Need some general rules of thumb for trying to decipher what clay body to select? This guide to clay body characteristics should give you some sense of the clay’s composition and behavior.
Defining the Terms
Carbon Coring: Black layer (also called black core) on the interior of the fired ceramic body caused by the early reduction of iron disulfide.
Coefficient of Expansion (COE): The fractional change in size as measured in linear expansion or contraction as clay and glaze react to thermal changes.
Glassy Matrix/Vitrification: Developed glass content in a clay body that lowers the absorption rate below 2% resulting in moisture resistance.
Particle Size: The measurement of individual clay particles given in mesh size or specific surface area.
Plasticity: An elastic property that allows clay to be formed, and retain its shape without deforming.
Water of Plasticity (WOPL): The measurement of an individual clay variety’s capacity to hold water on its platelets, thus forming a pliable ball.
Shrinkage (1, 2)
Shrinkage measurements and percentages are generally used by the potter as a guide to predict how much the body will shrink when fired to the recommended cone. However, shrinkage also gives an indication of how the body was formulated (what’s in it). Shrinkage is a direct reflection of how much water is in the clay. The amount of water is determined by the type of clay used; it is dependent on the water of plasticity (WOPL). A low-plasticity clay holds 26–30 grams of water per 100 grams of clay. A medium-plasticity clay holds 31–35 grams of water per 100 grams of clay, and a high-plasticity clay holds 35–38 grams of water per 100 grams of clay. Plasticizers (such as bentonites, hectorites, Macaloid (Bentone MA), and V-gum T) hold 38 grams of water per 100 grams of clay.
The plasticity of a clay body is determined by the additions of fine-particle ball clays or plasticizers. The more plastic a clay body is, the higher the percentages of these types of additions. However, high plasticity produced by ultra-fine ball clay or plasticizers also means higher shrinkage rates. As shrinkage rates begin to go over 12%, plasticity generally increases as this often correlates to increased ball clay additions. So if you are comparing white stoneware bodies, for instance, the body with the higher shrinkage rating will be more plastic than the lower-rated body.
The porosity of a clay body after being fired to the recommended cone, known as its absorption, is a little harder to use to determine formulation because so many variables affect it. The hard set rule is that porcelain will always have lower absorption rates than stoneware due to composition. If a porcelain body has a higher absorption rate than competing stoneware, it would automatically be suspect for use. Porcelain (in general, but not always) produces a translucent appearance after firing, which is in part due to higher flux levels, higher silica levels, and the use of very low-carbon, low-iron clay. The combination of these three factors produces a much more developed glassy matrix (vitrification), which also results in absorption rates of nearly zero.
Stoneware clay bodies typically consist of fire clays (large particles), intermediate particle-sized clays (200 mesh), and fine particle-sized ball clays (325 mesh), and have lower levels of silica and feldspar additions. Stoneware clay body formulation can go all over the place; there are no set rules for mixing. It can have a low or high percentage of fire clay and ball clay. The percentage of silica can vary, as silica can be added or omitted. Feldspar quantities typically are a third less than what is found in porcelain. Again, however, the lower the absorption rate, the higher the amount of added plastic ball clay and flux. Highly plastic ball clays typically have sub-micron particle sizes, which not only add plasticity, but also seal up the microscopic voids created by large-particle fire clays. So the general rule of thumb for choosing a stoneware clay body is to look more closely at absorption rather than shrinkage. Ball clay is used in all stoneware bodies, but lower absorption usually indicates that a sub-micron (highly plastic) ball clay has been used in lieu of a medium-plasticity (larger particle) ball clay.
Coefficient of Expansion (3)
The coefficient of expansion (COE) of a clay body is mostly defined by feldspar additions in porcelain, and by the percentages of fire clay versus combined ball clay plus feldspar in stoneware bodies. As the amount of feldspar increases in any given formula, the COE increases. The amount of silica can lower it, as does the clay content. Median COE values for porcelain typically run between 5.75 and 6.00, and those for stoneware typically run from 5.25 up to 5.65. If a stoneware body’s COE falls below 5.25, it is an indication that more clay has been used in the formulation, coupled with less feldspar. This would cause lower COE values, while causing the absorption rating to climb. Porcelain bodies with COE values above 6.00 usually indicate higher feldspar levels, which generally produce a higher degree of translucency.
Stoneware bodies typically have 10–13% feldspar additions.
Porcelain bodies typically have 25–30% feldspar additions, pending cone values.
The color of the unfired clay body says much about the composition; it is an indicator of the specific types of clay varieties used in formulation. Kaolinitic ball clay has very low carbon content and high alumina, and generally runs from light tan or light gray to light buff in color. It also tends to be a medium-plasticity body. High plasticity ball clays are higher in carbon content, so they range from brown to dark gray. When ball clay is added to either a stoneware or porcelain body above 10%, its color will affect the final color of the fired clay. So, porcelain that is white, light tan, or light gray contains either a plasticizer or a kaolinitic ball clay. On occasion, small amounts of bentonite are used, but those additions are so small they will not darken the clay body considerably.
The color of a porcelain clay body, more so than stoneware, is very revealing regarding its component materials. White porcelain uses Grolleg, super-standard porcelain, or air-floated kaolins, which are all white. In addition, plasticizers (plastic vitrox, or white kaolinitic ball clays) are added. These white porcelains tend to have higher percentages of fluxes as well, producing much higher degrees of translucency. If the body is buff or light tan, then EPK Kaolin, 6 Tile Clay, or OM 4 ball clay (or similar) has been added, which still produce a bright white body, but with less translucency. If a porcelain body is darker or gray in color, then higher carbon ball clays have been added for plasticity. The additions of these ball clays produce a fired clay with no translucency due to their titanium content.
Generally speaking, the darker a clay body becomes, the more high-carbon ball clay has been added. In stoneware clay bodies, a darker body is a good thing if it means higher amounts of sub-micron ball clay have been added. It can also mean higher amounts of fire clay are present, which can lead to higher absorption rates. Some stoneware bodies are not formulated for functional use; they are specifically formulated to withstand thermal shock from raku, salt, or wood firings. If you are looking for a stoneware body for functional use, look for a lighter tan or lighter gray color, which usually indicates higher levels of kaolinitic ball clays. One caution: red-colored or dark gray clays are typically higher in carbons and iron sulfide, making them more susceptible to bloating and carbon coring if fired incorrectly.
Clay bodies that give a very broad range of firing cones are typically unreliable for functional use. To vitrify at any given cone, a body has to have a set value of feldspar to silica/alumina. Clay bodies intended for functional use should not give any more than a two-cone range (cone 5–6 for example). A body that states it is suitable for firing from cone 04 to cone 6 is just flat misleading in regards to functional use. If the molar levels of flux required to vitrify a body at cone 04 were present in a cone 6 firing, the piece would slump.
Selection by Use
Another consideration for clay body shopping is defining intended use. Stoneware bodies are fairly adaptable for most uses because of their clay content. While all clay bodies have some clay memory, this property is exaggerated and often troubling in porcelain bodies. Memory is when a clay body wants to return to its original particle orientation after you have altered that orientation by forming. This means tiles are prone to warping, and handbuilt or sculptural pieces are prone to distorting. The simple correlation is that the higher the plasticity, the higher the memory and warping. Clay bodies with lower shrinkage rates (10–11%) work better for tile and low-profile handbuilding. Medium-plasticity clay bodies (11–12½% shrinkage) have a broad range of applications including sculptural. Clay bodies with high shrinkage values (12½–14+%) are best suited for throwing. High shrinkage-rated porcelains also work better for stamping, carving, and detailing work due to the finer particle distributions.
As with all things in ceramics, there are exceptions to the these general guidelines. Hopefully these will help you narrow down your shopping choices.
the author Thomas Anderson has spent the last decade researching technical research papers from various universities on clay formulation. He is currently writing formula limits for clay chemistry.