500 psi
Surface compression > 10,000 psi
Will break when hit by a hammer
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