How to Make Plaster Casting Moulds

You can make a plaster cast mold at home.

Plaster is a simple and effective mold-making material—it is inexpensive, easy to apply and dries quickly. Although it can occasionally crack if dropped or opened too forcefully, a cracked plaster mold repairs quickly with fast-acting glue. If you have a sturdy object to use as a form for a mold, you can make a plaster cast mold in an afternoon. Your plaster cast mold will be appropriate for casting duplicates in wax, clay, latex or other soft materials.

Things You'll Need

• Plaster powder
• Water
• Bucket
• Drop cloth or tarp
• Wooden wedge
• Hammer or mallet
• Brush
• Knife
• Mixing spoon or spatula
• Petroleum jelly
• Water-based clay
• Newspapers

Instructions

1. Protect your work table with several layers of newspaper, and lay a drop cloth down on the floor.

2. Add two parts plaster powder to one part water in a large bucket, and mix well with a spoon or spatula until no lumps remain. Tap the edges of the bucket a few times to release any trapped pockets of air in the mixture.

3. Cut a half-inch-thick strip of water-based clay with a knife, to serve as a barrier between the two halves of the mold. Squeeze the strip of clay onto your mold-form object, dividing the object into a front half and back half.

4. Coat the front half of your object in a layer of petroleum jelly to aid in the release of the object from the plaster cast mold. Paint a thin layer of plaster onto the front half of the object. Build up additional layers of plaster until the cast is 3 inches thick. Allow the cast to dry for two hours.

5. Remove the clay barrier and coat the reverse side of your object in petroleum jelly, followed by a thin layer of plaster. Build up successive layers of plaster until the cast is uniformly thick around the entire circumference of the object. Allow the cast to dry for two hours.

6. Place the point of a wooden wedge against the seam between the front and back halves of your plaster cast. Tap the wedge very gently with a hammer or mallet until the cast cracks open along the seam. Remove the mold-forming object from the plaster cast mold.

References

• Plaster Mold Making