drying shrinkage
Watch a mud puddle dry and you will see it pull inward, sometimes leaving a shrunken cake ringed by a gap where it used to touch the edges. A shaped ceramic does the same, only more so: as the water leaves, the whole body gets measurably smaller. Drying shrinkage is that reduction in size — the body contracting as the forming liquid evaporates and the particles draw together into tighter contact.
The cause is capillary stress: each departing layer of water lets the menisci pull the particles a little closer, and the body shrinks by very nearly the volume of water removed — all of it during the constant-rate period, and none once the particles touch. Ceramists quote it as a linear shrinkage, the fractional change in a length: (L_wet minus L_dry) divided by L_wet. For plastic clay bodies this runs from a couple of percent up to 8 to 10 percent; for a well-packed pressed powder with little liquid it may be under 1 percent. The wetter and finer the body, the more it shrinks. Drying shrinkage is separate from, and comes before, the much larger firing (sintering) shrinkage.
Shrinkage is not itself a defect — a body that shrinks uniformly just ends up smaller, and you allow for it in the mould. The trouble is that shrinkage is where all the drying stress lives: if one region shrinks before or more than another, the mismatch pulls the body out of shape or tears it. So drying shrinkage must be designed for (moulds and dies are cut oversize to the exact shrinkage) and, above all, made uniform. Controlling the amount and the evenness of drying shrinkage is most of what a drying schedule is trying to do.
Press a line 100 mm long into a wet clay tile, dry it, and measure again: it might read 94 mm — a 6 percent linear drying shrinkage. A mould-maker who wants a 100 mm finished green tile therefore cuts the mould about 106 mm, letting the shrinkage bring it home to size.
Linear drying shrinkage (L_wet minus L_dry) over L_wet is designed into moulds; the danger is not the shrinkage itself but any unevenness in it.
Do not confuse drying shrinkage with firing shrinkage. Drying shrinkage happens near room temperature as water leaves and stops once particles touch; the larger sintering shrinkage happens red-hot as pores close. A body's total size change is the two combined.