In ceramics, the permeability of clay slurries and plastics determines the rate as which water can move through the matrix
Ceramic slurries with high permeability cast quickly. Knowing how quickly is important in manufacture. Slurries with more fine clay particles are less permeable (but also less plastic and of lower dry strength). Testing equipment measures it as Bariod Permeability. This is done in a small pressure vessel with a filter membrane and outlet. After pressurizing for a set time the slip is drained and the filter clay examined (weighed, thickness measured).
Clays with low permeability dry more slowly (ball clays). Short, non-plastic clays (e.g. kaolins) dry much more quickly. Bentonite is among the least permeable clays. Bentonite powder, if wetted, can form a gel layer that blocks the passage of any more water.