Making a Negative Mould
1. Having completed the modelling on the positive mould, rub the whole mould with petroleum jelly.
2. Rebuild the frame around the mould with the wooden slats.
3. Mix the plaster and pour into the box to a thickness of two or three inches. Tap gently to ensure there are no air bubbles.
4. Build up the plaster over the positive mould and model.
5. Level off the top of the plaster so that the mould will sit firmly on a bench.
6. When the plaster is set, remove the slats and allow the plaster to dry thoroughly. This can take a day or more. If it is not sufficiently dry, it could break during separation.
7. Separate the moulds carefully. You will probably need to use a firm tool such as a large screwdriver.
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