The Split Ear: Waste Mould or Piece Mould?
Ananya has four days left before the practical submission. Her portrait head, modelled at life size in grey terracotta clay, stands 28 cm high on a wooden board. It has a pierced earring hole about 12 mm across, a deep undercut beneath the chin, and a nostril that turns inward. A fan has dried the surface to leather-hard, and two fine cracks have opened at the neck. The studio has 8 kg of plaster of Paris, one 20-litre bucket, soft soap, cooking oil, and no fibre reinforcement. The teacher is on external duty and has left a note: 'Decide the mould yourself; one good cast is enough for marking.' Ananya wants to make a two-piece piece mould so she can cast twice, but she has never made one. A waste mould would be simpler, yet it will destroy the clay original. The external examiner will look at the cast's surface and the sharpness of the ear hole. The room is 22 °C and the clay is still cool.
1. Which mould should Ananya make, and which details in the setup decide that choice? Give a recommendation with one condition that would change it.
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Decision: make a two-part waste mould, not a permanent piece mould. The deciding facts are: only one cast is needed for marking; Ananya has never made a piece mould; 8 kg of plaster is a thin margin for a permanent two-piece mould with a mother mould; and a failed piece mould would cost both the cast and the clay. A simple one-piece waste mould would still fail because the undercut chin and inward nostril create a lock. A two-part waste mould splits along a clay shim under the jaw and behind the ears, gives one good cast, and is meant to be broken off.
Condition that would change the answer: if the clay original had to be preserved, or two or more casts were required, the waste mould would be wrong. Then a piece mould would be justified, but only with more plaster, reinforcement, a test join, and a registration plan.
Ear hole: the 12 mm hole does not force a piece mould. It needs a greased metal rod or dowel through the hole as a core pin. The same pin is reinserted before casting so the hole stays open.
2. Sequence the plaster work, reinforcement, opening and drying so the ear hole and undercut survive. Include what she should do about the missing fibre.
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- Stabilise the clay: support the head in a sandbag or soft cloth; do not handle the cracked neck. Clean the surface and apply a thin coat of soft soap as a release; avoid cooking oil on the face because it can stain and retard set.
- Plan the split: mark a shim line under the jaw, up behind the ears, and over the crown. Build a 5-6 mm clay shim, press clay keys (buttons) into it, and add a clay registration edge. The shim controls where the two plaster halves meet.
- Mix in small batches: sprinkle plaster into water at about 2:1 plaster to water by weight; stir gently to avoid bubbles; use 1-2 kg batches. Apply a thin face coat with fingers or a soft brush, working it into the ear hole and chin undercut; then build up to 2-3 cm.
- Reinforcement without fibre: if clean cotton rag is available, tear strips, dip them in plaster, and lay them in the shell. If not, build a thick shell and bed the mould in a sand box during casting; a thin unreinforced shell will crack.
- Ear core: insert a greased 12 mm rod through the ear hole before the plaster sets; withdraw it when the plaster is green.
- Open and dry: after 30-45 minutes remove the shim and separate the halves gently. Remove clay with wooden tools. Dry the mould 24-48 hours at room temperature, not near a heater. Before casting, seal with soft soap, reinsert the greased rod, and cast.
3. The examiner may ask why Ananya did not simply fill the undercut with clay. Explain the trade-off and what would be lost.
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Filling the undercut with clay is the fast, low-risk option, but it changes the form. The chin undercut is a focal separation: it lets the jaw read against the neck and catches a shadow. Filling it would make the chin look merged into the neck and would flatten the portrait's likeness. Because a two-part waste mould can handle the undercut, there is no need to fill it. If time collapsed, a minimal fill could be justified only on a non-focal area, and Ananya should record that compromise in the viva. The examiner is testing whether she knows that a mould is not a neutral copy: every split, shim, and filled undercut is an interpretation. A piece mould would preserve the clay but carries a higher risk of misregistration; a waste mould sacrifices the clay but gives one clean cast. With one cast required, that trade-off is acceptable.