|Even if you love the sculpture or pottery clay you use, at one point or another, the need or desire to change clays can arise. You might want a clay body that shrinks less, or want to change from an iron rich clay body to a porcelain. Whatever the reason, whether you are buying commercial clay or mixing your own, it is always a good idea to test clays before purchasing large quantities. Even though commercial clays might have catalog descriptions that sound just right or another potter swears by a certain pottery clay recipe, once we apply our specific working and firing processes, results can vary. Today, Paul Wandless explains how a combination of simple tests can give you plenty of information that will make choosing and learning about pottery and sculpture clays a little easier. – Jennifer Harnetty, editor.|
Why Test Your Clay Body?
|What to Test For
should be done at multiple temperatures to yield the widest range of
information on the clay body. You need to understand the same general
characteristics at every temperature you fire to, and even at
temperatures you may want to fire to in the future. I test at every
potential cone I may fire to and keep records of all the results.The
three important general characteristics to take note of are shrinkage,
absorption and warping/slumping. Other important qualities to note are
color, texture, plasticity and hardness. Some results are determined
with visual and touch tests while others require simple formulas. All
require consistency of procedure so the results you achieve are created
under the same conditions.
|Firing Box and Stilts
When firing above a recommended temperature – either on purpose or by
mistake – clay bodies start to melt and fuse or stick to the shelves.
To test clay bars, you’ll need a shallow firing box to protect your
kiln shelves and to make handling and transportation easier. You’ll
also need clay stilts for the warping test.Using a high-fire clay, make a simple clay box that’s 8 X 10 inches
square with a 1-inch wall as shown at left. Make several boxes at the same time so you
can test multiple bodies in the same firings or just have them on hand
for future use.Make triangle stilts about the thickness of your pinky and long enough
to span the width of the clay bar. All stilts must be the
For the tests here, I’ve selected four bodies to test: two white bodies
Once all bars are cut and firing boxes are built, let them dry to bone
Before the Bisque
For each firing, place the bars side by side in the firing box, and
The weight calculation test is more specific. Weigh the fired bar on a
This post was excerpted from Electric Firing: Creative Techniques, which is available in the Ceramic Arts Daily Bookstore.