Published Sep 7, 2017
Even if you love the sculpture or pottery clay you use, at one point or another, the need or desire to change your clay body 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 clay bodies 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 Andrew 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 Poellot Harnetty, editor.
Why Test Your Clay Body?
Testing clay bodies provides you with information that you can observe, touch and feel first hand in your own environment. While a catalog photo shows what a clay may look like fired at one or several cones, it may not tell you what it will do at the cone you're firing to. Basic clay bar tests give you information more specific to your needs, and a 25-pound sample is usually enough to complete all the tests you need.
What to Test your Clay Body For
Tests 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 same height.
You'll need three clay bars for each body you're testing. For consistency and accuracy of results, use the same dimensions for all your clay bars. While some tests yield correct results regardless of the bar dimensions, you can always rule out size and dimension as variables that could cause any irregularities. I make 1/2 inch-thick bars measuring 2 inches wide by 6 inches long. If your work is thicker, make bars to match, but don't exceed 1/2 inches in thickness. After cutting the bars to size, draw a 10-cm-long line on one of them with a hash mark at each end (as shown at the top of page). This will be used for a shrinkage test later.
For the tests here, I've selected four bodies to test: two white bodies from Standard (#257 Grolleg Porcelain Cone 8 - 10 and #181 White Stoneware Cone 6 - 10) and two from Amaco (#29 Brown Stone Earthenware Cone 06 - 04 and #77 Terra Cotta Clay Cone 5). All four bodies have different characteristics at the temperatures to which I'm interested in firing them, and testing several bodies at the same time takes better advantage of each firing. Remember to write the clay number on the back of each tile.
Once all bars are cut and firing boxes are built, let them dry to bone dry. They can be stacked with newspaper layered in between and a board on top for a little weight to keep them from warping. It's important that the firing boxes and clay bars stay flat while drying to assure accurate test results.
Before the Bisque
At the bone dry stage, visually examine the bars for a color change if any. Use a ruler to measure in centimeters the shrinkage line to see if it has changed from its original 10 cm length. Record the results
For each firing, place the bars side by side in the firing box, and place one of them on the triangle stilts. The stilts should be placed about 1/4 to 1/2 inch in from the ends of the bar. If the stilts are too close, the bar may not warp or slump to its fullest potential. The shrinkage bar and the other regular bar are simply placed in the bottom of the box.
While the test firing can be done in any kiln, the results are most applicable if done in the same kiln used for your work. Indicate in your notebook or worksheet if it's an electric or gas kiln, oxidation or reduction firing, salt, wood, soda, etc. For best results, start firings at the lowest cone temperature and then progressively refire the bars at higher cones until the highest desired cone is reached. A sample for progressive test firings for a cone 10 clay body would be cone 06, cone 01, cone 6 then finally cone 10.
Clay Bar Tests
The following traditional tests give a good range of basic information that helps you to better understand your clay body. Keep good records in your notebook so you'll have the results for future reference. If there are other specific qualities, such as glaze fit or color effect, test for these also.
Visually examine for any warping or sagging movement in the center.
Remove the bar from the stilts, turn it over and place it on a flat surface so the gap (if any) can be measured. This test informs you at what temperature the walls may start to warp or deform or a plate may begin to slump. The information can be applied to the sculptural or structural applications of the clay body. An exact percentage for this is not as important as just knowing when the clay body starts to move.
Measure the length of the line in centimeters on the shrinkage bar, and subtract it from 10 (based on the original 10 cm line). For example, 10 cm - 8.5 cm = 1.5 cm. An easy way to convert this result to a percentage is to move the decimal to the right one place, so 1.5 cm means 15% shrinkage. Knowing the shrinkage rate helps in determining which glazes will fit the body and even which two bodies can be used together.
Absorption TestThere are two types of tests that can determine the absorption of a clay body. One is a simple visual test and the other is a weight calculation. For the visual test, place a few drops of a liquid (like ink) on the surface of the bar to create a stain. Let it soak in for an hour then wash off the surface with water. The darker the stain, the more absorbent the clay. This is not an exact test, but it gives a quick and useful general result.
The weight calculation test is more specific. Weigh the fired bar on a gram scale and record the result. Soak the bar in water for 24 hours, pat dry, then weigh again and record the result. Subtract the first weight (dry bar) from the second weight (water soaked bar) to get the weight of the absorbed water. Divide the weight of the absorbed water by the original dry weight and move the decimal two places to the right to find the absorption rate.
Example: Original bar weight of 4.2 grams is subtracted from soaked bar weight of 4.6 grams giving you an absorbed water weight of .4 grams. Divide .4 by 4.2 which equals .095 making absorption 9.5%.
Visually examine a bar to see if there has been a color change. The color change can sometimes be dramatic depending on the cone it was fired and is important for aesthetic purposes. It can also help you determine the best glazes to work visually on the surface.
Visually examine the bar to determine if the surface has changed. Run your fingers across the bar to see if it's the same, smoother or coarser. This information is important aesthetically and helps you determine if it meets your visual and tactile needs.
Using a nail, see if you can scratch the surface to see how hard or soft it is at the fired temperature. This test helps determine the surface durability at different temperatures.