Molds for many plaster crafts can be made from molding clay or carved into pieces of wood. Clay is best used when only a portion of an object is going to be replicated with plaster, simply by pressing the object with clay, then filling the hole with plaster. Wood carving works well for creating new objects from scratch. To make a three dimensional mold of a complete object, follow the steps below which show make a mold for plaster made from plaster itself.
Instructions
Making the First Half of the Mold
1. Examine the object which you wish to cast in plaster to determine where the mold parting line should be. For most objects, this will be the halfway mark along its shortest width. For example, the parting line of a human doll would be halfway between the front and back of the doll, rather than its waist.
2. Press down a piece of clay on a flat surface that is 1/2-inch deeper than the model, and 1-inch longer on four sides.
3. Hollow out some room for the object in the center of the clay. Then press the object firmly into the clay until it is halfway submerged in the clay along the parting line.
4. Press two marbles into opposite corners of the clay until they are submerged halfway. These will form the keys of the mold when it is complete.
5. Trim the edges of the clay so they are straight and the corners are square. Trim the top of the clay so that it is level along the parting line.
6. Paint a thin coat of orange shellac over the object and the top of the clay. Let it dry for twenty minutes, then dust it lightly with talcum powder.
7. Cut four pieces of 1/4-inch wood trim, sized according to your clay's length and width, and at least 1-inch higher than the highest point of the object. Place them around the clay to make a frame. Then use bar clamps to hold the boards in place.
8. Apply a thin layer of petroleum jelly over the surface of the object, clay and the inside of the wooden frame, covering the surfaces completely.
9. Mix some plaster according the manufacturer's instructions and pour it over the clay until it is level with the wood frame. Allow the plaster to dry as directed by the manufacturer.
10. Remove the clamps and wood frame. Gently lift the plaster from the clay. Turn over the plaster mold. Then remove the marbles and all loose pieces of clay. Leave the model in place.
Making the Second Half of the Mold
11. Place the wooden frame around the plaster. Then apply petroleum jelly to the plaster and wooden frame.
12. Mix some more plaster and pour it into the frame. Then wait for the plaster to dry.
13. Remove the frame from the plaster. Then turn the plaster on its edge. Insert the end of a flat screwdriver into the parting line. Tap the screwdriver with a hammer to split the two halves of the mold.
14. Remove the model from the mold by putting a small piece of wood on the plaster, then use it as a lever to gently pry out the model with a screw driver.
15. Examine one half of the mold and locate the position of the cavity closest to the edge of the plaster. Gently carve a pour hole into the plaster to that point in the cavity using a chisel.
16. Allow your mold to dry before using it. Remember to coat the mold with petroleum jelly before pouring plaster into the mold.
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