|Monthly Tech-Tip |
Understanding the theory behind sieve selection, how to properly sample a powder and how to carry out a particle size distribution test can give you valuable information about a material.
Particle size is one of the most important properties of clays and ceramic minerals. One gram of powdered material can have billions of particles with a surface area of many square meters. A knowledge of not only the average size but how the sizes are distributed, can be very valuable.
The sieve analysis test I recommend is performed by slurrying and soaking a clay sample in water and then washing it through a series of successively finer and finer interlocking screens (this can be done on a plastic or lump clay material by drying and then slaking it thoroughly in water first). This test is important in monitoring consistency of clay and material powders (unlike most of the other tests I recommend, this one does require that you purchase equipment you cannot easily make).
Particle size tests are important in many industries (e.g. cement) and sieves have thus become highly standardized. Usually screens are chosen so that each has an opening with double the area of the one below it. This is referred to as a series because the width of the 'root of two' opening is the root of two (or 1.414) times that of the previous. Screens can be referenced in mesh sizes (a 100 mesh screen has about 100 wires per inch) or dimensions of the opening. They are calibrated from a standard 200 mesh having an opening of 0.0029". A good brass sieve can cost $150 or more and must be treated with care.
Methods of recording and presenting the data are very important and sieve manufacturers provide helpful books. The W.S. Tyler company, for example, is a manufacturer of quality sieves and publishes a very interesting book called 'Testing Sieves and Their Uses'.
Insight-live.com defines a test abbreviated 'SIEV', it clearly defines the procedure to use to perform a particle size appraisal of a material. It also provides information on the reasons why the test should be a part of your standard testing regimen.
One of the most important aspects of doing sieve analysis tests is the matter of selecting a representative specimen to test. It does little good to graph in detail an analysis if the specimen is not typical. The diagram shown here illustrates one method.
Taking A Representative Specimen
In addition to controlling material consistency and spotting impurities, sieve analysis tests are valuable in making formulation decisions. For example, the strongest and best clays are made dense by a good distribution of particle sizes at plus 325 mesh. In this way, there is a good packing in the dried matrix and therefore maximum green strength and minimum low temperature absorption. Bodies with a distribution of particle sizes tend to be comparatively stronger in the fired state and tend to have more robust working and drying properties than those made from the same materials ground to a fine mesh size. Particles of 100 mesh or finer do not generally have a noticeable texture (they feel smooth) but are easily detected by a sieve analysis.
A sieve analysis test can also alert you to trouble. It will show if coarser or agglomerated particles are present in a body presenting the danger of pinholing and premature bloating (especially in oxidation firing if the particles are "undesirables" e.g. coal, plaster, concretions). The open nature of a coarser body can also invite pinholing and pitting in less viscous glazes.
Clays with sand or grog additions in a very fine plastic particle size matrix often lack intermediate sizes and can micro-crack around each large particle. This phenomenon, although invisible to the eye, is evident not only by the dead 'thud' when a piece is tapped with a pen but by a sieve analysis of the raw material. A sieve analysis can be employed to incorporate a coarser clay or a finer grog or sand.
The coarsest screen is at the top, the finest on the bottom. The opening for each is shown on the label. They are chosen such that each successive screen going down has an opening that is about half the area of the one above it. Using this series you can produce a practical measurement of the distribution of particle sizes in ceramic materials and bodies used in traditional ceramics (structural products industries, like brick, measure coarser particles than this, starting at perhaps 10 mesh and ending at 70). The 325 screen on the bottom is only used sometimes, it is difficult to finer-that-325 particles to pass through it because it blinds. It is not possible to shake powder through sieves that are this fine, samples must be washed through. We use the SIEV test to log results.
The coarsest screen at the top has an opening of 425 microns (that means that 425 micron and finer particles will pass through it, and, conversely, plus 425 micron particles will not pass). It's opening has an area of 180,000 square microns (425x425). Going downward, the openings on each screen have areas half that of the one above. Thus, for the second screen down, the opening area is 90,000 microns. Standardized sieves like this are essential to the study, classification and maintenance of powdered materials. In Insight-live the SIEV test uses this series of sieves.
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 buy these on ebay for a lot less than new ones, search for "tyler sieve". The finer sieves (especially 200) are fragile and easily ripped. It is good to have a 50, 100 and 150.