How is Pyrex glass unbreakable?

Answer 1

Direct exposure to firePutting it directly on an open flame. If you do it accidentally, let it cool thoroughly before using. I did it once, left it for 5 minutes, then poured room-temperature soup into it and it shattered. The same thing will happen if you put hot pyrex down in a wet (cool) sink.

Also putting it directly onto a stove burner. My mother did this once by accident (forgetting the burner was hot) and it exploded, throwing glass shards all over the room. Very dangerous.

Additional hazardsThere are a couple of other things you need to avoid. Shock will break Pyrex, just as it breaks any glass product. Glass is inherently brittle, and will not respond kindly to concentrated force. Lastly, if a Pyrex piece is scratched in the right way, it will break spontaneously. We often see older Pyrex wares with lots of little scratches in them. But a scratch presents what is called a stress riser in the glass. It is a place a break can "start" in that piece of Pyrex, and a scratch weakens the overall structure at that point. Dropping a heavy spoon into a scratched bowl might be all it takes to propagate a crack.