| Basic clay making lesson plans should include teaching | | | | the students' work projects will be thicker, then the |
| students the properties of different clay bodies. Some | | | | bars should be made to that thickness (but not exceed |
| clay bodies shrink less; others have more or less | | | | ½" in any case). After cutting the bars, draw a |
| absorption; some bodies have higher or lower | | | | 5" long line on one of the bars with a hash mark at |
| maturation points; and of course, clay bodies come in a | | | | each of the ends, to be used in the shrinkage test. The |
| wide variety of colors. While the hundreds of kinds of | | | | bars should be left until they are bone dry. Measure |
| commercial clays available from suppliers are | | | | the shrinkage lines on the bars to see whether they |
| described rather well in the catalogs in terms of what | | | | have changed from the original 5" length. Place the |
| they are and what they can be used for, nonetheless | | | | bars side-by-side in the firing box, and put one of them |
| actually working with them and firing them raises | | | | (without shrinkage line) on triangular stilts. The one on |
| specific issues which can only be resolved by actual | | | | stilts is used to determine warping or slumping. Start |
| testing. Students should learn early on how to test clay | | | | firing at lowest cone temperature, and then refire the |
| bodies in order to understand their material - its | | | | bars at higher cones, until the projected highest cone is |
| possibilities and limitations. | | | | reached. For example, sample bars to test for a cone |
| Testing a clay body provides a great deal of | | | | 10 body might be progressively fired at cone 06; cone |
| information which can be observed, felt and touched | | | | 01; cone 6; and at last cone 10. |
| first hand in the studio. While catalog photos and art | | | | Testing is easy, and is basic to ceramic art education. |
| lesson plans show how bodies may appear when | | | | Color change and surface texture change can be |
| fired at different cones, they may not reveal exactly | | | | determined visually. Hardness can be determined by |
| what the clay will do at the particular cone to which | | | | scratching with a nail. This test is used to determine the |
| you are firing. A simple clay bar test gives specific | | | | durability of the surface at different firing temperatures. |
| information relevant to the projects at hand. A 25 lb. | | | | The bars can be examined visually for warping or |
| sample is usually sufficient to complete all the tests | | | | slumping in the center: remove the bars from the stilts |
| needed. In order to yield a wide range of information | | | | and place on a table top upside-down so that the |
| about the clay body, tests should be run at different | | | | height of the slump can be measured. This test |
| temperatures, since even if only firing to a specific | | | | indicates at what temperatures the walls might begin |
| cone is contemplated, still the results from firing at | | | | to warp, or a plate might begin to slump. Shrinkage can |
| other cones can prove useful in the future. The three | | | | be determined by measuring the length of the lines on |
| most important characteristics of clay bodies are | | | | the shrinkage bars. Understanding the rate of shrinkage |
| shrinkage, absorption, and slumping / warping. It is also | | | | is helpful in determining which bodies can be used |
| important to note color, plasticity, texture, and hardness. | | | | together, and which glazes will work for the body. To |
| When firing at higher temperatures than recommended | | | | determine absorption, two tests can be used: simple |
| (whether by mistake or purposely), clay bodies melt | | | | visual inspection, and weight calculation. Put a few |
| and fuse to the shelves. In testing clay bars a shallow | | | | drops of ink on the bar surface to stain it. Allow it to |
| firing box is needed to protect the shelves of the kiln, | | | | soak in for a few hours, then wash the surface with |
| as well as to make for easier handling. Clay stilts are | | | | water. The darker the remaining stain, the more |
| also needed for a warping test. Make simple clay | | | | absorbent the clay. For more specific measurement, |
| boxes out of high-fire clay, 8" to 10" square with 1" | | | | weigh the fired bar; then soak the bar overnight, pat it |
| walls. Make triangular stilts about as thick as a little | | | | dry, and weigh it again. The difference between the |
| finger, and sufficiently long to span the clay bar's width. | | | | two measurements is the weight of the absorbed |
| For each clay body being tested, three clay bars are | | | | water, which is divided by the original dry weight of the |
| needed, roughly ¼" thick by 2" wide by 6" long. If | | | | bar to obtain the absorption rate. |