Ceramic Industry

Understanding Clay Body Formulation

July 30, 2009
This three-part series will discuss the many business and technical issues that should be considered when creating/evaluating a clay body formulation.

Boxed moist clay ready for use by the potter.

Purchasing pre-mixed moist clay from a ceramics supplier can offer many choices to the potter in terms of firing range, kiln atmosphere, forming characteristics and fired color. By offering a wide range of moist clays, the ceramics supplier is hoping for a satisfied customer who will be re-ordering more clay in the future.

A great deal of planning and testing is often behind a ceramics supplier’s choice of selling a pre-mixed clay. However, many stock clay body formulas were originally obtained from an individual potter’s clay body, which the supplier is now mass producing (hopefully with the potter’s consent). Within this group, there could be different levels of testing-or not. If a clay body works well for one person and it is formulated correctly, sometimes all is well and many potters can use it with good results. The formula gains popularity and stays in the catalog.

Whether a clay body formula comes into existence through this informal method or is carefully formulated, tested and adjusted before committing to full production, several business and technical considerations should be considered. This three-part series will discuss many of these considerations.

Is There a Need?
The first consideration for any ceramics supplier thinking about adding a new moist clay to their catalog is, “Is it going to duplicate an existing clay body?” There is usually no economic sense for a ceramics supplier to duplicate a moist clay that their customers are already buying. Of course, one exception might be when one supplier develops a formula similar to another supplier’s in order to gain market share.

While new moist clay formulas do come about, most ceramics suppliers have a vested interest in keeping their existing formulas since potters generally stay with a clay that is producing good results for them.

Name and Description
Moist clay is usually named or numbered depending on the history of the ceramics supply company’s method for clay body designation. However, each clay does have a catalog number for easy and accurate ordering and shipping. When choosing the name or number for a moist clay, it is often a matter of personal preference. Some clays are named according to who originally developed the formula, such as Pete’s Supreme, while others are purely descriptive, such as White Stoneware with grog.

Ceramics suppliers list the shrinkage and absorption rates for each moist clay they sell. The percentages are useful to compare one clay with another, but there can be a 1% or more difference in how the clay will shrink or absorb moisture in your own kiln. The percentages listed should only be compared very generally with other clays listed in the catalog.

Comparing these numbers with other ceramics suppliers’ clays is even more imprecise, as the size of the test kiln and the firing time to temperature are variable factors that can make comparisons meaningless. To arrive at more precise shrinkage and absorption rates, fire the moist clay in your own production kiln; small test kilns have different firing and cooling rates than larger kilns.

The next installment in this series will focus on moist clay performance and firing considerations.