All Glossary

Particle Size Distribution

Knowing the distribution of particle sizes in a ceramic material is often very important in assessing its function and suitability for an application.

Key phrases linking here: particle size distribution, psd - Learn more

Details

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 distributions of sizes in a clay, for example, produce the most density in the dried product. When the particles are all of similar sizes, a less dense matrix is produced. Clays of higher density are stronger in the dry form. Particle size distribution (PSD) measurements are an excellent way to characterize a material for quality control purposes. When problems occur in a ceramic process, particle size distribution information about the materials and body can help explain the causes. PSD information is also critical to spot wear in grinding equipment. Data sheets of most ceramic minerals contain at least some PSD information.

Related Information

Ball clay powders are minus 200 mesh. Right? Wrong!

These are the oversize particles (from the 79, 100, 140 and 200 mesh sieves) from 100 grams of a commercial Gleason ball clay. They have been fired to cone 8 oxidation. There is 1.5 grams total, this is within the limits stated on their data sheet even though the material is sold as 200 mesh grade. Firing the samples shows whether the particles contain iron that will produce specking in porcelains and whiteware. In this case there are a few. We do this test on many materials and this is typical of what we see.

Oversize particles in a typical manufactured porcelain body

Example of the oversize particles from a 100 gram wet sieve analysis test of a powdered sample of a porcelain body made from North American refined materials. Although these materials are sold as 200 mesh, that designation does not mean that there are no particles coarser than 200 mesh. Here there are significant numbers of particles on the 100 and even 70 mesh screens. These contain some darker particles that could produce fired specks (if they are iron and not lignite); that goodness in this case they do not. Oversize particle is a fact of life in bodies made from refined materials and used by potters and hobbyists. Industrial manufacturers (e.g. tile, tableware, sanitaryware) commonly process the materials further, slurrying them and screening or ball milling; this is done to guarantee defect-free glazed surfaces.

Example of a certificate of analysis for a kaolin

When companies ship materials they often include these with the shipment. The information reported is often very basic and properties important to ceramics are often not found.

The same clay in lump and powder form. Which is heavier?

Which one of these samples weighs more, the raw lump form of the clay or after it has been ground into a powder? Wrong. It is the lumps. Even though there is all that empty air space between those lumps, there is even more air spaces in the powder. The top one weighs 1662 grams (there is a 500g counter-weight barely visible), the bottom one is 1255. The finer I grind it the lighter it will be. If I were to fill in all the voids between the lumps on the top one with smaller sized lumps I could get alot more weight yet! It works the same on the ultimate particle level, when we combine powders of varying particle sizes we get a more dense and stronger dried product.

Screened to 80 mesh and feels absolutely smooth, but still speckles in reduction

The reduction was fairly heavy and this piece went to cone 11. The tiny ironstone concretion particles melted vigorously and flowed. This is why clean firing results requires 200 mesh materials!

Skagit Fireclay PSD test

Particles from each category in a particle size distribution test of Skagit Fireclay

A sieve shaker used on dry powder samples

These are used in structural product industries to measure PSD and are much coarser sizes than is typical in pottery, porcelain or stoneware.

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 of screen sizes for testing particle size is very wide (obvious here: the top screen has an opening of 56 mm, the bottom one 0.1 mm - the wires are almost too small to see). You can often buy these used on Ebay for a lot less than new ones, search for "tyler sieve". The finer sieves (especially 200) are fragile and more easily ripped. For potters it is good to have a 50, 100 and 150.

A root-of-two series of test sieves

The coarsest screen is at the top, the finest on the bottom. The top one has an opening of 425 microns (thus 425 micron and finer particles will pass through it, +425 micron particles will not). Its opening area is 180,000 square microns (425x425). Going downward, the openings have areas half that of the one above (thus, for the second, the opening area is 300x300=90,000 microns). Structural products industries, like brick, measure coarser particles than this, using 10-70 mesh. Using this series one can produce a standard measurement of the distribution of particle sizes in a material powder. The finer sieves (from 100-325) are only practical for wet processing (where the powder is water washed through the stack). In Insight-live the SIEV test (water washed) uses this series of sieves.

Micro photograph taken by an ordinary iPhone is still very useful

These are 40 mesh particles, about 400 microns in size. They are clearly visible as rounded, not angular. The carbon and high iron mineral particles are easy to spot. The rest appear to be quartz (having various levels of purity from amber to crystal clear). The interesting thing about these is where they come from: A very dirty-looking terra cotta clay. Something else interesting: At 50, 80 and 100 mesh the iPhone, even with its computational photography, was also able to capture the particles - and they looked exactly the same. Their presence explained a number of physical properties of the clay that otherwise seemed odd.

Stonewares dry better than porcelains

The plastic porcelain has 6% drying shrinkage, the coarse stoneware has 7%. They dried side-by-side. The latter has no cracking, the former has some cracking on all handles or bases (the lower handle is completely separated from the base on this one). Why: The range of particle sizes in the stoneware impart green strength. The particles and pores also terminate micro-cracks.

Chart of residue on 325mesh common for different glaze types

Particle size and LOI determine behaviour of over-fired bodies

These are four terra cotta body disks that have been fired to cone 10 reduction. The fluxing action of the iron has assisted to take them well along in melting. Notice that one is hardly bubbling at all, it is Redart clay that has been ground to 200 mesh (the lower right one is a body mix of 200 mesh materials also containing it). The upper left one is bubbling alot more. Why? Not just because it is melted more (in fact, the one on the lower left is the most melted). It is a body made from clays that have been ground to 42 mesh. Among the particles are larger ones that generate gases as they decompose. Yes, the particles in the others do the same, but their smaller size enables earlier decomposition and expulsion of smaller gas amounts distributed at many more vents. Some bodies cannot be fired to a point of zero porosity, they will bubble before they get there.

This appears to be a drying crack, but it is not

This clay normally dries well, but not this time. Strangely, this crack is not at the handle join, it is penetrating into the mug wall. Actually, this is not a crack, it is a split. Excessive slip around the join was not removed, that is bad when a body has larger particles, they permit water left on the surface to penetrate inward and begin a split. An aggravating factor was that the handle was allowed to dry faster than the mug itself, pulling at this join and opening the split even more.

Inbound Photo Links

Could these bentonite particles cause specking in a porcelain?

Large particle grogs are difficult to produce

How small can clay crystals be?

Links

By Tony Hansen Follow me on

Got a Question?

Buy me a coffee and we can talk