slip casting
Watch a paper towel touch a puddle: it wicks the water up and away, leaving the solid bits behind. Slip casting works on exactly that principle, run in reverse to build a wall. You pour a fluid, stable suspension of ceramic powder in water — called a slip — into a mould made of porous plaster. The fine pores of the plaster act like a sponge and pull the water out by capillary suction. As the water leaves, particles pile up against the mould surface and consolidate into a firm layer, so a solid wall grows inward from the mould face while you watch.
The kinetics are pretty and useful. The mould drinks water fastest at first, then slows as the growing cast layer itself becomes the bottleneck the water must seep through. To good approximation the cast-wall thickness L grows with the square root of time: L is proportional to sqrt(t). So doubling the wall thickness takes four times as long — the reason large slip-cast parts are left in the mould for many minutes to hours. A well-behaved slip is the hidden hero here: it must be highly loaded with solids yet still pour like cream, which takes careful deflocculation (a dispersant plus the right pH) to keep the particles repelling one another so the suspension stays low in viscosity and does not settle.
Slip casting is the ancient workhorse of complex hollow shapes that no die can press: sanitaryware (toilets, basins), teapots and figurines, and, in the advanced world, crucibles, thermocouple sheaths, and turbine components. Its virtues are cheap moulds and free-form geometry; its costs are slowness, labour, and green bodies of modest, sometimes uneven density. If the slip is poorly dispersed the particles pack loosely, and any trapped air bubble or plaster fleck becomes a flaw that firing will not remove.
To cast a thin-walled alumina crucible, a deflocculated Al2O3 slip at ~50 volume percent solids is poured into a plaster mould. Over about 30 minutes the plaster wicks out water and a wall grows to a few millimetres; the near-net crucible is then dried, lifted from the mould, and fired. Doubling that wall would take roughly two hours, because thickness follows sqrt(time).
A porous plaster mould sucks water from the slip, building a cast wall whose thickness grows as the square root of time.
The slip must be stable, not just wet: a poorly deflocculated, flocculated slip casts fast but packs loosely and unevenly, giving low, gradient green density. The chemistry that keeps it pourable belongs to the colloids field; slip casting only draws on it.