Clay Body Formulas

“Work out of your work. Don’t work out of anybody else’s work.”  Richard Serra

The Perfect Clay Body?    Part 1.

Is there a perfect clay body? No. I just thought we could face this fact at the beginning of the blog. Are some clay bodies better than others? Yes, but first we should define what is meant by the term, "clay body." A combination of clays, fluxes, and fillers form a working definition of a clay body. In the past one or two elements created a clay body. Each component served a function in the formula to determine, forming characteristics, drying shrinkage, surface texture, fired absorption, fired shrinkage and color.

Within each component many different individual materials can be used to fulfill the requirements for clays, fluxes, or fillers. The best clay body formulas will have the appropriate raw materials and the correct ratios of clays, fluxes, and fillers to achieve their desired result. Keep in mind in many respects a clay body’s performance can be quite subjective depending on the individual potters requirements.

Clays are grouped depending on their refractory qualities, particle size, oxide composition, loss on ignition, shrinkage rates, absorption rates, and other defining characteristics. The basic clay groups found in clay body formulas are fireclays, ball clays, kaolin’s, stoneware clays, bentonites, and earthenware clays. Each group of clays contributes specific attributes to the total clay body formula. Such clay qualities can be, particle size, green strength, fired strength, fired color, shrinkage, plasticity, texture, forming abilities and low amounts of warping in drying and firing.

When individual clays are not available a substitution can be made provided the substitute clay is of the same group. By choosing a clay within the same group, most of the clay characteristics will remain consistent. For example, in throwing bodies, Thomas ball clay can be substituted for Kentucky ball clay OM #4 ball clay. Both are from the same group of clays (ball clay) and have the best chance of matching. Other factors such as plasticity, green strength, particle size distribution, and metallic oxide content should also be considered to "fine tune" or get the closest possible match when a substitution is needed. This information can be found in the clay’s technical data sheet.

Fluxes help lower the melting point of heat resistant clays and fillers in a clay body. Increasing the amount of flux also increases the glass formation with the clay body. A primary goal of a flux is to cause the clay body to melt at a stable predetermined maturing range. In functional pottery the maturing range occurs when absorption, shrinkage, and fired color are compatible with the glaze producing a dense vitreous clay body. In high temperature functional pottery (2232 0 F. - 1222 0 C. and higher) the maturing range of most clay bodies can be two or three cones in length. However, many sculpture pieces are fired just below their maturing range due to less shrinkage and warping in the clay. 

Every temperature range has the appropriate choice of flux materials that will work correctly. Using the wrong flux can have serious consequences. If a low melting flux is used in a high temperature clay body over firing of the clay can result or substantially reducing its maturing range. Conversely, if a high temperature flux is used in a low temperature clay body it will not cause a vitreous clay body. In such instances the flux acts like filler in the clay body formula. The appropriate flux and the correct amount must be used in the clay body. An over fluxed clay body can, bloat, slump, shrink excessively, and fuse to the kiln shelves. Knowledge of the various fluxes will reveal groups of fluxes that work well in each of the temperature ranges. When increasing the flux component of a clay body formula always make test pieces and place them in a regular production kiln on top of an old kiln shelve. Be aware that firing clay and glazes in small test kilns can produce inaccurate results. The smaller kilns have faster firing and cooling times as compared with larger production kilns. Ceramic materials need end point temperatures to melt but also need the time to reach that end point temperature for complete melting.

 Fillers reduce clay body shrinkage and warping in the drying and firing stages. Flint, silica sand, sawdust, mullite, calcined kaolin, calcined alumina, and grogs of various sizes are the most widely used fillers. While flint can be called a glass former, it needs very high temperatures (3200 0 F. – 1760 0 C.). to go into a melt by itself. It is only when flint is combined with a flux that the melting temperature of flint is decreased. Flint does act as a filler in that it reduces dry shrinkage and warping in the clay body. If the amount of filler is too high, the clay body's plastic qualities are decreased. Clay bodies designed for slab forming and tile bodies usually have more filler or non plastic material than throwing bodies. Less warping and shrinkage in tile bodies are a higher priority than plasticity. 

Grogs are highly refractory calcined materials that are one of the most common fillers in clay body formulas. They reduce warping in the drying and firing stages and decrease fired shrinkage. In hand building and throwing body’s they add “tooth” or stand up ability in forming operations. In throwing clay body formulas fine mesh grog is commonly used. The particle size can range from 60 mesh to 48 mesh. In sculpture formulas larger mesh sizes are used in the 20/48 mesh range.

A perfect clay body is a subjective term but the chance of obtaining such an ideal goal increases when the correct amount and ratio of, clay, flux, and fillers are present. When designing a clay body take into consideration, the forming method, drying shrinkage, firing temperature, kiln atmosphere, glaze interaction, fired color, fired shrinkage and fired density. As a standard rule first mix a small amount of any test clay body or glaze with an old kiln shelve underneath to prevent any possible damage to the kiln shelves. When a good test result is achieved mix up a larger batch and test again until the new formula has proven itself over a few kiln firings.

Composing a clay body formula is often a personal choice based on theory, experience, and current knowledge of materials. A perfect clay body does not exist, but it does help to know what considerations are made when a formula is developed whether you mix your own clay or buy commercially prepared clay’s.

Many people have asked for the unabridged versions of these blogs. The information along with other topics can be found in my books;

+ What Every Potter Should Know

+ Safety in the Ceramics Studio

+ The potter Complete Studio Handbook


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