Identifying Tempered Glass

Last year I was gifted with a big glass donation from Red Butte Garden. As I was unloading all of the glass, one of the pieces cracked. That may sound like bad news, but it was actually awesome. That meant it wasn’t tempered. 1/4″ float glass is the workhorse of recycled glass art, so to have 400 lbs, already cut into nice sizes and ready to go is really a gift.

I should have known better. The top 10 or so sheets weren’t tempered, but the 11th was. Tempered glass is the angst-ridden, anger filled sibling of float glass, ready to explode if you provoke it too far.*

I realized it as soon as I ran a score across the glass.

There are several clues that I could have used to identify the glass. The real kicker though, is there are still times when all the clues point in the Not Tempered direction and you still end up with a big pile of tiny little glass chunks.

The “BUG”

Most tempered glass comes with an etched mark on it indicating it is tempered. A glazier friend told me it was known colloquially as the “bug”, but he may have been pulling my leg. Regardless, none of my glass pieces has a bug mark.

Tempered Glass Mark

The Edge

Tempering is a process that happens on purpose. The manufacturer knows going in what that glass is going to be, so the edges can be finished before the tempering process. Tempered glass typically has a nice finished edge.

Tempered and Not Tempered glass edges

The Score

When you try to cut tempered glass you get a lot of slivering and flaking. Almost like a little glitter trail.

Glass scores on Tempered and Not Tempered Glass

If you get all the way to this point without figuring it out, your choices are: throw it away (gasp!) or break it and use it as cullet.

Not to worry though, I’ll show you how to easily (and safely) break tempered glass and what to do with it in a later post.

* [Editors Note: Tempered glass isn’t, in fact, angry. It is float glass that has been heated to above annealing temperature and then cooled very rapidly to induce internal stress and surface tension.]

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