sedimentation
Sedimentation is simply particles sinking. Any particle denser than the liquid around it is pulled down by gravity, and given time it drifts to the bottom and forms a sediment — mud settling out of a river, grounds sinking in a cup of coffee. For a ceramic slurry this is a constant threat: the powder is far denser than water, so left standing it tends to settle, and how it settles decides whether you can still use it and what kind of cast body it will make.
How fast a lone particle sinks is given by Stokes' law: v = (2 times r^2 times (rho_p - rho_f) times g) / (9 times eta), where r is the particle radius, rho_p and rho_f the densities of particle and fluid, g gravity, and eta the liquid's viscosity. The lesson is in the r^2: settling speed goes as the square of size, so a particle ten times bigger sinks a hundred times faster. That is why fine, sub-micron ceramic powders settle slowly and coarse grit drops out first, and why a size distribution segregates as it settles — the coarse particles reaching the bottom before the fines. But the deciding factor is really the state of dispersion. A well-dispersed slip settles slowly, particle by particle, into a tightly packed, dense, hard sediment that is difficult to re-stir. A flocculated slip, whose particles have joined into big open flocs, settles fast but into a bulky, loose, high-porosity sediment that is easy to re-disperse. A strongly flocculated slip with a real yield stress may not settle at all, because its particle network holds everything up.
This makes sedimentation both a headache and a diagnostic. It is a problem when a stored slip separates or when a casting segregates coarse from fine, leaving a green body with a density gradient top to bottom; controlling it is why slips are kept dispersed, or deliberately given a mild yield stress, or simply stirred before use. But watching how a slurry settles is also a classic cheap test of its colloidal state: a slip that drops fast to a fluffy bed is flocculated, while one that settles slowly to a hard, dense cake is well dispersed. In slip casting the same physics is turned to advantage — the mould pulls liquid out and consolidates particles into the cast wall, which is sedimentation and filtration working for you rather than against you.
Pour the same alumina powder into two jars. In the dispersed jar the particles trickle down over hours into a thin, hard, dense cake stuck to the bottom. In the flocculated jar the particles fall in minutes as big flocs, piling into a tall, loose, cloudy sediment you can swirl back up with a shake. Same powder, opposite sediments — a five-minute read on the colloidal state.
A dispersed slip settles slowly to a hard dense cake; a flocculated one settles fast to a loose bulky bed — a simple test of dispersion.
Counterintuitively, a well-dispersed slip settles slower but packs into a denser, harder cake, while a flocculated slip settles faster but into a looser, more porous bed. Fast settling does not mean dense packing — the sediment structure is what matters for the green body.