Gypsum is a very common mineral found in marine evaporates and dry caves. The name “gypsum” comes from the Greek word “gypsos,” which actually means “plaster.” Gypsum plaster, sometimes called plaster of Paris, has a long history and a variety of uses. From mortar for ancient buildings to casting for statues throughout the ages, it has come to be a valuable resource used in homes, industries and artist’s studios all over the world.
In its mineral form, gypsum develops when water rich in calcium sulfate evaporates or fluctuates, leaving behind deposits of gypsum (hydrated calcium sulfate). When gypsum is heated, it dehydrates and becomes calcium sulfate hemihydrate. The dehydrated gypsum is then powdered and mixed with water to create plaster paste. The ratio of water to powdered gypsum as well as the formula of additives used, determines the thickness of the plaster and what uses it will best serve.
The ancient Egyptians used gypsum plaster 5,000 years ago to mortar the stones of the Cheops Pyramid. The sandy environment along the Nile is an ideal place for gypsum to form as the river’s annual flooding deposits calcium sulfate, which remains once the flooding is over. The Egyptians gathered the gypsum crystals, burned them to dehydrate them and then crushed them into powder. The process for making plaster has changed little since that time.
Plaster of Paris
After London suffered the ravages of fire in 1666, the king of France decreed that the wooden buildings should be protected from fire to prevent the same thing happening. There were large deposits of gypsum near Paris and so they used gypsum plaster to cover the wood because of its noncombustible nature. So, in the 1700s Paris became known as the capital of plaster, which is why gypsum plaster is sometimes called plaster of Paris.
Gypsum plaster is used most often in the building industry to create drywall, to plaster walls and to make the decorations featured on buildings. However, that isn’t the limit of its uses. It’s used to make molds from which products such as vases, pottery and statues can be created, and to harden casts when applied over bandages. It serves as an agricultural product used to condition soil and even as an additive in foods like tofu.
Manufacturers are relying more and more on “synthetic” gypsum which is a byproduct created by the processes used to desulfurize the flue gases in fossil-fuel power plants. The chemical compositions of natural and synthetic gypsum are identical. Some forms of synthetic gypsum are useless, such as phosphogypsum which contains radon. However, most are safe and non-toxic and their use in recycled products such as drywall prevents them from ending up in a landfill.
- Photo Credit Plaster head image by Supertrooper from Fotolia.com
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