CAKA

Calcined kaolin is kaolin that has been fired to drive off its chemically combined water. It stands in for part of the raw kaolin in a glaze, slip or kiln wash, bringing the same alumina and silica without the drying shrinkage.

Calcined kaolin is kaolin that has been fired until its chemically combined water is gone, and what comes out is a different material. It is no longer plastic, and soaking will not bring the plasticity back. The alumina and silica are all still there, a little more concentrated per gram now the water has left, but the drying shrinkage that comes with plastic clay is already behind it. A glaze with a lot of clay in it shrinks as it dries and can crack, then crawl in the firing, and swapping part of that raw kaolin for calcined takes the shrinkage out without changing the fired chemistry. The same holds for a slip going onto 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, though — with the water already gone, roughly six parts of calcined match the fired chemistry of seven parts of raw kaolin. And it will not hold a glaze up in the bucket: calcining destroys the plasticity that does that, so a glaze still needs some raw clay in it, or a suspender, or the batch settles hard.