Last week I witnessed what I can only call an "explosion" of a Pyrex baking
dish. My wife had left a Pyrex baking dish, warm from the oven, on the top
burner of our electric stove. The burner was, unfortunately, still on in
the middle position. A thin layer of food residue, on the inside bottom of
the dish, had turned black from the burner's heat and was giving off a
burning smell. I noticed this, turned off the burner, and placed the Pyrex
dish on an adjacent burner which was at room temperature since it was not
turned on. Fortunately, at that point, I removed myself several feet from
the stove. Within less than 30 s there was a very loud "bang" type noise
and the Pyrex dish, in my opinion, "exploded ". Pieces of glass went flying
over the stove top, the floor and about 6 feet in any direction. A piece of
glass hit my hand, cutting it rather badly and with enough force that I
still have a bruise from it. At that moment, I wasn't at my experimental
best, but I do remember that it appeared to me that momentum was conserved
since the distribution of glass was reasonably symmetrical about the
position of the dish just before the explosion.
While I'm only kidding about energy not being conserved, I really don't
understand the mechanism which caused such a violent event to occur. My
chemist daughter tells me that Pyrex glass should be capable of going from a
warm oven into a freezer. She would heat quartz (not Pyrex) tubing up to
600 deg C and then thrust it into ice water. I was only trying to go from a
warm (not hot) burner to one at room temperature. Of course, the burner
construction consists of a coil with spaces between the turns of the coil
which don't distribute heat evenly across the bottom of the dish. Even, if
the burner did distribute heat evenly, the bottom of the dish was heated
much more than the sides unlike the case of an oven where the dish would
(ideally) be heated evenly. So stresses in the glass have to be at the
heart of the explanation and the burner's heat must have supplied, at least,
some of the energy for the explosion (is there another source?), but I'd
appreciate help in understanding the underlying physics.
Don Polvani
Anne Arundel Community College
Arnold, MD