I’m very curious.
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In addition to the other comments, I’ll add that all glass can experience this effect to some degree – the surface cools and sets faster than the interior, which then cools and contracts, creating internal tension. This can make glass objects shatter during cooling or with use.
To prevent this glass is usually annealed – cooled slowly in an oven so that the surface and interior reduce temperature at the same rate, so internal stress tension does occur. Of course, some glass may be heat/cool treated (tempered) to create those stresses (glass designed to shatter safely).
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