Moulding, casting, finishing and mounting
This domain tests the sequence from original to finished cast: choosing the right mould, mixing plaster correctly, finishing without destroying detail, and mounting the piece so it is safe and presentable.
Mould types
- Waste mould: a plaster jacket over a clay or plasticine original; no keys, no separating agent needed because the original is destroyed; broken off to release one cast.
- Piece mould: the form is divided with shims into sections, each with registering keys; a separator (soft soap, oil or alginate) lets the pieces be removed and reused. Essential for undercuts such as ears, arms and drapery.
- Flexible moulds: alginate, gelatine or silicone for undercut detail; usually backed with a plaster mother mould.
Plaster and finishing
Sprinkle plaster into water (about 2:1 by weight), allow it to slake, stir gently to avoid bubbles, and pour from one point. The set is exothermic. After casting, fettle the seams, sand wet or damp to control dust, seal the surface, then patinate or wax. Mount on a base with a brass rod or epoxy, felt the underside, and check that the centre of gravity sits over the base.
Worked example
A portrait head with deep eye sockets and projecting ears cannot be pulled from a one-piece waste mould without losing the ears. Divide the head with a shim along the ear line into a front and back piece, cut keys into the shim edge, and cast in two halves. Join with fresh plaster slip and chase the seam.
Traps
No release agent in a piece mould; lumps of plaster in the mix; pouring before the plaster has slaked; demoulding before the cast is cold and firm; sanding dry plaster without a mask; a heavy cast on a small base.