How to Identify Tempered Glass from Non-Tempered Glass

I overheard one of our sales representatives talking to a customer. The customer wanted to learn how to identify tempered glass from non-tempered glass.

Our sales rep joked with the customer saying, “Take a hammer to it. If it busts into hundreds of tiny pieces, then it was tempered.”

Of course, few people want to purposely shatter a large piece of glass. (Well, I did. However, that’s a story for another time.) There are less destructive ways to be able to identify whether the glass has been tempered.

First, here’s a lesson about how tempered glass is made. Take a piece of annealed glass-glass that was allowed to cool slowly during the manufacturing process. Reheat the annealed glass in a furnace and cool quickly using forced air drafts.

A little known fact for the glass novice is that holes cannot be drilled into tempered glass. Most types of drilling would cause the glass to shatter. Holes, notches and/or carvings must be made before the tempering process.

Anyway, let’s get back to identifying tempered glass. If your glass is coming from the manufacturer, they are required to etch a tempered or safety glass identifier in the four corners.However, that's not always the case.

Another way to identify tempered glass is to view the glass through a pair of polarized sunglasses. In many cases, viewing the glass in sunlight, through polarized lenses, will reveal dark lines or spots. These are prime indicators that the glass is tempered. The lines are formed by the machine rollers during the cooling process.

Finally, look for dimples, bending or warping. These imperfections are caused during the heating process. Tongs used to remove tempered glass from heat often leave small impressions in the glass that can identified if you look closely.