Calcined Kaolin

Calcined kaolin is kaolin that has been fired until its chemically bonded water is gone. It is no longer plastic, and soaking will not bring the plasticity back. Firing removes the water and the plasticity but not the alumina and silica, so calcined kaolin supplies the same fired chemistry as raw kaolin, slightly more concentrated by weight, without the drying shrinkage that comes with a plastic clay. A glaze with a lot of clay in it shrinks as it dries, and it can crack and then crawl in the firing. Swapping part of the raw kaolin for calcined takes that shrinkage out without changing the fired chemistry. The same applies to a slip on damp or leather hard ware, and it is why calcined kaolin turns up in kiln wash recipes. The swap is not one for one by weight. With the water gone, about six parts of calcined match the fired chemistry of seven parts of raw kaolin. It will not keep a glaze in suspension either, because calcining destroys the plasticity that does that, so a glaze still needs some raw clay or a suspender or the batch settles hard.