If you decide to use calcium carbonate wads, you should wear safety glasses when unloading the kiln until you are certain that the wadding reacts slowly. Plaster of Paris, like slaked lime, is cementatious. When we add plaster to water, a chemical reaction takes place. Some of the water combines with the plaster until there are two molecules of water for each molecule of calcium sulfate. As this reaction takes place, the plaster rearranges itself into a more orderly crystalline structure.
Heat is given off.
The more excess water there is to be incorporated as pore space, the more porous the plaster becomes. Care must be taken in drying plaster. The reaction that makes plaster harden is reversible. As you heat plaster up, it gives off this chemically-combined water in stages. According to Dick Notkin, the first such dehydration takes place around 145 degrees F. Heating plaster above this point softens it slightly, and can make it soft and powdery.