Heat-treated glass is a term used to describe glass that has been processed through a tempering furnace to alter its strength characteristics. The process is done in order to 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 and fully tempered. Fabrication requirements, tolerances, and testing procedures for heat-treated glass are defined in the ASTM International document C 1048.
Heat-Treated Glass Flatness
Due to the process of heat-treating glass, the original flatness of the annealed substrate is slightly modified. This inherent condition of heat-treated glass results in roller wave distortion and glass bow and warp.
Viracon's tolerance for roller wave is a maximum of 0.003" (0.076mm) from peak to valley in the center of lites, and a maximum of 0.008" (0.20mm) within 10.5" (267mm) of the leading or trailing edge. There is no industry standard for heat-treated glass roller wave, however a tolerance of 0.005" is frequently specified.
Viracon's tolerance for localized warp for rectangular glass is 1/32" (0.8mm) over any 12" (305mm), or half of the ASTM C 1048 Standard Specification for Heat-Treated Flat Glass standard of 1/16" (1.6mm) over any 12" (305mm) span.
Strain patterns are a characteristic of heat-treated glass. To learn more about this subject see Viracon's Tech Talk on "Quench Patterns".