CLAYS- Prepared Clay Bodies
There is a certain amount of mystique and a bit of "black magic" involved with clay bodies. There is no "perfect" clay body. Anyone working in clay needs to realize and continuously appreciate the fact that the basic ingredients of all clay bodies are naturally occurring, removed from the earth with little if any refinement and ground to powder, mostly for industrial purposes other than for use by ceramic artists. Thus, regardless of the care and testing done by the mines and commercial clay body manufacturers, variations are to be expected from batch to batch over time.
Before use, it is recommended that testing be performed on any new batch of clay to determine its suitability for your particular technique and application. It must also be kept in mind that in every step of the forming process there are innumerable variables that are often immeasurable and all but impossible to adequately control and will affect the finished product.
SELECTING A CLAY BODY
Generally, most commercially prepared clay bodies can be made to work for almost any application with sufficient experience, practice and testing. However, certain fundamental characteristics can certainly lead to minimizing the time and frustration of repeated testing and failure. Beyond that, it becomes a matter of personal preference ("different strokes for different folks") whereby a particular clay is preferred over others within the same category. Every clay body is a bit different and very minor changes in shape, or in the forming, drying or firing process can make a previously unacceptable body suddenly ideal. The only way to determine which body is best for you is to test different bodies through your entire process.
FIRING TEMPERATURE - Of course, the desired firing temperature is important and especially so for functional ware to assure durability and that it will hold a liquid without weeping. Bodies should be fired to their maximum recommended mature temperature for functional ware. A Cone (or two) higher or lower may also work acceptably but test first. Otherwise, it should be noted that any body can be fired considerably lower and still retain more than acceptable strength for non-functional items that don't need to withstand, as an example, the abuse of repeated cleaning in a dishwasher. In fact, many sculptors purposely select a high fire body and low fire it to minimize energy costs as well as the possibility of breakage from the added shrinkage and thermal shock of the higher firing. It should further be noted that most glaze formulations are designed to "fit" a mature body. The most common glaze problem on an immature clay body is an increased tendency to craze which may, in aesthetic terms at least, even be desirable on non-functional pieces.
PLASTICITY & SHRINKAGE - Unless otherwise specified by the manufacturer, both plasticity and the shrinkage rate of a commercially prepared clay body should be within an acceptable range or the body would not be commercially viable.
STIFFNESS/SOFTNESS - It is difficult to judge a clay while it is in the bag. Many clays (most noticeably porcelains) soften considerably just by being thrown down on a hard surface while still in the bag. The stiffness is a matter of personal choice and special needs but any reasonably fresh commercial clay body should be acceptable. If in doubt, request it soft. It is always easier to dry it out than to add water.
COLOR - The final fired color of clay can vary considerably depending on temperature and firing. Generally, the higher the temperature the darker the clay becomes. The amount of reduction when firing in a gas kiln will darken a body. Even a white porcelain often turns gray. Considerable variation can occur in electric firings as well depending on such things as ventilation (the more air that circulates in the kiln, the less the darkening effect). Sand and grog can add a speckled look usually lightening the overall visual effect of a dark body and darkening a light body. Some clay bodies spot, especially in reduction gas firings.
TEXTURE - Sand and grog each add texture and surface interest. Vermiculite, pearlite, chipped feldspar, or very course sand is sometimes added by the user for special effects.
SIZE OF WORK - Some clay bodies have more "tooth" or "stand-up-ability" than others. Generally, the inclusion of sand and/or grog enhances this quality so for larger pieces more and larger particle-sized sand and grog is desirable.
SPECIAL NOTE TO TEACHERS
Traditionally, most school programs have used low fire white clays in conjunction with the dependable brightly colored low fire commercially prepared glazes. These glazes are designed to fit the low fire white (talc-base) clays. While this can give quite satisfactory results, the big disadvantage has always been that those clays are not particularly "strong" and are subject to breakage, especially when greenware is handled by inexperienced young hands. In addition, students often do not take the time to attach parts adequately resulting in disappointment when they break off in the firing. Many of our teacher customers have found that the use of a high fire stoneware can reduce many of these "accidents". Although the high fire clay is underfired, the right clay can be as strong or stronger than the talc-base clays. A very strong high fire clay for this purpose is Clay Planet's ORION STOUT stoneware clay which is only slightly off white at low fire temperatures so the glazes maintain their brightness and brilliance. The only disadvantage is some increased likelihood of glaze crazing which is often not a critical problem in young student work and the probability can be further reduced by bisque firing at a higher temperature such as Cone 2.
DISCLAIMER: STATEMENT IN LIEU OF ALL EXPRESSED AND IMPLIED WARRANTIES: CLAY PLANET'S ONLY OBLIGATION SHALL BE TO REPLACE SUCH QUANTITY OF PRODUCT PROVEN TO BE DEFECTIVE BY STANDARDS AND METHODS ACCEPTED WITHIN THE CERAMIC INDUSTRY. CLAY PLANET SHALL NOT BE LIABLE FOR ANY INJURY, LOSS OR DAMAGE, DIRECT OR CONSEQUENTIAL, ARISING FROM THE USE OR THE INABILITY TO USE THE PRODUCT. BEFORE USING, THE PURCHASER SHALL DETERMINE THE SUITABILITY OF THE PRODUCT FOR HIS/HER INTENDED APPLICATION AND HENCE ASSUMES ALL RISK AND LIABILITY WHATSOEVER IN CONNECTION THEREWITH.