Modelling Techniques (Clay, Plaster, Wax, Papier-mâché)
This domain tests the working properties of clay, plaster, wax and papier-mâché, and the sequence of steps that keeps a model sound. Questions often ask why a piece cracked, why a mould failed, or which material suits a given form.
Material rules
- Clay: wedge to remove air, build over an armature for tall or thin forms, keep unused clay wrapped, and hollow thick masses before bone-dry stage to prevent cracking and explosion in firing.
- Plaster: add plaster to water, sprinkle to form islands, allow to slake, then stir gently; use release agents on moulds; do not pour set plaster down drains.
- Wax: melt in a double boiler; keep the temperature low; use it for lost-wax patterns and for flexible detail; wax shrinks on cooling.
- Papier-mâché: tear paper into strips, apply paste in layers, dry each layer or dry slowly, and seal the surface before painting. Too many wet layers at once cause mould and warping.
Worked example
A student builds a 25 cm clay figure solid around a wooden core and finds a crack after two days. Reasoning: the outer clay dried and shrank while the inner clay stayed damp, so tension opened a crack. The correction is to hollow the figure when leather-hard, remove the core, join the sections with slip, and dry slowly under cloth.
Traps that cost marks
- Adding water to plaster instead of plaster to water.
- Using wet clay on a porous plaster mould without a release agent.
- Overheating wax until it discolours or ignites.
- Building papier-mâché layers too wet and too fast.