Most air dry clay models fail in one of two ways: they crack through a thick section as they dry, or a joined part falls off. Both are decided while the clay is still soft, which means the fix is a making decision rather than a rescue afterwards. Build hollow, join properly, and dry slowly.
Build hollow, or hollow it out
Any solid mass thicker than about two centimetres is a crack waiting to happen, because the outside shrinks while the inside is still full size. Build over a core you remove, build in hollow sections, or cut a finished solid form in half at leather hard, scoop it out and rejoin it. Sculptors have done the last of these for centuries and it is still the most reliable way to make a thick shape survive.
Joins that hold
Two pieces of clay pressed together part as they dry. Score both faces with a serrated rib or a needle, wet them with slip made from the same clay, press firmly enough to see the slip squeeze out, and smooth the join afterwards. On air dry bodies this matters more than on fired ones, because there is no firing to consolidate a weak join later.
Drying a model with thin and thick parts together
A figure with fine fingers and a heavy torso dries at two speeds and tears where they meet. Wrap the thin parts loosely in plastic for the first day so they cannot race ahead, then uncover progressively. Turning the model daily lets the base dry at the same rate as the top. This is patient work, and it is the difference between a model and a cracked model.
Armatures, and when not to use one
A wire armature supports a shape while it is soft and then refuses to shrink with the clay, which cracks the piece from the inside. If you use one, use it for a shape you will cut off and hollow, or use a soft removable core such as crumpled paper or foam that yields as the clay contracts. That one substitution prevents a failure people find baffling because it appears late and from within.
Questions people ask about air dry clay
Why did my air dry clay model crack?
Almost always a thick solid section, an unyielding armature, or one part drying much faster than another. All three are decided while the clay is soft.
How do I join two pieces so they stay joined?
Score both faces, wet them with slip made from the same clay, press until slip squeezes out, and clean up the seam.
Can I repair a crack after it dries?
You can fill it with slip or a filler, but there is no firing to fuse the repair, so the join stays weaker than the surrounding clay.