Mullite is present in the form of needles in
porcelain. because of its high melting point of 1840 °C. 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 finally replicated in the above studies (using
kaolinitic clay and then firing it at temperatures above 1100 °C) was kept a closely guarded secret by those crucible makers since the 15th century. Mullite morphology is also important for its application. In this case, there are two common morphologies for mullite. One is a platelet shape with low aspect ratio and the second is a needle shape with high aspect ratio. If the needle shape mullite can form in a ceramic body during
sintering, it has an effect on both the mechanical and physical properties by increasing the mechanical strength and thermal shock resistance. The most important condition relates to ceramic chemical composition. If the
silica and
alumina ratio with low basic materials such as sodium and calcium is adjusted, the needle shape mullite forms at about 1400 °C and the needles will interlock. This mechanical interlocking contributes to the high mechanical strength of porcelain. == Use as a catalyst ==