the falling-rate period

Keep watching that drying clay after the surface has finally stopped glistening. The water no longer sits as a film on the outside; the wet region has withdrawn into the body, and now each bit of water has to evaporate deep inside a pore and then travel out as vapour through the maze of already-dry pores above it. That is slow going, and it gets slower as the wet front sinks deeper. The drying rate, which had held steady, now falls and keeps falling. This is the falling-rate period, the second and final stage of drying.

The key change is that the particles have already come into contact — they are touching, packed as tight as they will get — so removing the remaining water no longer pulls them any closer. Two consequences follow. First, shrinkage essentially stops: the body is now at very nearly its final dried size. Second, the rate is no longer controlled by the outside air but by how fast water vapour can diffuse out through the dry pore network, so it depends on the body's thickness and pore structure. Removing that last, tightly held water is slow — often the longest part of the whole drying time.

Here is the reassuring news buried in a tedious stage: because the particles no longer move, the falling-rate period is comparatively safe. The great cracking danger — differential shrinkage between skin and core — is over once shrinkage is over. This is exactly why drying schedules are cautious early (through shrinkage) and then get bolder: once you are past the critical moisture content and into the falling-rate period, you can raise the temperature and lower the humidity to drive out the last water quickly, with far less risk of cracking.