Calcium sulfate and calcium chloride are flocculents. A flocculent is a material that increases the amount of water necessary to make a slip fluid. When my friends in Thailand were looking for inexpensive materials for lowfire and midrange glazes, I suggested glass cullet. I talked to Peter Pinnell about it, and he suggested the addition of a few percent plaster to help keep the glaze from settling. This seems to work well.
Calcium sulfate is a self-adjusting flocculent.
Its limited solubility keeps the amount in solution relatively stable. Used for molds, plaster is indispensable for slipcasting. But small amounts of plaster are enough to cause big problems in deflocculation of the slip. Barium carbonate is added to slips not only to eliminate scumming, but also to eliminate flocculation. Calcia is the oxide most commonly in the background of our processes.