Processed ceramic materials are typically ground to 200 mesh and feel very fine to the touch. With some you can detect some particle grains between your fingers. The amount of these "physical particles" can be measured by washing or shaking the ceramic powder through a sieve. Using water washing and standard wire mesh sieves it is normally only possible to determine the range of particle sizes of a powder sample down to 325 mesh (about 40 microns). Yet it is common for 90% (even 99%) of a powder to be composed of minus 40 micron ultimate particles. Even the physical particles we can measure on sieves are often agglomerates of hundreds or even thousands of ultimate particles. Almost all ceramic materials are composed mainly of ultimate particles. Ball clays, for example, have particles one tenth of a micron in size, 400 times smaller than 325 mesh. Understanding materials fully means being aware of these particles, their sizes, shapes, densities, etc. An interesting example to illustrate is a water-washed and processed large-particle-size kaolin intended for the casting process. It is likely that 99.9% of such a material will wash right through a 325 mesh screen, making it appear to be a very fine powder. It also feels exceedingly fine to the touch. However, in terms of ultimate particles and in relation to other clays, it has a very large particle size. On the other hand, a plastic kaolin may leave residue on a 200 mesh screen and appear to be coarser, whereas actually its ultimate particles could be 10 times smaller.
Can we ball mill a clay and make it more colloidal? Yes.
This 1000 ml 24 hour sedimentation test compares Plainsman A2 ball clay ground to 10 mesh (left) with that same material ball milled for an hour (right). The 10 mesh designation is a little misleading, those are agglomerates. When it is put into water many of those particles break down releasing the ultimates and it does suspend fairly well. But after 24 hours, not only has it settled completely from the upper section but there is a heavy sediment on the bottom. But with the milled material it has only settled slightly and there is no sediment on the bottom. Clearly, using an industrial attrition ball mill this material could be made completely colloidal.
Which clay contains more soluble salts?
Example of sedimentation test to compare soluble salts water extracts from suspended clay. This simple test also reveals ultimate particle size distribution differences in clays that a sieve analysis cannot do.
How small can clay crystals be?
Table salt crystals on a 60 mesh screen. It has an opening of 250 micro meters (these are the half of the crystals that passed this size). Notice on the right, several crystals are in the openings, about to fall through. Imagine that bentonite or ball clay crystals can be 0.1 um in diameter, that is 2500 times smaller on a side. That would be 2500x2500 on a layer the size of a salt crystal and the thickness of a clay crystal. Since the clay crystal is much thinner than wide, perhaps ten could stack to the same dimension. That means theoretically 2500x2500x25000 could pack into a grain of salt!
This is what labs use to measure particle size
To measure particle size in a slurry or powder you need sieves. This is the most popular type used in labs. They are made from brass by a company named Tyler. The range screen sizes for testing particle size is very wide. The top screen has an opening of 56 mm (that size and smaller pieces can fall through). The bottom sieve has an opening of 0.1 mm, the wires are almost too small to see. Coarser and finer sieves are available. You can buy these on ebay for a lot less than new ones, just search for tyler sieves. Keep in mind that the finer sieves (especially 325) are fragile and easily ripped. We use a series that bottoms out at 200.
The white one feels smoother, but it is actually far coarser. Why?
Large particle kaolin (left) and small-particle ball clay (right) DFAC drying disks demonstrate the dramatic difference in drying shrinkage and performance between these two extremes (these disks are dried with the center portion covered to set up a water content differential to add stresses that cause cracking). These materials both feel super-smooth, in fact, the white one feels smoother. But the ultimate particles tell the opposite story. The ball clay particles (grey clay) are far smaller (ten times or more). The particles of the kaolin (white) are flatter and lay down as such, that is why it feels smoother.
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