the constant-rate period
Set a shallow dish of water on the bench and watch it evaporate: the level drops at a steady pace, hour after hour, as long as the surface stays open and wet. A freshly formed clay body dries the same way at first. Its whole outer surface is covered by a continuous film of liquid, water leaves that film into the air at a constant rate, and — this is the crucial part — capillary action pulls fresh water up from inside to keep the surface wet, like a wick. This is the constant-rate period, the first stage of drying.
Because the surface behaves like a free water surface, the drying rate is set entirely by the outside air — its temperature, its humidity, and how fast it moves over the piece — not by the ceramic. The evaporation rate stays roughly constant (hence the name). But every drop of water that leaves from between the particles lets them slide a little closer, so the body shrinks by very nearly the volume of water lost. In a plastic clay body the constant-rate period can account for essentially all of the linear drying shrinkage — often a few percent, up to 8 to 10 percent for very plastic clays. The surface, drying and shrinking first, is where the trouble starts.
The constant-rate period is both the productive stage and the dangerous one. It is productive because shrinkage — the packing of particles — happens only here. It is dangerous because a wet, shrinking surface sitting on a wetter, not-yet-shrunk interior builds up stress; push the drying too hard and you crack or warp the piece right at the start. So a good schedule spends the constant-rate period going deliberately slow: warm, humid, gently moving air, keeping surface and core drying in step until shrinkage is safely finished.
Weigh a wet cast tile every ten minutes as it dries. At first it loses the same mass each interval — a straight, sloping line on the graph — and a ruler shows it steadily shrinking. That straight-line stretch is the constant-rate period; the graph bends only when the surface finally dries out.
During the constant-rate period the body loses water at a steady pace and does all its shrinking; the rate is set by the air, not the clay.
It surprises many people that in this stage the clay does not control its own drying rate — the room does. Two identical bodies in humid versus dry air dry at wildly different speeds, and it is the fast, dry-air case that cracks. Slowing the constant-rate period is the single most effective crack-avoidance move.