Q: Can I model directly in plaster of Paris, or do I have to make a mold?
A: It is generally easier to make plaster objects indirectly, using a mold. Quick molds may be made with potter's clay or with plaster that has been sealed with shellac and released with liquid soap. The problem with modelling in plaster is the short amount of time one has to work.
Plaster can be shaped directly at a certain point of the setting curve, before which it runs off, and after which it cakes up; but this only lasts a few minutes. After that it is more a matter of carving than modelling. This can be done quite easily when the plaster is still warm from setting, and gets progressively more difficult as it hardens. Semi-hard plaster can be worked with Surform tools and open-backed rasps (they resemble cheese-graters on a stick) that don't load up. When it is fully hard and dry, regular rasps and sandpaper can be used, but be sure to finish with any rasps and files before starting with the sandpaper, otherwise the grits left behind will dull the steel.
It is possible to add fresh plaster to plaster that has already set, but there is a tendency for the old plaster to suck out the water required by the new plaster, weakening it seriously. To combat this, the old plaster must be soaked in water, and the new plaster must be "killed"-mixed with a superfluity of water. Plaster is normally mixed by adding plaster to water until a small pile of dry powder forms on top of the water, and refuses to sink. When this happens the plaster is mixed for several minutes, then poured. Plaster mixed with more water than this will set, but will not be as strong.
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