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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 (HS) and fully tempered (FT). Fabrication requirements, tolerances, and testing procedures for heat-treated glass are defined in the ASTM International document C 1048.
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".
Raw glass that has not been heat treated is annealed glass. In a specification, the designation for annealed glass is AN.
Heat Strengthened (HS)
Heat-strengthened glass is twice as strong as annealed glass of the same thickness, size and type. If broken, heat-strengthened glass will break into large shards similar to annealed glass. Because of this, the tendency for the glass to vacate the opening is reduced.
The surface compression of heat-strengthened glass with thicknesses of 1/4" (6mm) and less is 4,000 - 7,000 psi. Surface compression for 5/16" (8mm) and 3/8" (10mm) heat-strengthened glass is 5,000 - 8,000 psi. (Because of reader repeatability and instrument tolerances, Viracon's tolerance for heat-strengthened glass surface compression is +/- 1,000 psi.)
While improving the strength and resistance to thermal shock and stress, heat-strengthened glass does not meet safety glazing requirements as outlined by the American National Standards Institute (ANSI) Z97.1 or the federal safety standard Consumer Products Safety Commission (CPSC) 16 CFR 1201, and therefore should not be used in these situations. In a specification, the designation for heat strengthened glass is HS.
Fully Tempered (FT)
Glass with fully tempered surfaces is typically four times stronger than annealed glass and two times as strong as heat-strengthened glass of the same thickness, size and type. In the event that fully tempered glass is broken, it will break into fairly small pieces, reducing the chance for injury. In doing so, the small glass shards make it more likely that the glass will become separated from the opening. The minimum surface compression for fully-tempered glass is 10,000 psi. In addition, it complies with the safety glazing requirements as outlined by the American National Standards Institute (ANSI) Z97.1 and the federal safety standard Consumer Products Safety Commission (CPSC) 16 CFR 1201.
In a specification, the designation for fully tempered glass is commonly abbreviated as FT.
Fully tempered glass may break without warning due to the expansion of nickel sulfide inclusions (NiS) present within float glass. To avoid the risk of spontaneous breakage in fully tempered glass, a common practice is to avoid the use of tempered glass whenever possible.
Although the incidence of tempered glass breakage due to these inclusions is rare, greater publicity of their occurrence has resulted in an increased awareness of this phenomenon. In fact, limiting the use of tempered glass in commercial building applications has become the recommendation of a number of glass suppliers, including Viracon.
In some situations however, tempered glass is required to meet safety glazing requirements or for added strength. In these cases, Viracon can perform a heat soak test to provide the added assurance that significant spontaneous breakage will not occur. For more information, refer to our technical document Heat Soak Testing.
HEAT SOAK TESTING