Easily make a mold of any oil clay sculpture.
Making a plaster cast of an oil clay sculpture is sometimes necessary, especially if you plan on selling more than one sculptured piece, or if you plan on casting a sculpture in plaster, ceramic or metal. However, clay sculptures are three dimensional so you need to make two halves to your plaster mold. Because of this you need to decide the best way to divide your sculpture, so that joint lines will be less visible and the plaster mold can easily be removed from the figure without breaking either the figure or the mold.
Instructions
1. Look at your sculpture with a detailed eye and determine where the easiest divide lines for a two part mold would be. Remember you want to keep joint lines in an area that is not easily visible on molded casts. Draw a line using a felt tip pen on the clay sculpture where you are going to divide the cast.
2. Hand form fresh, wet clay around the sculptured piece up to your parting line. Extend this clay approximately an inch from the widest point of the sculpture. Cut off the sides so that it has a rectangular shape. This will serve as one of the temporary sides of the plaster cast while the other side of the mold is being made. Allow the clay to dry to leather-hard (meaning it will still be visibly damp but will not deform when handled). This takes about a day.
3. Add petroleum jelly to the exposed part of the sculpture. Paint mold releasing agent onto the clay base and the exposed part of the sculpture, leaving the clay sculpture and base very oily. Cut four strips of heavy cardboard box the size of your clay base, but 4 to 5 inches higher than you clay base. Enclose the base with these strips and hold them in place with duct tape at each of the outer corners. Make the duct tape corner hold as strong as you can.
4. Mix your plaster according to the plaster package directions. Add plaster to your water instead of water to your plaster to avoid getting lumps. Cover the exposed area of the clay model with plaster up to the top edges of your molding cardboard retainers. Allow the plaster to set for 30 minutes. Remove cardboard retainers. Grasp the opposite sides of the plaster clay mold and loosen the clay from the plaster. Clean plaster of any clay with alcohol. Clean the sculpture of any clay with a damp cloth.
5. Place the sculpture in the plaster mold and place the plaster smooth side on the table with the reverse side of the sculpture facing you. This is the area that was previously covered in clay. Place you casting cardboard sides around the plaster sides and tape with duct tape again. Coat the exposed clay sculpture with petroleum jelly and then coat the bottom plaster cast with parting agent.
6. Mix your plaster according to the package instructions. Pour the plaster over the exposed sculpture and up to the sides of the cardboard retainers. Allow the plaster to set for 30 minutes. Remove the retainers and separate the plaster mold. Clean the sculpture and the mold. Remove leftover clay or debris from the mold with alcohol, Q-tips and clean cloths.
