How to Make Plaster of Paris Molds

Written by f.r.r. mallory
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How to Make Plaster of Paris Molds
Clay from a plaster mould. (F.R.R. Mallory

Plaster of Paris is a moulding material composed of calcium sulphate hemihydrates, calcium carbonate and crystalline silica. By mixing the powder form with water, you can make simple moulds to use for small craft projects--with the exception of hand prints. Because the plaster heats up while hardening and my become uncomfortable in the hands, a different type of moulding products is best for hand prints. And though children would be understandably attracted to the process of mixing Plaster of Paris, adults are better suited to this task.

Skill level:
Moderately Easy

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Things you need

  • Plaster of Paris
  • Oil or petroleum jelly
  • Two plastic cups
  • Plastic spoon
  • Paper bowl
  • Object to mould
  • Water
  • Modelling clay (optional)

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    Making Plaster of Paris Molds

  1. 1

    Lubricate the object to be moulded with oil or petroleum jelly. Plaster is activated by water and will attach to objects unless a release agent is used. The oil and water won't mix easily, so oil-based lubricants work well as release agents.

    How to Make Plaster of Paris Molds
    Oiling the object to be moulded.
  2. 2

    Place the object to be moulded in the paper bowl.

    How to Make Plaster of Paris Molds
    Object in the bowl.
  3. 3

    Pour plaster into a plastic cup and add water, mixing thoroughly.

    How to Make Plaster of Paris Molds
    Pouring plaster into a disposable cup.
  4. 4

    Pour the mixed plaster over the object so that the fattest part of the object is no more than halfway into the plaster (otherwise it won't come out without breaking the plaster).

    How to Make Plaster of Paris Molds
  5. 5

    Allow the plaster 30 minutes to harden. Larger pieces may take longer.

    How to Make Plaster of Paris Molds
    Drying the mould.
  6. 6

    Pull the object gently out of the plaster. Where the object was placed, you see a duplicate but opposite mould of the object. Allow the mould to harden completely.

    How to Make Plaster of Paris Molds
    Removing the object.
  7. 7

    Use modelling clay or other soft modelling material to press into the mould to capture the shape. The mould can be reused many times if handled carefully.

    How to Make Plaster of Paris Molds
    Using the mould.

Tips and warnings

  • Adults should mix plaster to keep the silica away from children.
  • Clean up the area well so that bits of plaster aren't crushed back into powdered forms.
  • Avoid breathing in plaster dust

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