Making sculptural moulds is an art form in itself. Just as there are a great variety of mould -making materials, so there are many different means of making a mould of an original sculpture these include hot rubber compounds, which have become very popular in recent years because of their flexibility. However, the most common and generally used medium for making moulds is Plaster of Paris: we will use this material as our main example.
Firstly, one has to consider the material that the original sculpture has been created. In most cases, this would be clay. Some sculptors carve their art directly into plaster, and mold has to be made from this material. Each different original material requires a different mold-making technique.
Making moulds from clay models
This technique is suited to making plaster moulds from clay sculptural models.
We will use a simple two-piece mold as our example. The best model for this type of mold is a rounded shape, like a human head. A two-piece mold cannot have too many undercuts. Undercuts are areas where the plaster can get trapped and result in a mold which is difficult to remove. As our example is in clay, undercuts are not too much of a problem as the clay is soft and malleable and can even be scooped out with a spoon. The situation is different however when the original material is something rigid, like plaster.
Once your sculpture has been completed in clay, it is time to make the mould. As stated, we are going to use plaster as the material for creating the mould. Preparing clay to accept a plaster mould requires nothing more than ensuring that the clay is moist. This will ensure that the clay model accepts the plaster and that the plaster retains the impression of the clay. Be careful that you do not make the clay too moist, as this may affect the setting process of the plaster.
Dividing the sculpture.
The most important part of making a two-piece mould is the careful division of the sculpture. Take a pointed instrument and draw a line from the bottom of the sculpture along the side, behind the ears, and down the other side. The important point here is to determine the easiest way of dividing the sculpture so as to avoid any undercuts or areas where the plaster may find a hold that may cause it to be unmoveable.
One also wants to avoid any lines that might scar the final sculpture when it is being cast. What one has to do at this point is to imagine how the two halves of the mould can most easily be separated once the plaster has set and is a rigid mass. This process take some experience and you should not be disappointed if the division of the sculpture is not absolutely perfect the first time.
After you have decided exactly where to divide the model, the next step is to build a separating wall on this line. A number of materials may be used for this wall, including brass foil that can be bought from sculptural supply stores. However, we will simply use clay in our experimental example.
The method of making a separating wall for your mould is as follows:
Flatten clay to about an inch in thickness, either with your hands or with a common rolling pin. Cut strips from this clay with a sharp knife. The width of the strips should be no more than 4 inches. Cut these strips into blocks of about 5 inches in length, depending on the height and wide of the sculpture. The idea is to take these neat strips of clay and make a wall along the line that you had previously sketched on the model. Make sure that this wall is strong, as there will soon be a fair amount of heavy plaster leaning against it.
Cover one-half of the plaster behind the separating wall with plastic. Mix your plaster; making sure that the consistency is correct. (Note: mixing plaster with water requires that the mixture must have a nearly equal amount of water and plaster. The easiest way of doing this is to fill a bucker to just below the halfway mark with water, and then to drop plaster onto the water until a small hill begins to form.)
Throw the plaster onto the clay surface with a flicking action of your hand and fingers. The idea is to throw this plaster fairly hard, so as to ensure that all the detail and crevices in your sculpture are covered with the plaster. This process may require two or three layers before you can move on to the other side. Practically, the plaster mould should be at least two inches thick, although I usually go to three of four inches. It may also be a good idea to wear old clothes while throwing plaster, as this can be a pretty messy business
Once the plaster on the first half of the sculpture has dried, one can begin with the same process on the second half. However, there are a few important points that need to be considered first.
Firstly, remove the separating wall of clay. This will reveal the edges of the first half of the mould. You are now going to throw layers of fresh plaster against the surface of the clay and the plaster edges of the first half of the mould. To ensure that the plaster does not stick to the plaster edges, you will have to apply a separating liquid. This is essential because if the two layers of plaster bond, you will not be able to separate them. The best separating liquid is a mixture of ordinary liquid wax and clay. Take a tin and mix clay with water until it is almost liquid. Mix in a small amount of liquid wax and paint this mixture onto the extended plaster areas.
To further ensure that the two layers of plaster will separate easily, it is useful to place clay fingers along the sides of the plaster mold. Roll three pieces of clay to about the width of your finger and place these on the middle and sides of the plaster mold. This is extremely useful as you can also pour water down these holes to help separate the two halves of the mold.
Begin throwing plaster as before, until the layers of plaster reach the same height as the plaster on the first side of the mold. As the plaster dries, take a sharp knife and trim back the excess plaster so that the joining line between the two layers of plaster can be clearly seen. Leave the mold to dry for at least three hours as an attempt to separate the plaster while it is still setting, may cause the mold to break
Removing the moulds
Once the mold is set and fairly dry, try to pry the two halves apart using the finger holds that you have made. If your preparation has been good, the two halves of the mold should slowly begin to separate. However, this is almost never the case in reality, and a certain amount of coaxing, and patience, are necessary to prise the mold apart. Pour water down the finger holes.
This should break the tension between the two layers of plaster. Try to prise the plaster apart again. After a time, you should see the joining line begin to open and the two pieces of the mold should separate cleanly. Now you have a perfect replica of your original sculpture, divided into two prices. This mold is ready for casting in other materials like cement fondue and fiberglass.