Potters often buy whatever selection of materials suppliers provide without thinking about the consequences or even the potential artistic possibilities. Using the correct particle size and distribution for the job you are doing requires some basic knowledge of material properties followed by some creative experimenting.
Defining the Terms
Mesh Size—A count of the number of strands per inch in a screen that is used to grade the particle size of the material. Theoretically, in a 100-mesh material sample, approximately 95% of the particles should pass through a 100-mesh sieve (this means that 1–5% of the particles will remain in the pan). A larger mesh-size number indicates smaller particles so 100-mesh particles are larger than 200-mesh particles. In ceramics, mesh sizes generally range from 25 to 325 mesh, smaller particles are referred to in microns.
Micron—A unit of length one-millionth of a meter or one twenty-five thousandth of an inch.
Particle Size—Also called grain size. It is relevant in terms of how long the particle will take to dissolve in a glaze melt and also in terms of plasticity, shrinkage, and strength of a clay body.
Sieve—A device with a screen or mesh bottom used to separate out coarse or unwanted particles. Most glazes are sieved twice through an 80 mesh sieve. Can be done dry or wet.
Size, on many levels, makes a big difference in both clay bodies and glazes. For example, if you have a glaze recipe that calls for 30% silica and you used 325-mesh silica, it would create a smooth and glossy glaze, but if you used silica sand (40 mesh), you would have a rough and crusty glaze.
Mesh sizing originally referred to the number of threads per linear inch of mesh, which presumed the same number of holes. Of course, thread size caused variance so uniform international standards measured in micrometers have been adopted. So a 100-mesh sieve has 100 square holes per inch which measure 0.152 mm (or 152 microns). Theoretically, in a 100 mesh sample, 95% of the particles should pass through a 100-mesh sieve. A higher mesh size number indicates smaller particles so 100-mesh particles are larger than 200-mesh particles. If you continue to add more and more threads to a screen, eventually you will clog all the holes. So anything beyond 325–500 mesh is usually described in microns.
Because the sieve hole is a square, the size of the diagonal is larger than the width and length. Larger particles can make it through the diagonal and that is why you often sieve several times.
Describing a material as 100 mesh is not very precise as you don’t know the size of the 95% of particles that passed through the sieve. A more precise notation has been established which uses -/+ signs. So a particle that is “-80 /+100“means that 95% of the particles passed through(-) the 80 mesh sieve but were retained(+) by the 100 mesh sieve. In ceramics, grogs are often listed more precisely as 12–48, (or -12/+48) which shows the range of particle size.
Today, because our grinding technology is vastly improved, the particle size (expressed as mesh size) is much different than it was just 50 years ago. For example, 50 years ago the standard silica for glazes was 200 mesh, which meant that 95% of the particles passed through a 200-mesh sieve. We do not know exactly how fine 95% of the particles were, we just know that they passed through the 200-mesh sieve. But now that same 200-mesh silica is much finer because our grinding ability is so much better. So the 95% of the particles that passed through the sieve are much finer and that affects the melting of those particles. Using 200-mesh silica now may be closer to using 325 mesh back in the day. Knowing the particle distribution may help and is generally available from many suppliers. For example, Minspar 200 lists that 87% of particles are finer than 30 microns; 72% are finer than 20 microns; 40% are finer than 10 microns; and 19% smaller than 5 microns. The graph below shows the variation in particle size of a sample of Minspar 200, ranging from 100 microns to less than 1.0 micron, with the vast majority between 3 and 50 microns.
Typical sieve analysis of a 50 pound bag of 200-mesh Minspar 200 (a soda feldspar mined by Imerys Minerals in North Carolina). Note that 3.6% of the material passes through the 200-mesh screen but is retained by the 325-mesh screen, so very little of the material is actually graded at precisely 200 mesh.
Although clays are described as 200 mesh, many are actually much finer than 325 mesh (40 microns). For example, in a 200 mesh sample of EPK, approximately 55-65% of kaolin particles are less than 2 microns, while approximately 81% of ball clay particles are less than 2 microns and many can be as low as 0.1 microns or 400 times smaller than 325 mesh.
Porcelain bodies are often difficult to throw because the particles (kaolin, feldspar, and silica) in the body are all approximately 200 mesh or less, making it a homogenous clay body. Stoneware bodies, however, are a mix of various clays and particle sizes. They contain fire clays, which are 25–50 mesh; ball clays, which are 200 mesh; and then grogs, which can be 12–80 mesh.
Grog is often added to sculptural bodies to give them strength and reduce shrinkage. Val Cushing lists a proportion of: 12% fine grog, 3% medium grog, and 15% coarse grog, for a total of 30%. This is how you can produce non-shrinking sculptural clay bodies. In a plastic clay body this proportion of mixed mesh-size grog helps fill all the voids and avoids micro-cracking around larger particles.
Grog is often listed by the largest particle size, for example 30 mesh. This tells you the size of the largest particle but not the smallest. Some grogs are listed as 30–80 mesh, meaning that the largest particles are 30 mesh and the smallest are 80 mesh.
Melting particles in glazes can be compared to dissolving sugar in tea. The particle size of the sugar makes a big difference in the amount of sugar that will dissolve into the tea and thus how sweet it is. For example, two sugar cubes will not dissolve as easily as the same weight of crystallized sugar and certainly won’t dissolve as easily as the same weight of powdered sugar. This is because there is more surface area per weight in the fine particles and they enter the melt easier. In glazes, the same principle applies; 325-mesh silica will go into the melt easier than the same amount of 200-mesh silica.
Mesh size can be related to other properties, like solubility. For example, nepheline syenite has two grinds available to potters, 270 mesh and 400 mesh. Theoretically the 400 mesh would go into the glaze melt better than the 270 mesh. But because it is slightly soluble, using the finer 400 mesh (more surface area) will also cause the glaze slurry to deflocculate quicker and the glaze will settle out and hard pan more easily. So there’s a trade off.
The mesh sizes in some materials are kept large to create specific effects, like granular illmenite or granular rutile. These create intentional speckles used in glazes like Jackie’s Speckled Lavender. Other materials, like silicon carbide, are specific sizes to encourage cratering in glazes. Some sculptural low-fire glazes may specify sand in the glaze to add texture. In the case of Bleeding Cake, the sand is used to get a surface that looks like Red Velvet Cake.
Mesh size can make a big difference with colorants. For example, when making a blue celadon, if you use 100-mesh black iron oxide (an old school way of mixing blue celadon with a coarse grind) you might get speckling. This can be corrected by ball milling the colorant in a small amount of the glaze for several hours then adding it to the whole batch. But by simply changing the colorant to synthetic red iron oxide (a very fine type of iron at 325 mesh) which goes into the melt more easily without ball milling, you get no speckling.
Routinely putting glazes through a ball mill (approximately 2 hours) is also a good way to smooth out a glaze batch by slightly grinding the particles, but grinding glazes for too long (beyond 4 hours) will reduce the particle/mesh size too much and cause the glazes to crawl. The most efficient grinding is when there is just enough material in the ball mill to fill all the voids and just cover the grinding media. If ball milled too long, the small particles in the glaze will shrink excessively when they melt and cause crawling. Ball milling is usually done wet with glazes but can be done dry.
This article was excerpted from the October 2011 issue of Ceramics Monthly, which can be viewed here.
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