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Ultimate Particles

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.

To effectively measure ultimate particle sizes advanced testing equipment is needed. These devices use xray or photographic techniques. For example, many devices simply take a micro photograph of an air suspended powder sample and then software analyzes the photo to produce the desired measurement. The rate of sedimentation in water can also reveal information about ultimate particles.

Can we ball mill a clay and make it more colloidal? Yes.

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?

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?

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

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?

The white one feels smoother, but it is actually far coarser. Why?

Large particle kaolin (left) and small-particle ball clay (right) DFAC tests (for drying performance) 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.

Out Bound Links

  • (Glossary) Colloid

    Colloidal particles are so small and light that they do not settle in water. Milk is colloidal. In true colloidal suspensions the movement of water molecules is enough to keep them from settling. Bentonite contains colloidal particles, but it also contains larger ones which also stay in suspension. ...

  • (URLs) Particle Characterization Instruments at Horiba Scientific


In Bound Links

  • (Glossary) Particle orientation

    Clay particles are flat and prefer to orient or arrange in a lattice-like pattern during mixing and forming operations. In bodies where they mix with other non-clay ingredients, they continue to arrange to the extent to which they can tolerate the disruptions of the others. The smaller the clay part...

  • (Tests) AVPS - Average Particle Size (Microns)
  • (Tests) MDPS - Median Particle Size (Microns)
  • (Tests) L1M - % < 1 micron
  • (Tests) L2M - % < 2 microns
  • (Tests) L10M - % < 10 microns
  • (Tests) L20M - % < 20 microns
  • (Tests) UPSD - Ultimate Particle Size Distribution
  • (Tests) L5M - % < 0.5 microns
  • (Glossary) Firing Shrinkage

    As kiln temperature increases bodies densify (particles pack closer and closer). As temperature continues to rise, some of the particles begin to melt and form a glass between the others that pulls them even closer. Some of the particles shrink themselves, kaolin is an example (in the raw state part...

  • (Glossary) Plasticity

    This term is used in reference to clays (or more often bodies which are blends of clay, feldspar and silica particles) and their ability to assume a new shape without any tendency to return to the old (elasticity). Plasticity is a product of the electrolytic character of flat clay particles (they ha...

  • (Glossary) Particle Size Distribution

    When minerals and mixtures of minerals are ground into a powder a particle size distribution is produced, that is, populations of particles of various sizes. The relative sizes of these populations can be measured to rationalize the behavior of the powder in the ceramic process. Wide distribution...

  • (Glossary) Surface Area

    Surface area (or SSA - Specific Surface Area) is a physical property you will see listed on the data sheets and certificates of analysis of many materials. Some materials (e.g. sedimentary clays) can contain particles that have a wide range of sizes, shapes, densities, surface texture, reactivities...

By Tony Hansen

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