Donovan Mirror and Glass

Serving Southeastern Wisconsin

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Tempered Glass

What is Tempered Glass?

Glass is tempered by heating it in a furnace until it is just past a transition temperature and almost in a softened state, then the glass is cooled very quickly. Under these controlled settings the outer surface of the glass drops below the transition temperature and completely solidifies. The inner portion of the glass remains above the transition temperature and slowly cools until it too falls below the transition temperature. As it cools it shrinks away from the outer surfaces slightly, causing the inner portion of the glass to be under tension and the outer surfaces under compression.

Beneficial Properties

Strength: The compressed surface layers of tempered glass give it strength, making it as much as four times stronger than annealed glass. Glass naturally has innumerable microcracks known as Griffith's flaws. These microcracks gradually widen when glass is placed under tension (because tension is a force that pulls apart). Because compression acts as an opposing force to tension the compressed surfaces of tempered glass are able to withstand much more weight (which creates tension) than ordinary glass.

Safety: The coexisting tension and compression within the glass is also what makes tempered glass shatter into many small cubes. When the outer layer of tempered glass is penetrated deep enough to reach the inner layer, the tension within the inner layer will instantly pull the glass apart in all directions. This is what produces all the small cubic pieces that are relatively harmless.


Tempered glass doesn't just crack or chip, it shatters instantly and completely. Consider a tempered glass table or shelf that supports a variety of objects. If the edge of the tempered glass were hit with a hammer, the entire glass would probably shatter and everything that was resting on the glass will fall. However, annealed glass may chip or crack and remain relatively intact, as will laminated glass. When glass must hold together even in case of breakage, tempered monolithic glass is not used. Such is the case with stair treads and second story or higher glass deck railings, as two examples.

Not all glass can be tempered. The stresses within the glass introduced in the tempering process cannot exist within any glass that has seeds (air bubbles). Unfortunately, this means several popular decorative glass patterns are not available tempered.

Tempered glass is highly susceptible to slight bowing because of the internal stresses. This can cause a slight distortion in reflected images, especially for large panels. It can also cause clearance issues with bypass shower doors if the panels are not installed with the any bowing in the same direction.

The tempering process also has a tendency to create roller marks. Again, this is more noticeable in larger panels, and the distortion is usually so slight that it is unnoticeable to most people.

How to Determine if Glass is Tempered

There is no way to tell if glass is tempered just by looking at it. For this reason tempered logos are lightly etched into the glass at one of the corners to identify it as tempered glass. These logos allow customers (as well as housing inspectors) to know that their application is up to code, for those applications requiring safety glass. It can be requested that no tempered logo be placed on the glass, but we would hesistate to do so for any application requiring tempered glass. This is an option for glass that does not need to be tempered, but has been tempered by customer request, such as glass shelving.

It may be possible to see the stresses within tempered glass by using polarized lenses, but it may not be possible to tell if the glass is tempered or just heat-strengthened. We are not familiar enough with this method to know its reliability.