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Q & A: spontaneously shattered glass

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Most recent answer: 05/20/2013
Hello, my question goes back about 22 years. Here's what happened, I was standing in my kitchen, when I heard a loud bang against our front storm door. I was there in seconds. The window was in the process of crumbling to the ground, I actually watched it fall. This is the weird thing the shattered glass in the house and on the porch, then started to jump around. As though it was energized. It reminded me of a whole bunch of mexican jumping beans. You could almost hear the residual energy. I thought the kids playing around the house, had hit it with a ball or a rock or something but they were as surprised as I was. The bang was very loud. There was no remnants of anything physical that caused this. It took several minutes before the pieces of glass stopped popping and jumping. My question is what could have caused this? I have perused the internet and I can't find another example of it anywhere.
- Andy (age 55)
Louisville, Ky.

I think we can explain this. it reminds of a time when the rear window of my parent's old Volvo suddenly shattered. My dad, then about 90, took it philosophically: "What can you expect when something is so far out of thermal equilibrium?"

If glass cools unevenly, it gets enormous strains built in. Think of the last part that cools enough to get rigid. It would normal shrink as it cools, but now it's stuck to other parts that are already rigid. So it's pulling them to shrink, they're pulling to expand. They're both under strain, like springs that are stretched or compressed. A lot of energy is released if they break apart. The shattering can be triggered by a very tiny scratch, or even by very gradual growth of some defect built in when the glass was formed.

Why am I pretty sure that's what happened to your window? For starters, it's a fairly common event. But also, there's a specific sign. Think of those pieces jumping around on the floor for several minutes. It takes some energy to make things jump. There must have been some sort of stored energy that could be released distributed throughout the glass. That fits perfectly the strain explanation.

Mike W. (posted without the usual check until Lee returns)

p.s. In case you're wondering what became of the '84 Volvo, the window was replaced and it ran fine until the fuel pump died last week. It's in the shop.


(published on 05/20/2013)

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