The water from the clay is drawn out of the slip by the pores in the plaster mold. For example, when it’s used as a casting mold, liquid clay (called slip) is poured into the plaster mold. One of the reasons plaster is so handy for potters is that is porous and absorbent. These capillaries are tiny microscopic open channels in the plaster that make the hardened plaster porous. ![]() The crystals also form capillaries in the plaster. The crystals form a matrix-like structure in the plaster that gives it strength. These crystals are what make the plaster harden. When the calcium sulfate bonds with the water, it changes from hemihydrate to dihydrate.ĭuring the setting reaction, crystals are formed in the mixture of water and calcium sulfate. This is a chemical reaction where the water recombines with the calcium sulfate particles. ![]() This is because, when it is mixed with water a ‘setting reaction’ takes place. In its partially dehydrated state, it becomes usable as an ingredient in plaster. Once it has become partially dehydrated it is called calcium sulfate ‘hemihydrate’. During the heating process, the calcium sulfate loses some of its water content. ![]() The heating process is called ‘calcining’. Part of the processing involves grinding it up to a powder and heating it. The main ingredient in each of these types of plaster is calcium sulfate, which is similar to chalk.Ĭalcium sulfate is a mineral that is mined from the ground and processed. There are a number of different types of plaster.
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