Plaster
Studio moulds are still largely plaster. This material rehydrates and sets into a rigid, porous solid. The reaction is exothermic, warming noticeably as it goes off.
Two core rules govern its use:
- The ratio matters, and more water is not more forgiving. Measure by weight, typically 70 parts water to 100 parts plaster (Plaster · 100 · 70). Extra water leaves the set plaster weak and overly porous.
- The sequence is not optional. Sift plaster into water. Let it slake for two minutes, mix gently, and pour promptly.
Plaster moulds must be thoroughly dry before reuse.
Release agents
Release agents prevent bonding between surfaces:
| Situation | Typical release |
|---|---|
| Plaster against plaster | Soft soap, or shellac then wax |
| Plaster against clay | Often none needed |
| Silicone against most surfaces | Usually none; seal porous surfaces |
| Resin against silicone | Usually none |
Key Operations
- Keys: registration notches cut into a piece so parts lock into exact alignment.
- Walls or shims: temporary barriers dividing the form during multi-part pours.
- Undercut removal: filling minor recesses with clay to simplify the mould.
- Venting and pouring: providing entry for material and an escape path for trapped air.
- Warning layer: tinted plaster placed against the positive in waste moulds to signal where to stop carving.
Workshop Safety
Plaster dust is a respiratory irritant. Plaster must never go down a sink drain, as it sets underwater and blocks pipes.
Common pitfall: treating small preparation steps as optional fussiness. Keys, vents, and warning layers prevent catastrophic cast failures. They are easy to add during setup, but impossible to fix afterwards.