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Casting with Plaster of Paris

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 ComposiMold Plaster of Paris Casting from a real seashell
Seashell in Plaster of Paris, Use ComposiMold's Plaster of Paris or ComposiMold's Plaster Additive

Plaster of Paris is low cost, easy to use, and makes good castings. Plaster of Paris comes as an easy-to-mix formula that mixes with water and dries to a dense, durable, and smooth and bubble free finish. Plaster of Paris is a great material to use for basic castings and molds and art projects because it is simple to mix and use. The Plaster of Paris sets in a few minutes, although it takes an hour before it is ready to be removed from the mold. It takes 24-48 hours to fully cure. Using Plaster of Paris is easy, but there are procedures to follow that will make you successful in your plaster castings.

ComposiMold Plaster of Paris Casting of frogs for a garden
Plaster Casting of Frogs

Plaster of Paris is not a plastic. It is a hard white substance made by the addition of water to powdered and partly dehydrated gypsum. It is hard like a cement or concrete but white and not as strong. So don’t use plaster for very thin castings. Basic shapes or designs that have a solid backing to them will be more successful. If you need some flexibility or stiffness in the final castings, then move to a plastic.

Creepy Plaster Casting of a Dolls Head
Creepy Plaster Casting of a Dolls Head

Materials you will need:

Plaster of Paris powder. For ComposiMold or ImPRESSive Putty Molds, please use our ComposiMold plaster that has been formulated to cure to a strong solid casting in these molds. By creating cure more quickly, the water in the plaster does not react with the ComposiMold or ImPRESSive Putty to give strong, hard plaster castings. You can also purchase a powder additive to add into any plaster of paris to obtain a similar affect.

ComposiMold Plaster of Paris Casting

Water: Use cold water to cure slower and give you more control over the reaction and give more time to fill the molds.

Mixing container that is large enough to hold to water and plaster. Plastic containers are nice because you can deform them afterwards to break away the hard plaster and re-use the container.

Measuring cup or any cup that can be used to measure out 3 parts of plaster powder to 1 part water.

Make your Plaster Castings

The ideal ratio for a Plaster of Paris mixture is 3 parts Plaster of Paris powder to 1 part water by weight or volume. Measure out the water and pour it into your mixing container. Some recipes suggest 2 parts plaster to water, but this will create a much weaker plaster casting. The thicker you can be while still being able to pour the better. Start with a 3 to 1 ratio and if necessary add small amounts of water to make it easier to pour.

Break any clumps of plaster up with a spoon. If you use your hands to mix, be sure to wear gloves.

When mixing the water and plaster, you should mix the powder into the water. Pour the plaster powder into the water spreading it out over the surface of the water. After it is poured all poured in, tap the sides of the mixing container and let the plaster powder fill with water to reduce bubble formation. The plaster powder will sink into the water. When you are finished adding the plaster, it will be slightly above the surface of the water and not easily absorbed by the water.

Gently stir the plaster and water to form a nice slurry similar to a thick pancake mix. Gentle stirring of the plaster will reduce the chance of bubbles being mixed into the mixture.

Pour the plaster into your molds. If you want to learn how to make your own molds, check out our mold making videos and get your free mold making e-book.

A couple other tips for plaster casting:

Do not pour any extra plaster down your drain. It can harden and destroy your plumbing. You can dilute the plaster so it does not create a hard plaster or let it harden and throw it away.

To make colored plaster, you can add colorants to the plaster mix while it is still in liquid/paste form. Remember that you will be adding color to white, so you will need more colorant to make darker colors. Any types of paint will work.

To paint the plaster casting after it has cured, use a layer of gesso or primer first to seal the plaster, and then you can paint the plaster any way you want.

Video of Plaster Casting of a Duck Decoy

ComposiMold Plaster of Paris Casting of an Ice Cream
ComposiMold Plaster of Paris Casting of an Ice Cream

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