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Name: Joan E.
Status: Student
Age: 60s
Location: N/A
Country: N/A
Date: February 2004


Question:
Would you please tell me why a Pyrex casserole that I have used for years would explode when I took it out of the oven. It was hot but I placed it on the top of the stove to cool. It exploded into very small shards of glass.



Replies:
Several things may have happened.

1. One tip-off is your comment "I have used for years...". Glass (Pyrex) and others can develop invisible (except using a pair of polarizing filters) stresses that finally cause the bowl to crack. Once a crack starts to propagate it can shatter into small shards.

2. If you had scratched or chipped the bowl ( and it could have been small enough that you might not even be aware of it) this too sets up stress sites that allow (1.) to occur more readily.

3. Even Pyrex can shatter if there is a large enough temperature difference at a small enough contact point. (My guess about what occurred is #2.)

Vince Calder


The reason your Pyrex exploded was due to thermal stress and thermal fatigue. Add on top of that, the brittle nature of glass is why it went into a thousand pieces.

Thermal Stress

Although Pyrex is designed to go in and out of the oven, it still has to deal with thermal stress and fatigue. When you heat up the Pyrex to cook that wonderful casserole, the structure of the glass likes to expand. When you take it out of the oven, it starts to cool and thermal stresses can be induced into the material depending on how fast it cools. If it cools too fast, the center part of the glass may stay warm, but the outer part cools fast. The difference in temperature causes the structure in the warm area to be a little longer than the structure in the cool area. This causes stress because the long crystal does not want to be pulled shorter be the short crystal and vice versa. Even if the Pyrex cools slow, it still has thermal stress issues, just not as bad as a fast cool down. That is why you really should never go directly from the oven to refrigerator or from the refrigerator directly to a hot oven.

Thermal Fatigue

All of this stress that is built up over the years pays a price on the Pyrex. This going up in stress and coming down in stress is called fatigue. Add on top of this that Pyrex is not a pure substance due to the nature of making the dish. That is, there are some small flaws in the glass such that, as you fatigue the glass, these small flaws grow into bigger flaws. Now, most of these flaws stay little and do not cause the glass to break because the glass can stop the cracks because of its microstructure. But as you fatigue the glass some of these flaws grow to what is known as a critical crack length. This is the smallest size of flaw that the glass can have until it can no longer stop the crack from moving through the glass. This is when the glass shatters. Because glass is a brittle material, once this critical crack length is met, it cannot stop any of the cracks from moving through the glass and it falls apart into a thousand pieces.

Now, this is not to say you need to get rid of all your Pyrex stuff. On the contrary, Pyrex is some of the best stuff for high temperature cooking. It just that you probably put that dish through a long life and its time was up. Go get another one and hopefully it will last you a lifetime. But, if you see a crack in the glass, you really should get rid of it immediately. Hope this helps.

Christopher Murphy, P.E.
Associate Mechanical Engineer
AFRL/PROE



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