Mould Making, Casting and Plaster Work
Plaster is the backbone of casting and press moulding. The exam tests the plaster-to-water ratio, mixing discipline, mould design and the faults that come from bad plaster.
Plaster chemistry
Pottery plaster is calcium sulphate hemihydrate: mixed with water it rehydrates to gypsum and sets with slight expansion (about 0.2–0.3 %), which takes fine detail. Beta plaster (pottery plaster) is softer and more absorbent; alpha plaster (hydrocal) is harder and used for master and case moulds.
The rules
- Ratio: about 100 parts plaster to 70 parts water by weight for pottery plaster; weigh, do not guess.
- Add plaster to water, sprinkle evenly, let it slake 1–2 minutes, then mix for 2 minutes without whipping in air.
- Working time about 10–15 minutes; set in 20–30 minutes. Warm water speeds it up; dirty water or old plaster slows it.
- Release agents: soft soap, shellac, oil or varnish on the model; seal new plaster before use.
- Mould design: 3–5 mm clearance for slip, keys and cottles, no undercuts, uniform 2–4 cm thickness.
Worked example
A mould box measures 20 × 15 × 6 cm = 1800 cm³. Set plaster density is about 1.5 g/cm³, so about 2700 g of set plaster is needed. With a 100:70 mix, plaster = 2700 ÷ 1.7 ≈ 1590 g and water ≈ 1110 g.
Traps
- Adding water to plaster - lumps and an unusable mix.
- Mixing too long - the set weakens and the surface stays soft.
- Casting into a damp or unsealed mould.
- Letting set plaster fragments get into the clay bin; they blow pots apart in the kiln.