# How to tell if you have tempered glass



## tzen (Nov 22, 2007)

About a week ago I broke the glass top to my 29g viv. I had carefully placed it standing up in an open dresser drawer, and then a minute later noticed how messy the room looked, and closed the drawer. So now I needed a new piece of glass for my lid.

There is a glass place about a mile from where I work that will cut glass to size for only about five bucks. They were the ones who I got the first top from. But I thought maybe I could recycle some old glass. Since I have experience doing stained glass, cutting glass is no problem. 
There is a Habitat for Humanity Restore in town, so i went there.
There was an old guy who helped me. We found a stack of 1/4 inch glass that looked like maybe it had been used for shelves, and it was just a few inches bigger than I needed. He said, "It might be tempered, so you wouldn't be able to cut it. You can sometimes tell by looking." We looked at the glass, and it looked like glass.
He said, "Sometimes if you tap on it, it makes a different sound." I tapped on it with my fingernails. It sounded like glass. We walked over to some windows that surely were not tempered and tapped on those. They sounded like glass in a window frame. Kinda lower and more resonant, which made sense, since they were much bigger, and not being held by my hand. We shrugged.
But the 1/4 glass was only 50 cents, so I bought two, in case I screwed up on the first try.

I can mention that tempered glass is also know as safety glass or toughened glass. For sheets of glass, they heat it above the annealing point, then quickly cool the surfaces with a blast of air. The inside stays hot and cools more slowly, making it contract more. The result is glass that is under internal tension. Much like prestressed concrete, this results in a glass that is much stronger. The internal tension also means that if any part of the glass gets compromised, the whole sheet shatters into little cubes. These cubes are good for not severing arteries, unlike the large fragments when regular annealed glass breaks. Also, tempered glass scratches more easily unless it has a surface treatment.


Anyway, I get my new $0.50 glass back home. I notice it has a few very minor scratches, but, hey, who knows how heavily they had been used.
I was apprehensive with my first score, but the glass cutter rolled across without any mishaps. Then I used my breaking pliers and squeezed gently. Breaking pliers have a very wide, downward curving mouth that applies pressure to break the glass along your score line. Nothing happened, so I I tried from the other edge, squeezing harder. Then I squeezed for all I was worth, but nothing.

The next way you can coax a stubborn score line is to tap the back side of it with the brass ball on the end of the glass cutter. So I tapped. But you can't tap very hard with only a 5 inch tool, so I tried the side of a socket wrench, then a ball peen hammer. Still nothing.

If you have a straight line and a large piece of glass, you can also do an "edge break," where you line up the line on the edge of the work bench, and with a sharp downward motion snap off the glass. I started gentle at first. After a while, I wife yelled down the stairs to find out what I was banging on so much.


So, although maybe it was stronger because it was quarter inch thick, I pretty much knew the writing was on the wall.


But I had one last trick to try before I wrote it off as a loss. I got out my wet tile saw with its diamond impregnated blade. I wore gloves and safety glasses. I think I got about a a quarter inch in before -PAM- little glass 2ct diamonds went flying out about 5 feet.

So to find out if glass is tempered, just use a tile saw.


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## tzen (Nov 22, 2007)

Oh ya.
The stresses in tempered glass also show up under polarized light, which I was reminded while I was fact-checking what i had wrote.

I put on my polarized sunglasses, and sure enough, the remaining piece I had bought had the classic wafflely appearance of car windows.

Doh. I had my sunglasses on my head when I bought the glass in the first place.


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## pl259 (Feb 27, 2006)

The first clue was that you couldn't snap the scored piece, with the same amount of force you'd used before on other sheets. I made the same mistake when I practically stood on a piece before it broke into a million pieces.


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## salix (Mar 28, 2008)

LOL, okay, I know it's not funny, but it was funny. :lol: 

I used to do stained glass as well, your description of cutting glass brings it all back to me. I didn't have special tools, mine consisted of the glass cutter, a pair of pliers and an old washcloth. I'd make my score, throw the washcloth on the edge, grab the glass with the pliers over the washcloth and it breaks like butter with only slight pressure.

Every break never ceased to amaze me!


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## tkromer (Dec 20, 2007)

imagining a tile saw hitting tempered glass pretty much made my morning. I've broken a piece with a drill before and a tile saw has to top that!


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## leveldrummer (May 30, 2008)

after drilling a few tanks for salt water, i gotta say, a glass hole saw will make quite a mess too, just by touching the glass, lol. and it will scare the crap out of you as well.


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## tkromer (Dec 20, 2007)

Yeah I was drilling with a hole saw and found one that I _thought_ wasn't tempered. Not as bad as a tile saw I would guess, but it was a mess for sure.


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## pl259 (Feb 27, 2006)

> The stresses in tempered glass also show up under polarized light...


BTW FWIW, this property is called birefringence.


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## tkromer (Dec 20, 2007)

That is a great "useless" word to file away


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## pl259 (Feb 27, 2006)

Most engineers are familiar with it. Believe it or not, I actually came across the word four seperate times last year. Twice at work, once here on DB, and another time on one of the kid's science projects.

Hey, the next time a DVD disc fails to work, you can blame it on "excessive birefringence".


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