Exploding Place Mat
Discussion
The Microwaved tea thread reminded me of something that happened at my house a couple of years ago; I googled it at the time, but can't remember the term for it but I'd never heard of it happening before and never seen it since.
We had some glass placemats that were years and years old - my guess is ~10 years old. We'd used them regularly, no special treatment just got wiped clean.
My GF went to move one slightly as it was a bit out of place and BOOM the thing just exploded into hundreds of tiny pieces of glass, which continued to fizz and pop for minutes after the initial event. We were finding bits weeks later still!
Apparently something to do issues when it was being made IIRC.
We had some glass placemats that were years and years old - my guess is ~10 years old. We'd used them regularly, no special treatment just got wiped clean.
My GF went to move one slightly as it was a bit out of place and BOOM the thing just exploded into hundreds of tiny pieces of glass, which continued to fizz and pop for minutes after the initial event. We were finding bits weeks later still!
Apparently something to do issues when it was being made IIRC.
Yep, that was mine.
In large panes of glass the problem is nickel sulphide inclusions in toughened glass; these are microscopic, but grow with time and eventually the localised stress is enough to 'set off' the stored strain energy in the glass and pop it goes.
Toughened glass is prestressed, by rapid cooling locking the core in tension and the outer faces in compression. That's what makes it so strong, because scratches in the outer face are limited in their ability to grow into cracks that mean failure in such a brittle material. The process also stores *a lot* of strain energy in the volume of the glass, so when it goes -impact, inclusion,or often chipppng/scratching an outside edge corner (ariss) -all thats left are little cubes and powder -that energy 'pays' for a lot of crack length!
NB commercially large panes for balustrades, balconies, big windows and overhead glazing (ie places failure could be catastrophic) are often heatsoaked for 24-48hours; this accelerates the growth of inclusions,weeds-out as early failures -so panes that survive the heatsoak can be expected to have a far lower failure rate/full service life.
In large panes of glass the problem is nickel sulphide inclusions in toughened glass; these are microscopic, but grow with time and eventually the localised stress is enough to 'set off' the stored strain energy in the glass and pop it goes.
Toughened glass is prestressed, by rapid cooling locking the core in tension and the outer faces in compression. That's what makes it so strong, because scratches in the outer face are limited in their ability to grow into cracks that mean failure in such a brittle material. The process also stores *a lot* of strain energy in the volume of the glass, so when it goes -impact, inclusion,or often chipppng/scratching an outside edge corner (ariss) -all thats left are little cubes and powder -that energy 'pays' for a lot of crack length!
NB commercially large panes for balustrades, balconies, big windows and overhead glazing (ie places failure could be catastrophic) are often heatsoaked for 24-48hours; this accelerates the growth of inclusions,weeds-out as early failures -so panes that survive the heatsoak can be expected to have a far lower failure rate/full service life.
We had an expensive Italian glass coffee table, constructed from three sheets of glass, glued together by bevelled edges. The kids used to sit on it, despite being told repeatedly not to, but I walked into it one night on the way out of the room which resulted in the glue bond between the top and one of the sides failing and the table collapsing, with the other side failing too. I cleaned off the glue residue and bought some high load clear glass glue to repair it, but never got as far as doing so, because as I was applying glue to the bevelled edge of one of the sides that I had laid on a smooth table top, I gently tried to move it forward a little by pulling it with a finger on top and it simply exploded into thousands of nuggets, just as you see in the street when some scumbag has broken into a car.
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