Heat-strengthened glass is produced by a process similar to that for tempered safety glass. It is, however, cooled more slowly, which reduce the pressure on the surface, which results in both lower bending strength and a different fracture pattern than that of tempered safety glass. The fracture pattern of heat-strengthened glass resembles that of untempered glass: a few radial cracks extend from the center of the fracture, and accordingly the fragments are large. Because the risk of serious injuries from cuts is larger than with tempered safety glass, heat-strengthened glass may not be rated safety glass.
Unlike tempered safety glass, heat-strengthened glass is not subjest to spontaneous breakage from inclusions of nickel sulfide crystals. In exceptional cases, it is possible to treat (grind) the edge of heat-strengthened glass, but normally this should be done before tempering.