Clay modelling covers everything made by hand without a wheel, from a child's figure to a sculptor's maquette, and the material choice runs from a non-hardening compound through air drying bodies to fireable sculpture clays. What they have in common is that the making decisions, not the finishing, determine whether the piece survives.
Choosing clay for modelling
Three questions settle it. Will it be fired, which rules in or out the whole fireable range. Does it need to be kept, which rules out the reusable non-hardening compounds. And how big is it, because anything substantial wants a grogged body: grog is fired clay ground back in, and it cuts shrinkage and cracking at the cost of being coarse in the hand. For small detailed work a smooth fine body is far more pleasant.
Build hollow and it will survive
The single most common failure in modelling is a thick solid mass cracking as it dries, because the outside shrinks against a full-size interior. Build over a removable core, build in sections, or cut and hollow the piece at leather hard and rejoin it. Sculptors have used the last method for centuries and it remains the most reliable answer to a heavy form.
Armatures that do not fight the clay
A rigid wire armature holds a shape while the clay is soft and then refuses to shrink with it, cracking the piece from the inside as it dries. Use one only for a form you will cut off and hollow, or use a yielding core such as crumpled paper that collapses as the clay contracts. This is the failure that most surprises people, because it appears late and from within.
The four tools that do the work
A wooden modelling tool for shaping, a loop or ribbon tool for taking material away, a serrated kidney for scoring joins, and a rubber kidney for smoothing. Almost everything else in a modelling set stays in the box. Smoothing is done at leather hard with a damp sponge or a rubber kidney rather than by dry sanding a finished piece, which is both a better finish and the safe way round the silica.
Questions people ask about clay modelling
What is the best clay for modelling?
For small detailed work, a smooth fine body. For anything large, a grogged body, which shrinks and cracks less. If it will be fired, it must be a fireable body rather than an air drying one.
How thick can a modelled piece be?
Roughly two centimetres before you should hollow it, whether or not it will be fired. Thicker than that and drying becomes the enemy.
Do I need an armature?
Only for shapes that cannot stand while soft, and then a yielding core is safer than rigid wire, which cracks the piece as the clay shrinks around it.