Glass placemat exploded on my kitchen table. What the heck happened?

February 1, 2013 5:28 PM

A couple of days ago, one of the placemats on my kitchen table exploded all over the table and also onto the ground. It's made of a glassy/plastic-y material. Meaning, it looks like glass, but it won't cut through skin unless you really dig it in. We found shards of it up to 5 feet away from the table. This occurred randomly in the early evening. No one in my house witnessed it. Nothing was placed on the placemat for that entire day. No one went near it. There were no sudden temperature changes in the house, although it has been colder than normal in the evenings here in Northern California. We've had the placemats 10+ years. The table is a glass table with a clear, plastic covering on top.

Pictures are here: http://s1286.beta.photobucket.com/user/everywhereishere/library/

Does anyone have any idea what happened?

We've thrown them all away already, but it still is quite puzzling.

posted by madonna of the unloved to grab bag (12 answers total)

Glass that cools unevenly can have a great deal of internal tension, causing it to shatter with seemingly no provocation. Here's a great video explanation, courtesy of the Corning Museum of Glass: Annealing and Tension in Glass
posted by qxntpqbbbqxl at 5:38 PM on February 1 [1 favorite]

It's probably tempered glass, and tempered glass has a reputation for randomly exploding (but only infrequently).

One of the nice things about broken tempered glass is that the little pieces aren't as sharp and pointy as regular glass, like you discovered.

Tempered glass can also be very strong, and that's why it's used for things like placemats and cutting boards.
posted by that girl at 5:39 PM on February 1 [1 favorite]

It looks like tempered glass, which is internally stressed by design so it shatters into safe little chunks, rather than shards. It is known to shatter spontaneously under certain conditions. Probably a manufacturing defect or tiny nick from normal use caused stress to build up over time, and an unusually cold night may have finished the job.
posted by Salvor Hardin at 5:40 PM on February 1 [1 favorite]

Spontaneous explosion of glass is a thing. Here's a post with lots of stories and some theories about it. It's a scary world.
posted by Corvid at 5:42 PM on February 1 [1 favorite]

I was at a party years ago and my friend's glass worker landlord took us into his shop and broke out some things he called Dutch Tears. Watching them explode was pretty incredible. It goes from one piece to millions of pieces all at once.

The idea that you can store so much force in a material as brittle as glass is pretty awesome.

That being said, that's probably a variant of what ended up happening to your placemats. My guess is that it was an increasing temperature variance that got worse during the cold nights. More so if some liquid is involved. Those pictures remind me of broken windshields, not broken plastic.
posted by Sphinx at 6:12 PM on February 1 [7 favorites]

Along with the internal stresses explanation, glass like this also has a bit of a memory effect. Every time it gets nudged, clinked or thermally stressed, it weakens slightly. Eventually, it explodes. Either because you dropped it that one last time, or in this case, because it just gave up the ghost. If it was literally just sitting there, then something probably momentarily tweaked one of its resonant frequencies.

I once saw a commercial glass coffee pot explode in someone's hand as they set it down, the same way they had been doing it for years.
posted by gjc at 8:47 PM on February 1

I watched a tempered glass shelf explode once, in a store I used to work at, for absolutely no obvious, immediate reason any of us who worked there could fathom. Nothing fell on it; no one had recently touched it. (It was holding expensive ceramic teapots, too, which sucked.)

Random glass explosions just happen sometimes.
posted by BlueJae at 8:54 PM on February 1

Glass is like cats - often beautiful, sometimes assholes. Unless specifically built and annealed to handle repeated heating and cooling, glass subjected to hot dinner plates won't last years and years. Even small scratches make glass objects prone to cracking - more so with tempered glass, which is designed to shatter under low stress rather than make sharp pieces under medium stress.

Apropos: the Pyrex brand of cookware is commonly believed to be made of borosilicate glass, which is much less prone to thermal shock than the more common soda-lime glass. This is in fact true in Europe, but for the last 30 years the Pyrex sold in the U.S. has been regular old tempered glass. It's an inferior product and after long use and many heating/cooling cycles it's prone to do the same thing your glass placemats did. After working with art glass for many years I've stopped using tempered glass in my kitchen.
posted by SakuraK at 10:40 PM on February 1 [2 favorites]

My ma had a set of lovely black tempered glass crockery when I was a kid. My brother thought it was a miracle, and loved deliberately dropping pieces because LOOK, IT'S GLASS, BUT IT WON'T BREAK, I AM A WIZARD!

It took about ten years for the stress to build up, but by the time we were teenagers the pieces of the set just started exploding for the same sort of minor taps and nicks discussed upthread. I had a teacup explode in my hand while washing it, from the force of applying a sponge. Tempered glass isn't that sharp at all but there was enough force there I have a few prime scars from it.

It's a bit like a chain of dominoes, really. Tempering lines them up, and then every drop, knock and scratch knocks another one over. A cold night would absolutely have done it, especially if it had a few minor knocks or scratches over dinner. It really doesn't take much at all for that last domino to fall.
posted by Jilder at 11:20 PM on February 1 [1 favorite]

I'm going to trade in all my newer Pyrex for old, old Pyrex from the antique stores.

I did have an old white glass mixing bowl from an antique store several years ago which was used to make countless cakes for the grandkids over the years. One day I mixed up some stuffing mix in that bowl and put it in the microwave. There was a POP and when I opened the microwave, the bowl had cleanly separated about halfway down the sides - I had a top ring and a small bowl on the bottom. They never dreamed of microwaves when they made those dinosaur-age bowls.

I'll bet those placemats were pretty, but when that one exploded it would have sent me racing to hide under the covers.
posted by aryma at 12:05 AM on February 2

I had a glass explode in my hand at a bar last year - there was a bit of ice inside, my warm hand on the outside, and cool air blowing in at it from a nearby window. it was the weirdest thing! I was listening to a friend talk, so I wasn't shaking the ice around or anything, it just randomly blew up, covering me with those tiny glass shards.
posted by 5_13_23_42_69_666 at 12:09 AM on February 2

Thanks for your answers, everyone! I feel slightly less creeped out by it now. I can be superstitious, and I was taking this as a bad omen of some sort, so I'm glad that there's science behind it. I'm just glad no one was using it when it happened. No more glass placemats for us!
posted by madonna of the unloved at 12:48 AM on February 2