Sketch, maquette and model
A sculptor plans in steps. The sketch is a drawing on paper from the front and side. The maquette is a small 3D model, often made of clay or wax, that you can hold in your hand. It lets you test the pose and balance cheaply. The model is the full-size clay figure. The mould is made from this model.
Why not skip the small one? A mistake in a tiny maquette costs minutes. A mistake in a full-size model can cost days.
What is casting? The mould idea
Casting is making a copy by pouring something liquid into a mould and letting it become hard. The mould is a negative: its hollow is the reverse of the model. The copy is the positive.
Things that can be poured: plaster, clay slip (thin runny clay), wax, resin, and melted metal. Casting lets an artist make one fragile clay figure last for years, or make many copies.
Waste mould in plaster
Plaster of Paris is a white powder. Mixed with water it is runny, then heats up slightly and sets hard in about 20 minutes. To make a waste mould the sculptor covers the model with plaster. Often a thin coloured layer goes first and a white layer on top. When the cast is ready, the sculptor chips the mould away. If the chisel reaches the colour layer, the sculptor knows the cast is near.
It is called waste because it is thrown away. You get only one cast from it. It is good for deep, undercut shapes, like a hand with open fingers.
- Wet the surface or use a release agent (soap or oil) so plaster does not stick.
- Add plaster slowly to water, not water to plaster, to avoid lumps.
Piece moulds and rubber moulds
A piece mould is made of two or more plaster pieces that fit together and open along seam lines. Because the pieces come apart without breaking, the mould can be used again and again. Pieces are placed so that no part of the model locks the mould in (no undercut across a seam).
A rubber mould uses silicone or latex. Rubber is soft: it stretches to come off the model and springs back, so it can copy very fine detail, like skin texture or leaves. Because soft rubber droops, a hard plaster jacket is made around it.
| Mould | Uses | Good for |
|---|---|---|
| Waste | One | Deep, tricky shapes |
| Piece | Many | Simple shapes, many copies |
| Rubber | Many | Fine detail |
Terracotta: hollowing and firing
Terracotta means baked earth: clay fired to a reddish-brown colour at around 900 to 1000 °C. Clay holds water. When it dries, it shrinks. A thick, solid lump dries slowly and unevenly, so it can crack, and any trapped water turns to steam in the kiln and can burst the piece.
So terracotta figures are hollowed: the wall is kept about 1 to 2 cm thick and even. A small air hole is left so steam and air can escape. Steps: press or slip-cast clay into the mould, let it firm up, remove it, hollow it out, dry it slowly for days, then fire it in a kiln.
Fired clay can also be kept safe with simple care: store it dry, handle it with both hands, and clean it with a soft brush. Keeping a work in good condition is called conservation.
Try it: make a plaster footprint
- Press a toy or a coin into a tray of damp sand. Lift it out. The hollow is your mould (the negative).
- Mix plaster of Paris and water to the thickness of cream and pour it into the hollow. Ask an adult to help. Wait 30 minutes.
- Lift out the cast and compare it with the toy. Is the shape the same? Which side shows detail best?
- Predict: what would happen if the sand were dry and loose?
Key formulas and definitions
- Model → mould (negative) → cast (positive)
- Waste mould = used once; piece / rubber mould = used many times
- Hollow terracotta: wall about 1–2 cm, one air hole
- Terracotta firing temperature: about 900–1000 °C
- Key terms: maquette, negative, positive, undercut, release agent, slip, jacket
Worked examples
1. A sculptor wants to make 20 identical small figures. Which mould should she choose and why?
A piece mould or a rubber mould. Both can be used many times. A waste mould is broken after one cast, so she would need 20 moulds.
2. A plaster mould has a coloured layer under a white layer. Why does this help when the sculptor chips the mould off?
When chips of the coloured layer start to appear, the sculptor knows the cast is very close and works gently. This protects the cast from the chisel.
3. Why must a thick clay figure be hollowed before firing?
Thick clay dries slowly and unevenly, and trapped water turns to steam in the kiln. Both can crack or burst it. A thin, even wall dries evenly and the steam can escape through the air hole.
Common mistakes
- Thinking the mould is the copy. The mould is the negative; the poured and set material is the copy.
- Forgetting a release agent, so plaster sticks to plaster or to the model.
- Leaving terracotta solid and thick, or with no air hole, then seeing it crack in the kiln.
- Planning a piece mould with a part that locks the pieces together (an undercut over the seam).