Heat Treatment

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. 

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. There is no industry standard for heat-treated glass roller wave.

Annealed (AN)
Raw glass that has not been heat treated is annealed glass. 

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.) 

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 a specification, the designation for fully tempered glass is commonly abbreviated as FT. 

Heat Soaking
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, we can perform a heat soak test to provide the added assurance that significant spontaneous breakage will not occur.