If there are any facts that I failed to include please use the comments section to ask questions or add any other useful information. One serious caution is to dispose it as the solid and not put it in a drain as it will clog the pipes. But, it is a safe material with great casting potential. Pour material slowly and use a mask if you feel the need. Plaster of Paris is a fine powder and should not be inhaled. Food color would be a poor choice as it is not color fast. But, use caution as some colors can cause an almost instant set time. It is also easy to color the plaster with poster paint or tempera. In fact, there is a product called modrock that is coated with plaster and simply moistened to use. Plaster of Paris can be strengthened by using glass fiber, gauze bandage, or other suitable material. But, by using cool water, and short mixing times, I have had material that had a working time of 30 minutes. It is also possible to purchase Plaster of Paris with retardants that are designed to increase working time. The best methods to increase the working time is to use cold water, use short mixing times, or by using an excess of water. But, adding other chemicals will change the material characteristics and may lead to failure due to interruption of the crystal lattice. I have seen everything mentioned from baking powder to vinegar. There is a great deal of speculation on the web about increasing set time. In order to fully cure, the cast will usually take between 48 and 72 hours with good ventilation to allow any excess water to escape. But, set time is not the same as cure time. If it is cool to the touch it has at least set. Typically, it can be de-molded in about 30 to 45 minutes. Plaster of Paris sets in a very short period of time depending on the temperature of the mix and the air temperature. This provides a dense cast, but there are still interstitial areas and it is porous. It sets or hardens in a monoclinic crystal lattice which means that the particles are all facing the same direction. Once set, it is not water soluble, and it has a very low rate of expansion, about 0.1 %. This allows it to be sanded, trimmed, and damaged easily. On the Mohs scale of mineral hardness, with talc being 1 and diamond being 10, Plaster of Paris is a 2. Plaster of Paris is a very soft mineral although it can be relatively strong when it is used as a cast. When it is used as a cast for broken limbs it is used over bandage material and not in direct contact. So, it is not used as a casting medium for human parts casting. It can become very hot and can burn exposed skin. In order to make it a castable solid we add water so that it becomes the dihydrate again! The Plaster of Paris gives off the energy that it has stored and when setting into a solid provides an exothermic reaction. 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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