PYRO
An aluminium silicate, mined as a rock rather than as a pure mineral · Lowers thermal expansion and holds fired shrinkage down · Builds mullite from about 1100°C, which a fired body draws its strength from · Refractory on its own account, so it widens a body's firing range
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Pyrophyllite is an aluminium silicate mined as a rock rather than as a pure mineral, so a bag carries a good deal besides the mineral itself. In a body it lowers thermal expansion and holds shrinkage down.
Pyrophyllite goes into whiteware and tile bodies, and into stoneware where thermal shock is the problem. It loses its combined water between about 500°C and 900°C, and from about 1100°C it builds mullite, the needle-shaped crystal a fired body draws much of its strength from, leaving an amorphous silica behind in the spaces between. Part of what holds the shrinkage down is that it expands a little on the way up, as it gives up that water, before contracting again higher in the firing, so it works against the body rather than with it. It is refractory on its own account, which widens the range a body will fire over.
