For glaze recipes that contain excessive, high-shrinkage ingredients like clay, bentonite, Gerstley borate, or magnesium carbonate, a portion of the recipe's kaolin can be replaced by calcined (pre-fired) kaolin to prevent applied glaze cracking and consequent crawling in the fired glaze. Use 86% of the kaolin amount removed.
For more information on calcined kaolin, see the Digitalfire.com Reference Library of ceramic materials.