Why did my pyrex bowl suddenly break?
Question
I was wondering if you could help me understand something that happened in my kitchen last week: I was warming up some leftovers in the microwave when the food caught fire, presumably because I'd overlooked a piece of tin foil from the wrapping. I switched off and unplugged the microwave, and waited several minutes for the flame to extinguish before opening it. I had just opened the door to let out some of the smoke when the Pyrex plate I was cooking in literally exploded, spewing glass and burnt food all over the kitchen and me! The plate was microwave safe and I'd used it in there many times before. My question is what could have caused it to explode like that?
Answer
Pyrex expands when you heat it, and if it's heated for long enough it will expand all the way through.
When it cools suddenly, such as when you opened the microwave door, the outside of the Pyrex dish will shrink before the inside does. This will put the outside of the dish under tension, as it will be pulled apart by the expended Pyrex in the middle of the dish.
If there are any cracks in the outside of the dish, such as scratches or damage built up over many heat/cool cycles, this crack can travel through the dish and the tension in the surface can be released as a great big crack, and the plate will split.
This can be quite explosive, as the forces involved are immense, similar to the forces involved in water expanding as it freezes, which can blow cast iron pots apart and erode mountains. These forces should be more than enough to make the dish fly apart.
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