- Calcined kaolins are fired 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 kaolin is valuable in glaze formulations that contain high amounts of kaolin to source alumina, and which are thus subject to shrinkage and cracking-during-drying problems. By substituting some of the raw kaolin for calcined (such as Glomax), 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% loss on ignition (or LOI) that the calcined material does not.
- Calcined kaolins are 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 likewise be minimal impact on the fired properties.