Making & Casting from a (waste) mould
A waste mould is a mould that will be wasted or destroyed after usually only one casting. Waste moulding & casting is a traditional sculpture technique. By sculpting in clay and finishing in plaster, both materials are used to their best advantage.
Making the mould
1. You start off with your clay sculpture, still wet and malleable. Draw a rough guide line around the sculpture as a reference as to where the mould should come out to.
2. Locate the parting line by locating the highest points and with the least detail and avoiding all undercuts always taking into account that the halves of the mould don’t become interlocking and can easily separate. To make a barrier as the parting line (so you can take the mould apart) use shim e.g. Brass shim and insert it along the sculpture in a line to create a wall.
3. Mix up some plaster at approximately 1:4 ratio with room temp water (to give you more work time/”pot life”). You want a thick, smooth, liquidy consistency with no lumps.
4. Once the wall is built up and the plaster is mixed, the next step is to apply the plaster. On one side of the shim barrier wall, you’ll need to throw/flick the plaster on (this method means that the plaster will get into all the detailed places and eliminate air bubbles in the mould. You can use your fingers or a brush to apply the plaster.
Optional - you can make the first layer of plaster coloured so that later on you know when you’re getting close to the casting.
5. You’ll need to keep layering up the plaster, eventually the plaster will get to a thicker consistency, at which point you’ll need to use a pallet knife to apply the rest of the plaster in thicker layers and smooth out the mould.
6. Remove the shim wall and brush with a releasing agent like vaseline or moulding soap (so the two halve can later be separated).
7. Repeat the steps on applying the plaster to the other side of the casing.
8. Leave to dry overnight.
Taking the mould apart
9. Once the plaster mould has completely cured, you can use a pallet knife (or something similar) to pry open the mould and separate the two halves. 10. When you have the two halves, you can start casting from the mould.
Casting from the mould
11. To cast plaster in a plaster mould you’ll need to add a barrier so the plaster doesn’t stick to itself, you can do thins by applying a layer of vaseline or mould release soap/soft soap. 12. Mix up some more plaster (or whatever to medium you want to cast in) and pour slowly into the mould, making sure to be aware of any air bubbles and get rid of them by lighty tapping the mould on the side or the table – which will make the air bubbles rise and pop.
Optional – you can re-enforce the casting by adding material and other mediums e.g. fibre mesh, when the plaster cast is still pourable.
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