The composition of a clay body formula is often a personal choice based on theory, experience and knowledge of materials. It is always advisable to mix a small test batch of any clay body formula and note the results in your kiln. Any new formula-whether purchased pre-mixed or mixed in your own studio-should prove itself over a few different kiln firings.
Several variations of the ZAM Super Body formula have been mass-produced by ceramic suppliers in large volumes over the years with good results. The formula has a dry shrinkage rate of 6% and is designed for throwing and hand building. It matures from cone 6 to cone 10 (c/6-c/10) and yields a medium brown color in reduction atmospheres and a light tan in oxidation atmospheres. It has a total fired shrinkage of 12.0% at c/6 (2232°F) and 12.5% at c/10 (2345°F), and an absorption rate of 2.6% at c/6 and 0.50% at c/9 (2300°F).
A single clay body that functions perfectly for every potter does not exist, but knowing what considerations are made when a formula is developed can help pottery producers identify the source of quality problems and/or optimize their own clay body formulations. Following are the clays, fluxes and fillers used in the ZAM Super Body formula, along with the reasons each material was chosen.
marginal-performance clay. Fireclays can contain excessive silica in small particle sizes, which can cause cooling cracks; a high organic content, which can cause black coring or bloating if the clay if it is not subjected to an oxidation atmosphere at the beginning stages of the firing; lignites (also called brown coal or low-quality-coal) or coal, which can also cause black coring; calcium nodules, which can cause lime pop; and sand, which can impart a gritty quality to the clay. Other impurities that have been found in fireclays include tree branches, metal bolts, paper, rocks and cigarette butts.
With all of these drawbacks, why use fireclay? The coarse or large particle size of the clay makes it ideal for adding "tooth" or "stand-up" ability in throwing and sculpture bodies. Fireclays also lower the dry shrinkage, fired shrinkage and warping potential of a clay body, and their refractory or heat-resistant nature imparts high-temperature-resistance to the fired clay. Unfortunately, potters buy less than 1/10% of all mined clays, so they have very little influence on the quality of the clays that are supplied. As long as fireclays meet or exceed the major user industry's requirements (and they currently do, even with all of their impurities), potters and ceramic supply companies do not have the economic purchasing power to demand a better grade of fireclay from the mines.
Hawthorn Bond fireclay, mined by Christy Minerals, High Hill, Mo., is used in the ZAM Super Body because it is readily available to most potters throughout the U.S.
Using two different fireclays in the formulation can help mitigate the clay's negative aspects, and inspecting the dry clay before the mixing operation can help prevent large and obvious contaminants from entering the clay mix. Some pottery supply companies screen the fireclay, which can significantly improve its quality. The screened clay costs more, but it is usually worth the extra price since it can reduce losses. Many potters choose not to use any fireclay in their clay body formulas because of its poor quality. Each potter must decide if the good qualities of fireclay in the clay body formula are worth the risk.
Fireclays also contribute iron and manganese particles that can provide random patterns of brown and black specks in the clay bodies fired in reduction kiln atmospheres. Using no fireclay or low percentages can lower the maturing range of the clay body and cause it to become soft with little grit or tooth when formed on the wheel. Clay bodies with no fireclay component often feel and throw like cream cheese. Alternatively, using excess fireclay will make the fired clay too porous and non-plastic, and will decrease the fired strength of the finished ware. However, the amount of grit or tooth needed in a moist clay body is subjective and must be decided by the individual potter.
Lincoln fireclay, supplied by Laguna Clay Co., City of Industry, Calif., is used as the second fireclay in the ZAM Super Body also because it is readily available to most potters in the U.S.
Stoneware clays have greater plasticity than fireclays but are not as plastic as ball clays. They make an excellent choice for a clay body formula because of their reliability, particle size and adaptability with other clays in the formulas. Using low percentages of stoneware clay (e.g., <30%) will allow the other clays in the formula to dictate the clay body's performance, usually with less than optimum results. Conversely, using too high a percentage of stoneware clay (e.g., >50%) will detract from the qualities of the other clays in the formula. The correct amount of flux, other clays, and fillers must be used to ensure a balanced clay body.
Too little feldspar in the clay body will reduce the amount of glass formation within the fired clay, which can lead to improper glaze fit (exhibited by "shivering," in which the fired glaze flakes off the clay like paint chips) and the potential for moisture to seep through the porous fired clay form. A common method of adjusting a clay body that does not hold liquid is to add 5% increments of a flux such as feldspar. Once the correct amount of flux is achieved, the absorption rate will decrease with greater vitrification in the fired clay body. Most functional stoneware pottery should have absorption rates of 3% or lower. A glaze should never be considered as a "sealer" or waterproof coating, as moisture will always seep through small imperfections.
In some instances, potash feldspars, such as Custer (supplied by Pacer Corp., Custer, S.D.) or G-200 (supplied by Zemex Industrial Minerals Co. [Feldspar Corp.], Atlanta, Ga.), are used instead of soda feldspars, such as F-4 (Zemex) and nepheline syenite, because potash feldspars tend to be less soluble. Soda spars can sometimes break down in the moist body, causing the clay to become thixotropic or rubbery in the forming process. The clay can also feel soft and have a "Jell-O" consistency when worked on the potter's wheel or in hand-building operations. As an increasing amount of water is applied to form, the clay becomes very soft and decreases its ability to hold a curve. Eventually the clay cannot support its own weight, and the form slumps or deforms. However, soda spars can be used to increase the fired clay body's melting capability.
As with other body components, the choice of whether to use potash, lithium or sodium based feldspars is in part determined by the other raw materials in the formulation. For example, spodumene, a lithium-based feldspar, can bleach any iron in the clay body and cause a red/brown fired color, while potash or sodium feldspars can cause the fired clay color to become more brown. Some experimentation will likely be needed to determine the effects of different types of feldspars in a specific clay body.
When a darker color clay body is required, it is always better to incorporate high-iron-bearing clays than to add straight red iron oxide to the clay body formula. Adding metallic coloring oxides to the clay body for color will cause the moist clay to take on water very fast during the throwing or hand building process, resulting in a very soft clay. Additionally, the more water a clay body takes on in the forming operation, the greater the shrinkage, warping and cracking potential during drying and firing. Some metallic oxides can also easily over-flux the clay body during firing, especially in reduction atmospheres. If red iron oxide must be added to the formulation, it should be limited to 2% or less of the overall body components. Low-temperature earthenware clays should not be used in greater than 10% amounts in stoneware bodies, as their fluxing action will negatively react with the clay body and cause possible bloating, warping and excessive fired shrinkage.
Grog is classified by its particle size-the lower the number, the larger the particle size. Grog 8/12 mesh looks like small pebbles, while grog 100 mesh is a fine powder. Grog 48/f ranges from 48 mesh (about the size of beach sand) with varying smaller particles to a powder size, and it was chosen for the ZAM Super Body because of its wide particle size distribution. As a rule, particle size variation is preferred when choosing grog due to the mechanical advantage of interlocking sizes and shapes. Different sized grog particles touch and combine to create a cohesive clay body and grog bond. Large-particle-sized grog (over 20/48x mesh) can cause surface irregularities in ware trimmed on the potter's wheel; however, leather-hard clay can be burnished at the trimming stage to force the grog particles below the clay's surface.
Grog decreases dry and fired shrinkage-every 10% of grog added to a clay body decreases fired shrinkage by approximately 0.75%. However, using too much grog will yield a gritty moist clay that will be "short" or non-plastic in forming operations. In most stoneware throwing bodies, grog amounts of greater than 15% adversely influence the plasticity and handling qualities of the clay. Additions of grog below 100x mesh (powder consistency) can also significantly decrease the plastic qualities of a clay body.
Conversely, little or no grog in the clay body decreases the amount of tooth when the clay is being formed on the potter's wheel. The moist clay has difficulty standing up and will slump during throwing operations.
As with other components in the clay body formula, the appropriate ratio of grog (non-plastic) to plastic particles depends on the required clay body function.