Clever… use a marble.
I’ve seen many a marble with air bubbles and impurities. I would not toss one in the oven.
I thought that was pretty clever, until BigD145 mentioned the air bubble thing. I’m sure a ball bearing would be safer and probably as cheap. I suppose if a person really wanted to use a glass marble, it might be a pretty good idea to bake the marbles in a safe container first to determine if any will fail.
Marbles are formed at the melting point of glass. The air bubbles should be a partial vacuum, and be incapable of overpressure below 1400F. That said, I still wouldn’t toss them in the oven either.
A crumpled ball of cupcake paper should do the same job. Or maybe use some tin foil.
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