PPP: Stoneware Clay Body Formulas Part 2: The Perfect Body

Understanding the considerations used in selecting specific clay body components can help pottery producers optimize their own formulations.

A stoneware cup with a matte blue glaze, fired at cone 9 in a reduction atmosphere.
A "perfect clay body" is a subjective term, but the chances of achieving this goal increase when the appropriate ratio of clays, fluxes and fillers are used. When designing a clay body, factors such as the forming method, dry shrinkage, firing temperature, kiln atmosphere, glaze interaction, fired color, fired shrinkage, absorption and raw material availability should all be considered, as outlined in Part 1 of this series, "The Basics" (Ceramic Industry/Pottery Production Practices, June 2005).

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.

15 Parts Hawthorn® Bond Fireclay, 40 Mesh (Clay)

Fireclays are the weakest part of any clay body formula with regard to quality and reliability. In fact, most problems in clay bodies that contain fireclay can usually be traced to this

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.

10 Parts Lincoln Fireclay, 60 Mesh (Clay)

Fireclays also serve a useful function in their ability to add large particle sizes to the total mix. Some of the best clay bodies have small, medium and large clay platelet sizes, contributed by ball clays, stoneware clays and fireclays, respectively. These platelets mechanically interlock when moist, providing greater contact between the clay surface areas and therefore increased plasticity.

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.

A stoneware cup with a matte green glaze, fired at cone 9 in a reduction atmosphere.

40 Parts Goldart Stoneware Clay, 200 Mesh (Clay)

Goldart stoneware clay, supplied by Resco Products, Inc., Cedar Heights Clay Division, Oak Hill, Ohio, is the backbone of the ZAM Super Body formula. Stoneware clays are relatively clean, with a medium platelet size and, as their name implies, can be fired to stoneware temperatures of c/6 to c/9. As a group of clays, they can almost function as the total clay component in a formula. Although Goldart stoneware contained high concentrations of sulfur at one time, these concentrations have been kept under control for the last 15 years.

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.

15 Parts Zamek Ball Clay, Air Floated (Clay)

Ball clays such as the Zamek ball clay, supplied by Old Hickory Clay Co., Hickory, Ky., contribute plasticity to the clay body, but they also increase the rate of shrinkage during drying and firing. Using low percentages (e.g., <10%) of ball clay will produce a clay body that is "short" or cracks easily when bent in the forming process. However, low plasticity and low shrinkage are ideal characteristics for tiles and dry or hydraulically pressed items. Stoneware bodies containing an excessive amount of ball clay (e.g., >25%) often feel "gummy" when moist and can leave loose slurry on a potter's hands after pulling up a form on the potter's wheel. The clay body can also feel "rubbery" or soft when force is exerted against it during any forming operation. Determining what percentage of ball clay to use is ultimately a matter of finding the right balance between the level of plasticity that is desired and the amount of shrinkage that can be tolerated in the ware.

10 Parts Flint, 200 Mesh (Filler)

Adding flint to a clay body decreases warping and shrinkage in the drying stage. However, high percentages of flint can cause a decrease in plasticity. In stoneware throwing bodies, the amount of flint should generally be kept below 14%, while in porcelain clay body formulas, flint can reach 25% levels. Flint also reacts with feldspar during firing to augment the development of glass formation within the clay body. While clays and other materials such as talc, feldspars and frits have a silica component, the amount is usually not enough to achieve the optimum effect in the vitrification or melting stage of the clay body. The presence of flint in the clay body also promotes a better glaze fit and decreases the probability of glaze crazing defects. Both 200x and 325x mesh flint are used; however, the coarser 200x mesh is typically preferred for clay bodies because it reduces the possibility of the moist clay becoming too "gummy," while the finer 325x mesh is frequently used in glaze formulas.

7 Parts Custer Feldspar (Flux)

Feldspar is a major flux or glass former in a clay body. However, using too much feldspar in the clay body (e.g., >25%) can cause the ware to slump or attach to the kiln shelf at the unglazed foot. High levels of feldspar can also cause the body to shrink excessively and warp during firing, and over-fluxing caused by excessive amounts of feldspar can darken the fired clay color if any iron is present in the formula. The fired clay can become brittle, weak, bloated or, in extreme situations, can exhibit a "Chernobyl" effect during firing, becoming a molten ceramic mass on the kiln floor.

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.

A stoneware cup with a clear glaze, fired at cone 9 in a reduction atmosphere.

3 Parts Sheffield Clay or Redart (Flux/Clay)

Any raw material that can offer more than two or three functions in a clay body should be strongly considered because it can provide a higher degree of integration among the various components. Sheffield Clay, a low-temperature earthenware clay mined by Sheffield Pottery, Inc., Sheffield, Mass., is one example of a material that serves more than one major function in the clay body. The clay has a high iron content, and it can contribute a brown color in reduction and a medium cream color in oxidation atmospheres. It also promotes some fluxing action and adds different platelet sizes to the total clay body, which ensures a greater platelet size distribution. Redart, a low-temperature clay mined by Resco Products, Inc., Cedar Heights Clay Division, can be substituted for Sheffield Clay on a one-for-one basis and also adds color, platelet variation and flux to the 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.

8 Parts Grog, 48/f (Filler)

The most reliable grog is manufactured from virgin deposits of an alumina/silica refractory material and is calcined or fired to extremely high temperatures (above c/32 [3073°F]). Grog can also be manufactured from crushed firebrick kiln linings, but these might contain contaminants of iron, manganese, chrome or copper, depending on their point of origin.

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.

Making Adjustments

The stoneware clay body formulation listed in this article has been used successfully by numerous pottery producers. However, it is not a universally "perfect" clay body. Part of the fun of producing pottery is learning about the different ceramic materials and experimenting with various options to create unique, high-quality finished products. Understanding the various factors that should be considered when developing a stoneware clay body formulation can give potters the freedom to make adjustments and create their own "perfect body."

Editor's note: Part 1 of this three-part series, "The Basics," appeared in the summer (June) edition of Pottery Production Practices and can be found online at www.ceramicindustry.com. Part 3, which will discuss the role of plasticity in stoneware body formulas, is scheduled for the spring (February 2006) edition of Pottery Production Practices.

About the Author

Jeff Zamek received bachelor's and master's of fine arts degrees in ceramics from Alfred University, College of Ceramics, Alfred, N.Y. He taught ceramics at Simon's Rock College in Great Barrington, Mass., and Keane College in Elizabeth, N.J. In 1980 he started his own ceramics consulting firm and has contributed articles to Ceramics Monthly, Pottery Making Illustrated, Clay Times, Studio Potter and Craft Horizons, in addition to Pottery Production Practices. His books, What Every Potter Should Know ($31.45) and Safety in the Ceramics Studio ($25.45) are available from Jeff Zamek/Ceramics Consulting Services, 6 Glendale Woods Dr., Southampton, MA 01073. These books can also be ordered online at www.ceramicindustry.com, through Ceramic Industry's online bookstore.

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