Tempered glass has a minimum surface compression of 10,000 pounds-per-square-inch (psi) and minimum edge compression of 9,700 psi, according to ASTM C1048. That makes it about four times stronger than annealed glass. Heat-strengthened glass has surface compression of 3,500 to 7,500 psi, about twice as strong as annealed glass, with no edge compression standard.
Ironically, the rapid temperature change that gives tempered glass its compression strength may also cause it to shatter, seemingly without warning. On rare occasions, tiny inclusions, including nickel-sulfide, may be present in glass, which can expand during heat treatment, then stop when the glass is cooled and resume growth when the glass is exposed to high in-service temperatures (such as on the sunny exposure of a building). This “phase change” can cause tempered glass to shatter. Heat-strengthened glass is cooled more slowly; consequently, inclusions do not experience a phase change, which essentially eliminates the possibility of spontaneous glass failure