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Mullite (100 mesh) - 1 lb.
Mullite is a mineral of long interlocking needle-like crystal structure that is very resistant to thermal shock failure (has a low thermal expansion). It is also has a low thermal conductivity and is very refractory thus the theoretical formula (of 71.8% alumina and 28.2% silica)bears little resemblance to the real world material (we have provided a typical non-theoretical analysis). It is found rarely in nature, however it can be synthesized by calcining kyanite, bauxite or alumina/kaolin mixtures of proper Al2O3:SiO2 ratio. Mullite crystals can also be formed within special purpose porcelains by incorporating similar minerals into the recipe and firing to the necessary temperature and heating curve to decompose them into mullite. These include andalusite (cone 13), kyanite (cone 12), sillimanite (cone 20). The resulting bodies display low thermal expansion and are useful in spark plugs, laboratory ware, etc. and in thermal shock resistant refractories. Firing of ordinary stoneware bodies provides the necessary temperature and adequate kaolin to produce mullite crystals from the decomposition of kaolinite (kaolinite looses some silica and the remaining higher alumina reorients itself to a higher melting compound). The resulting lattice of crystals is potentially much stronger than the simple glass-weld bonds of low-fire ceramics. Source: Digital Fire.
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