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.

Painting on Silicone Casts

 Finishing silicone casts with paint is a challenging task as regular paints tend to crack or chip off very soon. EnvironMolds brings a special dye and paint system that will easily incorporate into the silicone.

Silicone rubber is a versatile material that is used to make both molds and casts to suit different applications. It is easy to use – just mix in the required quantity of base and catalyst in the prescribed ratios and you are ready to go. Moreover, as silicone does not stick to anything but itself, the mold making and casting process is relatively straightforward and effortless. The rubber can be specially formulated to be food or even skin safe, thus lending itself well for making food and body molds.

However, the problem arises when the artist tries to paint on a silicone cast. Indeed, you may feel like finishing the cast with a beautiful color of choice, however, regular paints will start cracking, chipping and wear away with time.

Alas, the smooth finish and waterproof qualities of the silicone weighs against it in this regard. Even if you use oil-based paints for their better adherence, the inherently flexible nature of the rubber will cause the paint to wear off easily.

At times, people try making the silicone surface rough by soaking it in denatured alcohol or rubbing it with sandpaper. While this can work for regular silicone caulking, the coarse facade will end up marring the silicone cast and render it unpleasant to the eye.

It is only special silicone paint and dye that can adhere to the silicone rubber and yield the desired finish. EnvironMolds ( offers the special Cirius silicone formulation for painting and dyeing silicone creations. The silicone base of the dye comes to the rescue here as silicone will definitely stick to itself! Try bending and twisting the silicone rubber cast as much as you wish – but the color will not crack, peel or even rub off as it has become chemically incorporated into the silicone!

How to use?

You can mix a few drops of the silicone dye with the base before adding the catalyst to give an intrinsic uniform hue during the casting process itself. Swirling one or two drops of dye in the mix can also create a lovely marbled effect in the silicone cast.

On the other hand, there is also the option of diluting the dye with a silicone medium for coloring after the cast is ready. It can then be painted, sprayed or air brushed on the silicone surface to get the desired finish. However, it is important that the cast should be cleaned and prepped properly prior to painting. The brushes should also be cleaned quickly after use.

This silicone paint is considered compatible with both tin and platinum cure silicone variants. However, the dye can interfere with the setting of platinum silicones.

Twelve different hues are available, including the famous ‘Quinn Blush' used by renowned doll rebirthing artist Terry Quinn to add skin blush to her silicone dolls.