GLOSSARY: Different Types of Heat Treatments to Glass

Glass Tempered

There are some terms of different glass heat treatments that you need to know when specifying architectural glass. Often times, they are easily confused with one another or used interchangeably, so in today’s post, we thought we’d try to offer some clarity (no pun intended) on the types of heat treatments to glass.

Annealed: this is glass that has no heat treatment applied to it. It is regular glass, sometimes referred to as “plate glass” or “float glass.”

Tempered Glass: This is glass that has been heat-treated through a tempering oven to become fully tempered. It has the characteristic small break pattern and is 4 to 6x stronger than annealed glass of the same thickness. Tempered glass, according to ASTM C 1048, which is the industry standard for heat-treated glass, must have a surface compression level greater than 10,000 psi. It qualifies as “safety glass” in building codes.

Heat Strengthened Glass: This is sometimes referred to as “partially tempered” or “heat-toughened” as the time in the tempering oven is shorter, the break pattern isn’t as small as fully tempered glass and the result is only an estimated 2x stronger than annealed glass of the same thickness. ASTM C 1048, which is the industry standard for heat-treated glass, requires heat strengthened glass to have a surface compression within the range of 3,500 psi to 7,500 psi. It does NOT qualify as “safety glass” in building codes.

Heat-Soaked Glass: This is glass that has undergone a special process after being fully tempered where the glass is then placed in a special oven at 550-degrees Fahrenheit for several hours to test for nickel sulfide exclusions. If nickel sulfide exclusions are present, the glass will likely break in the chamber. In many instances, only a selection of pieces in a large order is tested in this manner to be representative of the entire project, as a way to inspire confidence of a reduced likelihood of spontaneous breakage. This type of glass testing is much more common in large curtainwall projects rather than interior glass uses.

Gardner Glass Products is proud to be able to provide fully tempered glass for architectural uses that comply with ASTM C 1048. We are certified in glass thicknesses of 4mm5mm, 6mm, 10mm, and 12mm in a variety of glass types, including low-iron, tints, and acid-etched finishes.

To learn more about the differences between Fully Tempered Glass and Heat Strengthened Glass, watch this informative video by Vitro/PPG 



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