How to Plaster Mold
Molds, which are negative impressions of objects, are used to make identical positive copies so that the objects can be reproduced in bulk or in different materials. The most common type of mold is the plaster mold, most often used to re-create pieces of fine art, stage sculptures and theatrical props. Plaster molds are usually chosen when the copies are to be cast in liquid latex, foam latex, fiberglass or plaster itself.
Things You'll Need
- Model object
- Pencil
- Modeling clay
- Mold release spray
- Large bucket
- Stirring stick
- Gypsum powder
- Water
- Paintbrush or other applicator
- Burlap
- Scissors
- Rope
- Drill or utility knife
Instructions
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1
Find a nonporous model object that you would like to copy with a plaster mold, or create an object of your own design from plasticine clay. Coat the model with a mold release spray and let it dry completely.
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2
Divide the model into two equal halves by drawing a midline around it in pencil. Apply modeling clay to this line, and build up a dividing wall that is at least as tall as you would like your mold to be thick.
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3
Shake a small amount of gypsum powder into the bottom of your bucket. Add water to the gypsum, stirring as you go, until you have a thick, soupy mixture. Mix the plaster well to reduce air bubbles.
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4
Paint an impression coat of plaster onto one side of your model object, applying plaster right up to the modeling clay wall and filling in all the small details on the model. Let the impression coat dry completely, for at least 30 minutes.
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5
Mix together a larger batch of gypsum and water, following the same guidelines as Step 3. Apply another coat of plaster over your impression coat. While the second coat is still wet, cut the burlap into many short strips and layer them over the wet plaster. Press the burlap down gently so that it soaks up the plaster. Let this layer dry completely.
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6
Paint a third layer of plaster over the burlap strips, then let it dry. Keep adding overlapping layers of plaster and burlap to the model until you have the mold thickness that you desire. Let the mold cure, or dry and harden completely, for six hours.
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7
Repeat Steps 3 through 6 on the other half of your model.
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8
Pull the mold halves apart with your hands, letting the modeling clay separate, and gently remove the model object. Clean out the remaining clay from the inside of the mold, then fit the halves back together. Tie them in place with a piece of rope.
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9
Drill or carve a pouring hole into the top of your plaster mold, along the seamline.
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Tips & Warnings
If working with a clay model, coat the clay with a clear sealer like Krylon Crystal Clear before creating the plaster mold. Mold thickness is usually determined by the size of the object you are molding. Large objects will require much thicker molds in order to support the shape.
