Understanding Clay Body Formulation, Part 3

Clay mixer at a ceramics supply company. (Photo courtesy of Sheffield Pottery, Inc., Sheffield, Mass.)

This is the third in a three-part series. The first two installments of the series are available by following the links below.

Stability in Storage
Many ceramics suppliers ship their moist clay two 25 lb, 3 mil plastic bags that are enclosed in a cardboard box. The bags are sealed either with a twist tie, rubber band or-because of cost cutting-a twist of the plastic bag as it is inserted into the box. The name of the clay manufacturer and clay name or catalog number are usually stamped on the box. Since the plastic bags are permeable, the average shelf life of the moist clay is four to six months before it becomes hard.

Raw Material Availability
An important factor in any clay body formulation for the ceramics supplier is the transportation cost of the clays, feldspar, flint, grog or any other raw material that forms the clay body. Shipping charges can equal or exceed the actual cost of the raw materials.

It is uneconomical to develop a clay body formula using a West Coast material when the ceramics supplier is located on the East Coast. It is also not profitable to use a distinctive clay from a distant location to achieve a special handling or firing quality in a clay body. In most instances, there is not enough demand among potters for such a specialized clay body.

Ceramics suppliers tend to choose raw materials that will remain in production for reasonably long periods of time, but there is no guarantee on this central element in clay body production. Periodically, a ceramics supplier will have to reformulate a clay body formula because of the discontinuation of a raw material. Depending on the individual ceramics supplier, this fact is often not revealed to the potter.

Economic Considerations
For ceramic suppliers, the economics of making moist clay or buying it from a distributor is a major factor in keeping their business viable. Aside from low shipping costs, ceramics suppliers are looking for clay body formulas that have the widest appeal to potters while being easy to produce. A long production run of a single clay body formula is an ideal situation for a supplier, preferably followed by a formula of the same temperature range in approximately a similar color.

Nylon, fiberglass and paper fibers, while achieving unique results in a clay body, can require additional time and money to clean mixers and pug mills-all of which mean less clay being produced and higher labor costs. The same economic considerations apply to the use of any coloring oxide or stain in the clay body, as extra time is required to prepare the machines for the next batch of clays. If the machines are not cleaned properly the next batch of clay is often contaminated.

The actual retail price of the moist clay is also a central factor in its profitability. Many potters rate the price of moist clay as a high priority and are reluctant to pay a few cents more per pound. They think they are saving money, but they don’t realize that it’s the quality of the clay that is most important. However, a ceramics supplier has to be aware of pricing their moist clay in relation to the competition, as many potters still shop only by price.

Raw Material Quality
Clays are mined and refined to the specifications of larger industry standards, which might not insure their problem-free use by potters. Very few ceramics suppliers will take the additional steps of refining clays further to insure better-quality moist clay for their customers. However, a few suppliers do screen their clay, and this extra refining adds to the cost due to the equipment, labor and time involved in this quality-enhancing procedure. Some segments of the market are willing to pay the extra cost for improved quality moist clay.

Aside from screening the clay, ceramics suppliers might also be reluctant to use additives, such as Epsom salts, bentonite, nylon fibers, fiberglass fibers or paper pulp, in their stock clay body formulas due to the limited number of potters willing to pay the extra costs of production. Usually, potters who request such a unique clay body are charged an extra premium due to the added costs of the materials and machinery cleaning expense, as well as the time required to fit the special clay body into the production cycle.

Recommendations for Choosing a Clay Body
No ceramics supplier has a perfect track record of selling defect-free clay. However, some have better quality control procedures than others and provide moist clay with fewer defects. Potters should first investigate their local suppliers that have the lowest incidences of defective clay.

In part, this information can be obtained by asking other potters what their experience has been with a given ceramics supplier. Ceramics is a small world, and a few phone calls to potters can return big dividends in useful information on which suppliers to frequent. There is no defect-free clay production, so always find out how the supplier handles the eventual customer complaints. Do not expect that you will be treated differently from other potters who have experienced unfair services.

Choose the moist clay body that fits your firing temperature, forming method and kiln atmosphere. Many problems can be avoided just by using the appropriate moist clay for your individual situation.

The ceramics supply company’s catalog and staff, as well as other potters, are good resources to help you evaluate a moist clay choice. Most ceramics suppliers offer several moist clays in a particular firing range. Start by ordering small samples of each clay and test each for its handling properties, firing results and glaze interaction to see which one best suits your needs.


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