Gypsum vs Plaster – Know the Difference

 Gypsum is usually considered to be the same as plaster. However, it should be noted that gypsum occurs naturally and plaster is derived from the gypsum by treating it with heat to eliminate the water.

There are very many materials used for making molds and casts. Plaster and gypsum are one of the common casting materials as plaster casting is quite a popular medium with casting artists.

The terms gypsum and plaster and used interchangeably and most people consider them the same thing. The confusion abounds as both of them basically contain the same calcium sulfate compound. However, there are intrinsic differences between the two materials.

Gypsum is a naturally-occurring material containing calcium sulfate dihydrate. It is formed in the seas at the right temperature. It is considered a rock-forming material and can grow up to large sizes. Gypsum has a crystal structure but contains water. The crystals are either transparent or translucent – they can be colorless or white, though other shades like grey, red or yellow are also found on occasion. Gypsum is soft, flexible and slightly soluble in water.

When gypsum is heated at temperature between 150°C to 180℃, it loses water and takes the form of calcium sulfate hemihydrates. Certain additives are added during the heating process and it is then ground to powder form which is called plaster of Paris.

Therefore, plaster is a man-made material – the white powder is basically derived from gypsum. It is mixed with sand and water – and some fibers at times – to form a smooth solid. The moist plaster is used to mold different items and will turn hard as it dries to retain the desired shape. Therefore, plaster of Paris is commonly used as a protective coating on walls and ceilings as well as for making decorative castings. It is also used to make orthopedic casts for setting broken bones and the like. At times, the same plaster is used to make toys, statuettes, chalk and even cosmetics.

However, it should be kept in mind that plaster of Paris in its original form is not actually suitable for casting purposes. This is because it cannot hold details well and also tends to turn chalky and flaky over time. The plaster powder is again treated with heat to make it suitable for fine art casting. Only then will it be able to create hard casts that can hold detail, not turn chalky and also resist chipping and cracking. The cast will have a natural white finish and will turn out to be extremely durable. This plaster cannot be used directly on the skin as it tends to emit heat during the curing process.

EnvironMolds offers a range of plaster options for making plaster casting along with plaster bandages. The latter can be used to make shell molds that support flexible molds like silicone rubber, latex rubber and alginate. They can also be used directly on the body to make form molds, like of the abdomen for belly casting.

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