First patented in Britain in 1874 and officially mandated for use in the U.S. as safety glass in buildings in 1965, tempered glass has without a doubt saved countless people—especially children—from serious or even fatal injuries over the years.

If you have patio doors, shower doors, or windows in places that need safety glass and the units are newer than the mid-1960s, then chances are good they have tempered glass. Glass used in major appliances is also almost always going to be tempered or a variation such as glass ceramic.

Architects, designers, and builders fully understand the need to stay on top of applicable codes and code changes regarding the use of safety glazing products whether in residential, architectural, or commercial applications. In addition to knowing the basic code requirements, understanding similarities, differences, advantages, and disadvantages of the various products available can be of significant value when the time comes to specify products.

Thermally tempered glass

If you heat a 3-mm or thicker sheet of glass to about 1200 °F or 660 °C or near transition temperature in a specially designed furnace and then rapidly cool it using high-pressure blowers in a pre-set pattern, you should end up with tempered glass. Three millimeter or thicker is required because thinner glass physically can’t be thermally tempered using conventional tempering furnaces.

Glass at transition temperature is in a slightly expanded state. Rapid cooling of the hot glass locks the surface in the expanded state while the still plastic interior of the glass is attempting to return to its original size. This conflict between a lite’s surface and center results in extreme stress in the glass as both separate surface compression and interior tension.

This interaction between the layers is what gives tempered glass its increased strength and its unique break…