Strain patterns are a characteristic of heat-treated glass. To learn more about this subject see Viracon’s Tech Talk Quench Pattern Characteristics.
Annealed (AN)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.
Heat SoakingFully 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.