a drying crack
A dried-up riverbed or a clay flat in the sun breaks into a network of cracks with curled edges — the mud shrank, could not shrink evenly, and tore itself apart. A ceramic body can do the identical thing, on a smaller scale, before it is ever fired. A drying crack is a break that opens in a green body during drying, when the tension from uneven shrinkage grows larger than the weak body can hold.
The chain is always the same. Drying pulls water out; capillary stress squeezes the body; if the surface dries faster than the interior, the shrunken skin is stretched in tension over the un-shrunk core (differential shrinkage). A green body is very weak — its green strength is only a few MPa at best, and it is far weaker in tension than in compression, like all ceramics. When the local tensile stress from shrinkage mismatch exceeds that meagre strength, the surface splits. Cracks favour the fastest-drying spots and stress-raisers: exposed edges and corners, sharp inside angles, thickness changes, and any forming flaw or void already lurking in the body.
Drying cracks are among the most common and most maddening ceramic defects precisely because they are born invisibly and often only announced later — a hairline seeded during drying can lie hidden until firing opens it wide, so the kiln gets blamed for a wound inflicted before it. The cures all attack the same root: shrink slowly and evenly. Dry in humid, gently moving air; retard fast-drying edges (wax, tape, or shielding); avoid abrupt thickness changes and sharp corners in the design; and add binder to raise green strength so the body can better resist the shrinkage tension.
A mug cracks right where the handle meets the wall. There the thick handle stays wet while the thin wall dries and shrinks fast; the wall pulls on the still-large handle joint, tension concentrates at the sharp inside corner, and the weak green clay splits. Rounding that corner and slowing the drying keeps it whole.
A drying crack opens where shrinkage tension beats green strength — usually at edges, corners, and thickness changes that dry unevenly.
A crack you cannot see is still a crack. Much drying damage is sub-visible until firing widens it, so a 'firing crack' is frequently a drying crack in disguise. Prevention in the drying room beats hunting for the culprit after the kiln.