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Plaster of paris5/7/2023 ![]() ![]() So, when we buy Plaster of Paris, we are buying the hemihydrate of calcium sulfate. Since it takes in energy as heat it is an endothermic reaction. The modified material is now calcium sulfate hemihydrate, Plaster of Paris, and has only ½ a molecule of water, 2CaSO4♲H2O + Heat → 2CaSO4 The dihydrate is ground and roasted at 150 C (300F) to drive off 1 and ½ molecules of water as steam. It is a naturally occurring mineral found in many locations around the world but was originally named after a large deposit in Montmartre near Paris. ![]() This simply means that there are two molecules of water with each calcium sulfate. Plaster of Paris is made from calcium sulfate dihydrate, (CaSO4.2H2O), frequently called gypsum. So, while I am not an expert, I have used this material for years with good success and continue to find new uses. Plaster of Paris has been in use for well over 5 thousand years and is still misunderstood. The basis for this post is that I have checked several sites for the instructions and found them wrong, misleading, or lacking in good information about this versatile casting material. If you would like to understand more about the chemistry, manufacture, physical characteristics, and some tips for re-enforcing, coloring, dangers, and proper disposal, please read on. If you only want the instructions for mixing Plaster of Paris please go directly to the video. This video is also available on YouTube here: ![]()
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