Clay Making Lesson Plans - Testing Clay BodiesFrances Osborne Austin Tx

By Alice Lane | Submitted On March 26, 2009

Basic clay making lesson plans should include teaching students the properties of different clay bodies. Some clay bodies shrink less; others have more or less absorption; some bodies have higher or lower maturation points; and of course, clay bodies come in a wide variety of colors. While the hundreds of kinds of commercial clays available from suppliers are described rather well in the catalogs in terms of what they are and what they can be used for, nonetheless actually working with them and firing them raises specific issues which can only be resolved by actual testing. Students should learn early on how to test clay bodies in order to understand their material - its possibilities and limitations.

Testing a clay body provides a great deal of information which can be observed, felt and touched first hand in the studio. While catalog photos and art lesson plans show how bodies may appear when fired at different cones, they may not reveal exactly what the clay will do at the particular cone to which you are firing. A simple clay bar test gives specific information relevant to the projects at hand. A 25 lb. sample is usually sufficient to complete all the tests needed. In order to yield a wide range of information about the clay body, tests should be run at different temperatures, since even if only firing to a specific cone is contemplated, still the results from firing at other cones can prove useful in the future. The three most important characteristics of clay bodies are shrinkage, absorption, and slumping / warping. It is also important to note color, plasticity, texture, and hardness.

When firing at higher temperatures than recommended (whether by mistake or purposely), clay bodies melt and fuse to the shelves. In testing clay bars a shallow firing box is needed to protect the shelves of the kiln, as well as to make for easier handling. Clay stilts are also needed for a warping test. Make simple clay boxes out of high-fire clay, 8" to 10" square with 1" walls. Make triangular stilts about as thick as a little finger, and sufficiently long to span the clay bar's width.

For each clay body being tested, three clay bars are needed, roughly ¼" thick by 2" wide by 6" long. If the students' work projects will be thicker, then the bars should be made to that thickness (but not exceed ½" in any case). After cutting the bars, draw a 5" long line on one of the bars with a hash mark at each of the ends, to be used in the shrinkage test. The bars should be left until they are bone dry. Measure the shrinkage lines on the bars to see whether they have changed from the original 5" length. Place the bars side-by-side in the firing box, and put one of them (without shrinkage line) on triangular stilts. The one on stilts is used to determine warping or slumping. Start firing at lowest cone temperature, and then refire the bars at higher cones, until the projected highest cone is reached. For example, sample bars to test for a cone 10 body might be progressively fired at cone 06; cone 01; cone 6; and at last cone 10.

Testing is easy, and is basic to ceramic art education. Color change and surface texture change can be determined visually. Hardness can be determined by scratching with a nail. This test is used to determine the durability of the surface at different firing temperatures. The bars can be examined visually for warping or slumping in the center: remove the bars from the stilts and place on a table top upside-down so that the height of the slump can be measured. This test indicates at what temperatures the walls might begin to warp, or a plate might begin to slump. Shrinkage can be determined by measuring the length of the lines on the shrinkage bars. Understanding the rate of shrinkage is helpful in determining which bodies can be used together, and which glazes will work for the body. To determine absorption, two tests can be used: simple visual inspection, and weight calculation. Put a few drops of ink on the bar surface to stain it. Allow it to soak in for a few hours, then wash the surface with water. The darker the remaining stain, the more absorbent the clay. For more specific measurement, weigh the fired bar; then soak the bar overnight, pat it dry, and weigh it again. The difference between the two measurements is the weight of the absorbed water, which is divided by the original dry weight of the bar to obtain the absorption rate.

Ceramic art education starts with knowledge of the fundamental properties of clay bodies. Art lesson plans should include simple tests which make clay making lesson plans both fun and informative.

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