Pyrex is a glass with boron oxide added to the mix. This is another molecule that, like silicon dioxide, finds it hard to form crystals. More importantly, as an additive it counteracts the tendency of glass to expand when heated or contract when cooled. When different parts of a piece of glass are at different temperatures, expanding and contracting at different rates, stresses build up within the material as the different parts of the glass strain against one another. These stresses cause cracks to grow and ultimately shatter the glass. If this happens in a vessel containing boiling sulfuric
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