Making copies of existing objects is done with an old technique, one that is easily performed by any hobbyist at home. Plaster is a versatile material in this process, used in both mold making and casting. You can make plaster copies of a statue or statue model by creating a two-part plaster mold, then pouring liquid plaster into the hardened mold. When the plaster dries and the mold is separated, you will have a perfect copy of your statue model.
Place your plaster or clay statue model in a clean, well-protected working area. If you are working from a ceramic or metal model, coat the model with a mold release agent. If you are working with a clay model, coat the model with Krylon Crystal Clear or another clear sealer. Let the model dry before continuing.
Find your model's dividing line, from one side of the base to the other, and build up a thin wall of modeling clay over the line so that your model statue is divided into two equal halves. Make the wall several inches tall, or at least as tall as you would like your mold to be thick.
Mix together a small batch of gypsum and water in a bucket, creating a soupy mixture of plaster. The mixture should be creamy, like a thick milkshake, so that it sticks to the model. Using the paintbrush, apply an impression coat of plaster mixture to one side of the statue. Make sure that this layer of plaster has as few bubbles as possible, as the bubbles can compromise the mold's strength. Allow the impression coat to dry completely, so that there are no tacky spots.
Apply a second coat of plaster mixture to the statue model. While this coat is still wet, lay a few strips or squares of burlap over the model and let the plaster soak into the burlap. Wait until the plaster is reasonably dry, then apply another layer of plaster over the burlap strips. Keep applying coats of plaster and burlap until you have a fairly thick, sturdy mold half, then let the plaster dry. Repeat Steps 3 and 4 on the opposite side of the statue model, and let the entire mold cure for 6 to 8 hours.
Pull apart the mold halves at the dividing line and remove all of the modeling clay. Carefully remove your statue model. If your model was made of plaster, be very careful about pulling the mold halves away from it, as they will not want to come apart easily.
Strap the mold halves together with a piece of rope or, if the mold is small, with rubber bands. Drill a casting hole in the top of the mold, then take the mold apart again. Clean the inside of the mold and coat it liberally with a mold release agent. Strap the mold together again, so that the edges and the casting hole line up. You should now have a mold that is ready for wet plaster to be poured in through the top hole.
You can get bubbles out of your plaster by thoroughly sifting the gypsum into the water and stirring the mixture before you apply it to the model. You may wish to use an electric mixer.
The thickness of a mold often depends on the size of the object being cast. Larger objects will require thicker molds, while small objects often need a mold that is only a few layers thick.
If you are creating a mold from a model that is made from a breakable material like glass, you may want to make a silicone RTV rubber mold instead, as the rubber will not damage the object. With rubber, you will pour the material into a molding box and press the model into the wet rubber, rather than painting the mold around the model.