Calcined kaolin (also called metakaolin) is a powdered white non-plastic material. It is raw kaolin that has been fired (in a rotary calcining kiln) high enough to remove the 12% (approx) crystal water. If you are a potter you can make your own calcined kaolin by simply bisque firing any raw powdered kaolin (in a small enough bisque vessel and slow enough ramp that the heat penetrates well). The material is a good example of how we can alter the mineralogy of a material to affect its working properties while maintaining the chemistry to maintain fired properties.
Calcination makes the powder whiter and more chemically inert. That makes it useful in a wide variety of products and industries. But strangely, it is often under-utilized in traditional ceramics (people do not realize its true value). Kaolin is pure clay mineral, having a fired chemistry of 1 part Al2O3 and 2 parts SiO2. But the raw clay crystals are hydrated, having 12% crystal-bound water. This is the secret to their plasticity. Al2O3 is essential to the chemistry of the vast majority of glazes and kaolin is an ideal source (because all glazes also need the SiO2 that it supplies, and, it readily decomposes in the melt). The other principle affordable and readily meltable supplier of Al2O3 are feldspars, however they also supply lots of KNaO (and in many cases oversupply it to get the needed Al2O3). Raw kaolin also imparts suspending properties to the glaze slurry. And it hardens the dry glaze layer. But there is a problem with raw kaolin: Once percentages pass 20% in a recipe shrinkage becomes too high (causing crawling). In these cases substituting part of the raw kaolin for calcined kaolin solves the problem, maintaining the chemistry of the glaze but reducing the shrinkage and cracking. This enables controlling the physical properties of the glaze slurry without impacting the chemistry of the fired melt. Of course, mixing the raw and calcined materials must take into account the LOI of the raw material (12% less calcined is needed).
Calcined kaolin has other uses in ceramics. It is refractory and softens at about cone 35. It is thus useful in refractory castables and furniture, thermal insulation bodies, low expansion bodies, permeable ceramic compositions, and investment casting (see molochite).
Calcined kaolins are also useful in tuning the shrinkage and plasticity of slips (engobes) which are applied to wet, leather hard or dry ware. Engobes contain higher clay percentages than glazes and it is more important to control their drying shrinkage. Thus they can be substituted for part of the raw kaolin to tune drying while maintaining fired properties.
Source; Digital Fire