Heat-treated glass is a term used to describe glass that has been processed through a tempering furnace to alter its strength characteristics, provide greater resistance to thermal and mechanical stresses and achieve specific break patterns for safety glazing applications as compared to annealed glass.
The process of heat-treating glass is taking annealed glass, cutting it to its desired size, transferring the glass to a furnace and heating it to approximately 1,150° F. Once at this temperature, the glass exits the furnace and is then rapidly cooled, or quenched. Air is blown onto the glass surface on both sides simultaneously. This cooling process creates a state of high compression at the glass surfaces while the central core of the glass is in a compensating tension. The only physical characteristics of the glass that change are the improved strength and resistance to thermal stress and shock.
There are two kinds of heat-treated glass, heat-strengthened (HS) and fully tempered (FT). Fabrication requirements, tolerances, and testing procedures for heat-treated glass are defined in ASTM C1048 Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass.