Toughened or tempered glass was first developed in 1938.
Glass is toughened in a thermal tempering process in which the glass is heated to a temperature of about 680° C, then rapidly cooled with drafts of cold air. The process alters the tensile balance within the glass, and induces constant compressive stress in the body of the glass as well as on the surface. This increases the strength of the glass, as well as its pressure resistance. Thermally tempered glass is about five times stronger than non-tempered glass. If it is broken, it crumbles into small chunks without sharp or jagged edges. Tempered glass is also very heat resistant, tolerating temperatures of up to 275°C without softening.
