Mullite
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Mullite, or porcelainite, is a rare clay mineral, aluminum silicate (Al6Si2O13). It is produced artificially during various melting and firing processes, and is used as a refractory.
Researchers recently discovered [1] that potters in the Hesse region of Germany unknowingly used mullite to create crucibles, enabling alchemists to heat their crucibles to very high temperatures. The formula for making it (using kaolinitic clay and then firing it at temperatures above 1100 degrees) was kept a closely guarded secret.