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Re: your mail




On Mon, 18 Nov 1996, fred brace wrote:

A student came in today with a story about hearing a loud crash and
running into the kitchen to find that the oven view window had broken into
a whole bunch of little pieces and that the pieces seemed to be vibrating
up and down. The oven hadn't been on. Any explanations?


Amazingly enough, this is a fairly common occurrence. Tempered glass has
enormous stress within it, and if it is manufactured a bit wrong, or if it
becomes badly scratched or slightly cracked, it can "spontaneously
disassemble". I've heard about Pyrex (borosilicate) measuring cups
exploding for no reason, while sitting on a table. I've heard about car
rear windows shattering into tiny fragments spontaneously. Then there's
"Prince Rupert's Drops."

"Tempered" means that the glass surface was strongly cooled during
manufacturing while the inside of the glass was still molten. This
creates large stresses within the glass, and makes it very strong, since
it will not break unless it is bent to such an extent that the internal
forces are cancelled.

The glass in your oven window was most certainly tempered glass. Was this
a new oven? I'd expect that the spontaneous explosion would tend to occur
in a newer oven, since if the reason was faulty glass, older ovens with
faulty glass would have exploded long ago.

The "jumping" fragments are also common. It occurs because the internal
forces in the fragments are extremely nonuniform, and the "explosion" is
continuing as the larger fragments spontaneously shatter into smaller
ones, and the pieces fly apart. I've seen this myself. As a kid I was
playing with friends around a dead car in an abandoned lot. It's windows
had been shattered , and the rear window was very interesting, the
fragments were tiny and cubical. Some of the fragments were larger, and
we kids found that if we snapped these in half, the halves would go
"crackle" and spontaneously and rapidly break into smaller and smaller
pieces which jumped about as they broke.

There is a famous physics demonstration associated with this phenomena:
"Prince Rupert's Drops". To perform this, you melt a large glass rod with
a torch, and allow the liquid glass to drip into a bucket of water. Most
of the drops will shatter as they cool, but a few will not. These
unshattered drops are tempered glass, they contain enormous stresses
(which can be examined by placing the glass between crossed polarizers.)

The fun part: carefully hold the the stressed glass droplet by its sharp
point, wrap it in a paper towel for safely, then place it on a table and
whack it on the side with a hammer. It will not break, as long as you
don't damage the pointy part of the drop. The internal forces are much
higher than the peak force created by the hammer impact. But then
carefully snap off the sharp point, and the glass will shatter into tiny
fragments (which sometimes jump about as each fragment breaks into smaller
ones.)

This demo is straightforward, but definitely practice with it first before
showing it to students. Some types of glass and some sizes of droplet
don't work well, so first make a bunch of them to get a "feel" for the
process.


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