Mullite, or porcelainite, is a rare clay mineral with the chemical composition Al6Si2O13, a form of aluminium silicate. It is produced artificially during various melting and firing processes, and is used as a refractory.

In 2006 researchers at University College London and Cardiff University discovered that potters in the Hesse region of Germany since the late Middle Ages had used mullite in the manufacture of a type of crucible (known as Hessian crucibles), that were renowned for 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 Celsius) was kept a closely guarded secret.

Mullite is present in the form of acicular crystals in porcelain.

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Mullite

21st Century Technology Cracks Alchemists' Secret Recipe