How to Make a Plaster of Paris Cast

Written by tanya lee
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How to Make a Plaster of Paris Cast
Sculptors often use plaster of Paris to make casts of their work. (Plaster head of the ancient Greek philosopher Socrates image by Supertrooper from Fotolia.com)

Plaster of Paris is a form of gypsum. The first uses of plaster date back at least 9,000 years to Anatolia (present-day Turkey) and Syria. The Egyptians used it 5,000 years ago as a mortar for monuments such as Cheops pyramid. They also used it to make models taken directly from the human body. In the 1700s, the walls of the wooden houses of Paris were covered with plaster as a way to protect against fire; hence, the name plaster of Paris. Today plaster of Paris has many uses in industry and architecture. It is a versatile, inexpensive, easy-to-use craft material most often used for making casts from moulds.

Skill level:
Easy

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Things you need

  • Mold
  • Mold release
  • Plaster of Paris
  • Water
  • Mixing bowl or other plastic container
  • Spoon or flat paddle
  • Measuring cup

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Instructions

  1. 1

    Spray your mould with an appropriate release material so that you can get your cast out of the mould without breaking it. Molds for plaster of Paris can be made of latex rubber, alginate, plaster, moulage, urethane rubber, silicone rubber, plastic, waxed cardboard, or a wide range of other materials.

  2. 2

    Measure then pour your water into a plastic container such as a mixing bowl or food storage container. The proportion of water to plaster of Paris is 1 to 2, or one part water to 2 parts plaster of Paris. To make a figurine about 5 inches high, you would need approximately 1/2 cup of water and 1 cup of plaster of Paris.

  3. 3

    Measure the amount of plaster of Paris you need and use a spoon to scoop it up and sprinkle it on the water. Sprinkle slowly so the water has time to absorb the plaster of Paris as you go.

  4. 4

    Let the mixture rest for 2 or 3 minutes, then mix it slowly and gently with the paddle or spoon. The reason for mixing it slowly is to get as few air bubbles as possible in the mixture. The mixture should end up being the consistency of cake batter. If it is not, add a little more plaster of Paris or a little more water.

  5. 5

    Tap the container several times to get the air bubbles to rise to the surface.

  6. 6

    Pour the plaster of Paris slowly into your mould. Fill the mould just a little over full.

  7. 7

    Let the plaster of Paris set in the mould. First it will get very hot, then it will begin to cool off. When the cast is cool, remove it very carefully from the mould. The cast is still soft at this point, so you can easily repair any little blemishes that have resulted from the casting process.

  8. 8

    Wait two or three days for the cast to cure before you paint it.

Tips and warnings

  • Do not pour the extra plaster of Paris into any part of your plumbing system; it will clog your drains.
  • Do not pour the extra plaster of Paris into any part of your plumbing system; it will clog your drains.

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