Clean kaolins are calcined by firing the powder in a rotary calcining kiln to a temperature high enough to effect loss of crystal water (and accompanying mineral change). Calcined kaolin normally converts to mullite during this process.
Calcined kaolins soften at about cone 35 and are used in refractory castables and furniture, thermal insulation bodies, low expansion bodies, permeable ceramic compositions, and investment casting (see molochite).
Calcined kaolin is also valuable in glaze formulations which contain high amounts of kaolin to source alumina and which are thus subject to shrinkage and cracking off problems. By substituting some of the raw kaolin for calcined the physical properties of the glaze slurry can be controlled without impacting the chemistry of the fired melt. Mixing the two materials must of course take into account that raw kaolin has a 14-15% LOI which the calcined material does not have.
Calcined kaolins is also useful in tuning the shrinkage and plasticity of slips which are applied to wet, leather hard or dry ware. Because their shrinkage is very low, they can be substituted for part of the regular kaolin and there will be minimal impact on the fired properties.