|Monthly Tech-Tip |
In ceramics, the permeability of clay slurries and plastics determines the rate as which water can move through the matrix
The term "permeability" can occur in multiple contexts in ceramics. Most often it refers to how well water can penetrate a clay (either in its dried solid form, or as a powder). Bentonites, which can have incredibly high surface areas, are among the least permeable clays. And they take the longest to dry. Bentonite powders, if wetted for example, can form a thin gel layer only millimeters thick, that blocks the passage of any more water. Koalins, which have large particles and can be quite non-plastic, are very permeable to water. And they dry quickly. Some non-clay minerals can also exhibit resistance to the penetration of water.
Interestingly, clays that have already been brought to the wet, plastic form, are quite impermeable. But when dried out completely, they will slake rapidly in water.
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 more plastic and of higher 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).
Texas talc (left) quickly absorbs all the water poured on it. Montana talc (right) resists whetting of the particles much more, the water is just sitting on top and has not penetrated at all.
Plainsman A2. In spite of three times the normal rain fall this summer, its natural high impermeability made it shed almost all of the water. But there is a back story. When mined this clay was quite wet, about 10% moisture - some of the lumps were 100 lbs! The outer ones dried over a period of weeks and, in the process, fractured down to the inch-size you see here. But the inner section of the pile took years to dry out. By now it has broken down so fine that a completely dry sample put into water will slake in minutes to produce a smooth slurry.