Making plaster cast moulds can use a variety of techniques, depending on whether you are using the plaster mould to cast metal, concrete, plastic or simply a positive figure in more plaster. Starting with a simple two-part mould is the best way to become familiar with the technique. From there, you can refine your technique to create multiple-part moulds to allow greater detail in the object you are casting. The process starts with a model, which is the figure you want to cast more of in the plaster mould.
Create your model out of any rigid substance, such as wood, soapstone or clay. If you use clay, allow it to dry completely, and fire it in a kiln if you need additional strength. For your first project, use a model that only has a slight amount of detail and no undercuts (places in which a part of the model has space beneath a raised area). You will need to be able to pull the model straight out of each half of the mould without breaking off any details or appendages.
Press at least 3.8 cm (1 1/2 inches) of clay into the bottom of a container that is at least 2.5 cm (1 inch) larger than your model on each side and at least 5 cm (2 inches) deeper than your model is tall. Use more clay if your model is more than an inch tall.
Press your model, face down, into the clay until it is exactly half-buried (any rounded parts are submerged only up to the point they begin rounding back the other direction). There should be approximately 2.5 cm (1 inch) of clay remaining below your model.
Place a piece of 1.3 cm (1/2 inch) dowel so it creates a rod from one edge of the container to the top of your model, again exactly half-submerging it in the clay. This will create the tunnel you will pour the casting material into.
Smooth down the clay around your model until it is relatively flat, removing any excess. With your thumb, create an impression about 6 mm (1/4 inch) deep in each corner of the clay. Smear a thin coating of petroleum jelly over the entire surface, including the model, clay, sides of the container and inside your thumb impressions.
Put on latex gloves and mix the plaster of Paris according to the directions on the package. Stir it by hand to feel for and break up any lumps in the plaster. When it is mixed thoroughly, pour it into one corner of the container and let it flow across the model. Tap the sides of the container to release any air bubbles, then allow it to set until it is hard and dry.
Invert the container and gently flex it until the entire contents come out. Peel away the clay and discard it, then clean any excess off the face of your model. Leave the model and dowel in place in the plaster and place it, face up, back in the container. If the container does not have straight sides, you may need to trim down the plaster with sandpaper or plaster-forming rasps.
Coat the face of the model, dowel and plaster with more petroleum jelly, including the four raised impressions formed from your thumb indentations. Mix and pour another batch of plaster to create the second half of your mould.
Release the mould from the container after the plaster has hardened, and pry the two halves of the mould apart. Gently remove the dowel and your model and clean off the petroleum jelly with a damp rag. Use a small amount of mild washing up liquid if the jelly won't come off with just the rag.
Use the mould by placing the empty halves back together until your thumb imprints lock into place. For casting with relatively cool materials such as foam, concrete and plaster, you may need to apply more petroleum jelly or another type of mould release to keep the casting material from sticking, then seal the mould together with a thick rubber band. For casting with metals, mould release is not needed (and may in fact cause harmful fumes), but the mould will need to be held together with a stronger material, such as a plumber's strap pulled tightly around the mould and bolted together.
Use clay around more of the object to begin with, then slowly remove it in sections to create multiple-part moulds for more complex objects.