Drying Means the Water Has to Leave
Guide 1 set the scene: a freshly formed green body is a fragile heap of powder held together by a little liquid and, often, a little binder, and it has to survive the dangerous middle passage from wet shape to furnace. The very first thing that has to happen on that journey is drying — the plain removal of the liquid. Whether the piece was pressed slightly damp, cast from a slip, or thrown as wet clay, water is sitting in the gaps between the particles, and every drop of it has to get out before real firing can begin. Fire a body that still holds water and the water will flash to steam inside it and blow it apart.
Here is the surprise that this whole guide turns on: drying is not one smooth process but two distinct acts with a sharp turning point between them. In the first act the body actually shrinks as it dries, and the particles are pulled steadily closer together. In the second act the body has stopped shrinking, and the water that is left retreats and evaporates from deep inside. The turning point between the two — the moment the shrinking stops — is where nearly all the danger lives. Understand these two acts and you understand why a part warps, cracks, or comes out fine.
A useful mental picture: think of the wet body as a sponge that is also slightly sticky. As water leaves the outer surface, more water is drawn up from the inside to replace it, the way a paper towel wicks a spill. The particles do not simply sit and wait for the water to go — they are actively squeezed together by the water as it retreats. That squeeze has a name and a size, and it is the real subject of this guide: capillary stress.
Act One: The Constant-Rate Period, Where the Body Shrinks
The first act is the constant-rate period, and its name tells you the key fact: water leaves the surface at a steady, constant rate, roughly as fast as it would evaporate from an open dish of water. Why so steady? Because the whole outer surface stays wet. Every drop that evaporates from the surface is immediately replaced by water wicked up from the interior, so the surface never dries out — it behaves exactly like a free water surface, and it evaporates at whatever pace the surrounding air, temperature, and airflow allow.
And here is the crucial thing about act one: as the water leaves, the particles move closer and the whole body shrinks. This is drying shrinkage, and essentially all of it happens right here, in the constant-rate period. Picture wet sand at the beach: while it is soaked, the grains are held slightly apart by water films; as it dries, the grains draw together and the sand pack tightens and shrinks. A wet clay body can shrink several percent linearly on drying — a plate visibly smaller off the shelf than it was on the wheel — while a barely-damp pressed body, which held little water to begin with, shrinks only a trace. The wetter the forming route, the bigger the drying shrinkage.
It is worth pausing to name the mechanism, because it is not just 'water evaporates.' The particles are being actively pulled together. As the liquid retreats into the ever-narrower gaps between particles, the water surface curves into tiny concave menisci bridging particle to particle, and each of those curved surfaces is under tension, tugging the neighbouring particles toward one another. Multiply that tug over billions of contacts and the body as a whole is squeezed inward. The water is not passively draining away; it is doing mechanical work on the powder, and that work is drying shrinkage.
Capillary Stress: The Invisible Hand
That tug from the curved water surfaces is capillary stress, and it is not a metaphor — it is a pressure with a real number. A curved liquid surface of radius r generates a pressure difference of about P = 2γ/r, where γ is the surface tension of the liquid (the same thing that lets a water strider stand on a pond). The tighter the curve — that is, the smaller the pore the water is bridging — the larger the pressure. Because a drying body's pores are microscopically small, that pressure is far larger than you might guess.
Put in numbers. For water, γ ≈ 0.072 N/m. In a fine ceramic body the water bridges sit in gaps of order 0.1 micron, so r ≈ 1 x 10^-7 m, and P ≈ 2 x 0.072 / (1 x 10^-7) ≈ 1.4 x 10^6 Pa, about 1.4 MPa. Widen the pores tenfold to 1 micron and the stress drops tenfold to about 0.14 MPa. Now compare that to the green strength of the body itself — often just a fraction of a megapascal to a few MPa. The capillary stress is the same size as the strength holding the part together. That is not a coincidence; it is the whole tension of drying in one comparison.
The Turning Point and Act Two: The Falling-Rate Period
Act one cannot last forever. As the particles draw together, they eventually touch and jam into a rigid, close-packed skeleton — they simply cannot move any closer. At that moment the body stops shrinking, even though it is still full of water. That special moisture level is the critical moisture content, and it is the great turning point of drying. Reaching it is actually good news: the most dangerous part is over, because once the particles can no longer move, further water loss no longer changes the body's dimensions, so it can no longer set up shrinkage stresses.
DRYING RATE vs MOISTURE (the two acts of drying)
rate
|
| ___________________ ACT ONE
| | \ constant-rate period:
| | water leaves the |\ surface stays wet,
| | surface as fast | \ body SHRINKS,
| | as it evaporates | \ particles pull together
| | | \___ ACT TWO
| | | \____ falling-rate period:
| | | \___ front retreats inward,
|___|____________________|________________\____ shrinkage STOPS
wet <---------------- ^ ---------------> dry
CRITICAL MOISTURE CONTENT
(particles now touch and jam)Past that turning point comes the second act, the falling-rate period. Now the outer surface can no longer stay wet — the liquid can no longer be wicked all the way up to it — so the drying front retreats inward, into the pores. Water evaporates deep inside the body and the vapour must then diffuse out through the maze of already-dry pores near the surface. Because that escape path gets longer and longer, the drying rate falls steadily away, which is exactly what gives the period its name. The reassuring part is that all of this happens with no more shrinkage: the skeleton is set, so this act, though slow, is comparatively safe.
Why It Warps and Cracks — and Drying Slow and Even
Now we can see exactly where drying goes wrong, and it is always in act one. The trouble is that a part does not dry all at once — the surface, exposed to the air, dries and shrinks first, while the interior is still wet and full-sized. The dry, shrinking skin is stretched over a fat wet core that refuses to shrink with it, so the skin goes into tension, and a ceramic is weak in tension. If that tension exceeds the meagre green strength, the surface cracks. The same uneven, one-sided shrinkage also bends a part out of true — a slab dries and shrinks on top before the bottom, and it curls. This mismatch is the differential shrinkage that guide 3 will take up in full; here the point is simply that its engine is uneven capillary stress during act one.
Two things make this worse, and both come from geometry. Thick parts are dangerous because the surface can dry long before water from the deep interior can reach it, so the surface-to-core mismatch is large — a thick block dries far more treacherously than a thin sheet. Nonuniform parts are dangerous because thin sections dry and finish shrinking while thick sections are still going, so the two fight each other across the join. This is why an even wall thickness is a drying virtue, and why a sudden change from thick to thin is a classic place for a drying crack to start.
- Start slow and wet. Keep the surrounding air humid at first so the surface evaporates no faster than water can be wicked up from the interior — this keeps the whole part shrinking together through act one, when all the danger lives.
- Dry evenly, not just slowly. Shield exposed edges and thin sections that would race ahead, so the surface and the core stay close in moisture and the shrinkage stays uniform.
- Once the part reaches the critical moisture content and stops shrinking, you can safely speed up — raise the temperature and lower the humidity, because the falling-rate period no longer makes shrinkage stresses.
- For a thick or complex part, stretch the schedule out — hours to days — because the safe drying rate is set by how fast water can travel from the deep interior, not by how fast the surface could evaporate.