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The following narrative is to provide the reader with general information concerning heat and chemical strengthened glass. It is advisable to consult with a qualified engineer to determine whether or not a particular glass can be tempered or if tempered glass is appropriate for a particular application.

In general, most glasses can increase their strength and heat resistance through heat treatment or by a chemical process. Heat-treated glass can be classified as either heat-strengthened glass or fully tempered glass. The differentiation between heat-strengthened glass or fully tempered glass is accomplished by measuring the degree of residual compression and/or the edge compression that exists in the material after heat treatment. Chemical strengthening of glass is accomplished through a chemical ion-exhange on the surface of the glass.

In the heat treatment process, the basic characteristics (color, clarity, chemical composition, and light transmission) do not change. The compression strength, material hardness, expansion coefficient, softening point, thermal conductivity, solar transmittance and the stiffness does not change. The only physical characteristic that changes is the material's bending strength. Heat-treated glass is stronger than annealed glass of the same size and thickness under uniform loading. As a result, the material is more resistant to thermally induced stresses, cyclic wind loading and impacts by wind borne objects.

Heat strengthened glass is defined as having a residual surface compression greater of 3,500 psi and less than 10,000psi, or an edge compression greater than 5,500 psi and less than 9,700 psi. As a result the breakage pattern differs depending on the surface compression and surface quality. Under low compression the breakage resembles annealed glass with large shards. At the midpoint of the compression range, the glass breaks become smaller with more fragments. At the higher end of the range, the breakage resembles that of fully tempered glass.

Under ASTM-C 1048, fully tempered glass is defined as having a residual surface compression in excess of 10,000 psi or an edge compression in excess of 9,700 psi. The breakage pattern in fully tempered glass is a multitude of cubical shaped piece called "dice". It should be noted that even though a fully tempered sheet of glass meets this definition, it still may not be classified as safety glazing glass. To be classified as safety glazing glass, the size of fragments in a controlled breakage must meet the requirements of CPSC 16 CFR 1201 or ANSI-Z 97.1.

In general, all heat treated glass will break if the compression layer is penetrated. Surface scratches and edges chips that do not penetrate the compression layer can slowly propagate over time and result in breakage.

In heat-strengthened and fully tempered glass, a strain pattern, which is not normally visible, may become visible under certain lighting conditions. The heat-treatment process may induce bending or slight warpage in the glass sheets. It also cause distortion around the tong location, i.e., the clamping area where the glass was held during the tempering process. Heat- strengthening and heat tempering is not usually effective on sheet having a thickness of 1/8 inch or less.

Chemical strengthening is used for certain optical application or on glass sheets that have a thickness of less than 1/8 inch. The compressive strength of a chemically strengthened glass can reach as high as 100,000 psi. The compression strength will vary greatly between pieces because of surfaces flaws or variations in the chemical solutions and heat gradients in the chemical tanks.

The heat-strengthening or tempering process occurs after the glass sheets are fabricated to the dimensions required. Fully tempered and heat-strengthened glass cannot be cut after tempering. Chemical tempered glass can be cut but the cutting process causes total loss of the added strength within one inch of the cut. It is not advisable to sandblast, acid etch or grind the glass after tempering because the surface compression layer may be weakened or penetrated thus causing spontaneous breakage. It is also not advisable to try to temper deeply sandblasted glass.

Tempered glass can be furnished with holes, notches, and bevels provided the inner surfaces of the holes or notches do not have surface fractures or cracks and the surfaces are smooth. Guidelines exist as to where the holes and notches can be located to avoid high breakage during the tempering process.

Finally, it should be noted that the effects of the tempering or the heat strengthening process can be reversed by re-annealing (stress relieving) the glass.