drying

You have just shaped a wet pot, a cast slip, or a pressed damp cake. It is soft, heavy with water, and if you fired it now it would explode. Drying is simply getting that liquid out — but it is not the trivial step it looks like. A shocking share of the cracks that turn up in a finished ceramic were actually born here, quietly, before the kiln was ever lit. Drying is the removal of the forming liquid (usually water) from a green body, done in a way that does not tear the body apart.

Drying is not one smooth process but two very different regimes back to back. In the first, the constant-rate period, the whole surface stays wet, water evaporates as fast as if it were an open dish, and the particles slide closer together as the water between them leaves — so the body shrinks. In the second, the falling-rate period, the particles have touched and can pack no closer; the water surface (the menisci) retreats down into the pores, evaporation slows and slows, and shrinkage stops. The switch-over happens at the critical moisture content. Almost all the danger — the shrinkage, and the stress that can crack the piece — is packed into the first period.

Because drying shrinkage and drying stress happen only while the body is still wet and weak, the whole art is to dry the wet stage slowly and evenly and only speed up once shrinkage has finished. A thick tile dried too fast forms a dry, shrunken skin over a wet, un-shrunken core; the skin is stretched, and if that tension beats the body's feeble green strength, it cracks or warps. Every traditional and advanced ceramic — a slip-cast sink, an extruded honeycomb, a tape-cast capacitor layer — passes through this pre-firing gauntlet, and a good drying schedule is what gets it through intact.