I think it’s very important to know what type of glass you want or possibly need (depending on the situation). That is what I found this brief article reviewing the characteristics of tempered safety glass.

Tempered  glass is physically and thermally stronger than normal glass. The greater contraction of the inner layer during manufacturing induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. For glass to be considered toughened, this compressive stress on the surface of the glass should be a minimum of 69 megapascals (10,000 psi). For it to be considered safety glass, the surface compressive stress should exceed 100 megapascals (15,000 psi). As a result of the increased surface stress, if the glass is ever broken it only breaks into small circular pieces as opposed tolarge jjagged shards. This characteristic makes tempered glass safe for high-pressure and explosion proof applications.

It is this compressive stress that gives the toughened glass increased strength. This is because annealed glass, which has almost no internal stress, usually forms microscopic crackson the surface, and any applied tension gets magnified at the surface, reducing the applied tension needed to propagate the crack. Once it starts propagating, tension gets magnified even more easily, causing it to propagate at the speed of sound in the material. Consequently, annealed glass is fragile and breaks into irregular and large sharp pieces. Any cutting or grinding must be done prior to tempering. Cutting, grinding, and sharp impacts after tempering will cause the glass to fracture.

The strain pattern resulting from tempering can be observed with polarized light or by using a pair of polarizing sun glasses.

All credit due to Wikipedia for information regarding tempered safety glass.

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