A customer asked me to design and build a “new” front door using her existing door and two pieces of old glass that she has.
The old door is in great shape with VG fir, raised bottom panels, and leaded glass upper panels. The old glass is (was?) tempered glue chip. The plan was to combine the door and glass into a stain grade unit with two sidelights.
Last night, I was taking measurements of the glass so I could make my final design drawings. After measuring the first piece, I picked it up so I could move it aside to measure the second piece. Just as I was lowering it, it shattered into a pile of shards. I didn’t drop it, or bump it, it just exploded!! It had been in my shop for a few weeks and I had moved it a couple of times without problems. I suspect that there was a minor crack or weak spot that had gone undetected and that the way I held it somehow stressed it and it blew.
Here’s a before and after picture. That’s the end of me trying to reuse old glass. I’m thankful that it happened in my shop rather than after it was installed in her home.
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Tempered glass can do that. My brother-in-law once had it happen when he closed the rear gate on a station wagon. The glass disintegrated into several thousand little bits.
Tempering puts the surface of the glass into extreme compression, and the interior into extreme tension. This makes the glass more resistant to impact, but gives it a lot of internal "stored energy." The slightest scratch at an edge can lead to the kind of explosion you experienced. As long as the scratch remains in the outer, compressed layer, the glass is fine. But as you move it around, the sheet flexes a bit, and the scratch gets ever so slightly deeper. As soon as it hits the inner layer, boom!
-Steve
Edited 10/23/2007 12:14 pm ET by saschafer
Edited 10/23/2007 12:14 pm ET by saschafer
Thanks for the explanation, Steve. I knew that old glass was more fragile than new, but that was the first failure I've seen that didn't involve some noticeable cause. Spooky stuff!! - lol
I can't remember the last time I tried using old glass, but this episode has made a change in my fundemental company policy..................."NO OLD GLASS!!"
I got a few minor scratches on my hands and forearms, but I shudder to think that something might shatter and really cut someone up.
"...I shudder to think that something might shatter and really cut someone up."
Fortunately, when tempered glass does break, it tends to break into lots of small, relatively blunt-edged pieces, so the chances of getting cut are reduced. (This is in contrast to plate glass--you can make some pretty serious weapons out of broken plate glass.)
-Steve
I noticed that, but got a few minor cuts when it happened - and a couple more as I cleaned up the mess.
This is a business for me and I sometimes have to take a firm position with customers about using tempered glass in some situations. Until yesterday, I would have been willing to use old, tempered, glass. Not now. - lol
Dave.. I have made a 'few' doors using old glass.
My first time.. I measured the glass and cut the parts for glazing.
I never knew then that the old glass was fatter on the bottom! YES.. I think gravity messes with it! The pane must have been 1/6 to 1/8 inch bigger on the bottom.. Really!
EDIT: inch bigger on the bottom ... change to thicker on the bottom.
Edited 10/23/2007 8:30 pm by WillGeorge
Several years ago, I saw a TV show that talked about really old glass. Apparently, the manufacturing techniques of the time weren't good enough to make nice flat, clear, pieces. IIRC, their inability to make large pieces gave rise to divided light windows.
They also said that it was common for really old glass to be thicker at the bottom. Glass never loses all of it's elasticity and "flows" over time. It can take decades (or centuries), but it wants to flow.
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