It is important to know how to test a clay body to understand the different attributes it might have. Testing shows how workable it is, how much water it will absorb, and how much it will shrink when fired.
This is a measure of the way a clay body can be manipulated, and is mostly a subjective matter. Your opinion of a clay body’s working character will be influenced by the way you wish to use it. If you intend to create your forms on a pottery wheel, you may want a very plastic and bendable clay body. On the other hand, if you are hand forming large, complex pieces, you will want a tough, durable clay body that resists cracking when you manipulate and fire it.
To test for working character, roll out a coil of clay about 1 centimeter thick and 14 centimeters long. Bend it into the shape of a script e (1). If the clay body doesn’t crack at the bend, it has a good working character. If it cracks at the bend, the working character of the body is poor. If you form your work on the potter’s wheel, continue your test by throwing a sample of the clay on the wheel. If you hand form your pieces, make a test slab, curve it, and paddle it to see how it reacts to this treatment.
One of the things you need to know about a clay body is the rate it absorbs moisture at your firing temperature. Most fired clay bodies absorb some moisture but the absorption rate can vary a good deal. Following are two procedures for testing absorption. The first is an informal method that only takes a minute or two; the second is far more accurate but takes more time.
Informal Clay Body Absorption Test: Lay a light coating of water on a piece of ceramics that has been fired to maturity but is not glazed. The base of a fired pot will do nicely if it is completely free of glaze.
The object of the test is to observe the rate of absorption. If the water is absorbed into the clay body in a span of 20–40 seconds, the body is highly absorbent. If the water is absorbed in a span of 2–3 minutes, the body is moderately absorbent. If absorption takes longer than 2–3 minutes, the body is mature. If the water stays on the surface without any absorption, the body has little or no absorption.
Computational Clay Body Absorption Test: This highly accurate test is carried out on a test tile made especially for the purpose. The test tile should be fired to maturity and not glazed.
- Make a test tile 2×2 inches or larger and fire it. Remove the tile from the kiln as soon as it cools.
- Weigh the tile and write the weight on it (use a pencil or an indelible-ink pen).
- Immerse the tile in water for 24 hours or longer.
- Remove the tile from the water and use a sponge and a towel to remove the surface water.
- Weigh the tile again.
- Subtract the original weight from the weight of the immersed tile.
- Divide the difference by the original weight.
- Move the decimal point two places to the right. The result is the percentage of absorption.
Example: Weight of the tile after immersion: 9.8 grams
Weight of the tile when drawn from kiln: 9.4 grams
0.4 ÷ 9.4 (original weight) = 0.042
After moving the decimal point two places to the right, the result is 4.2, meaning the absorption rate of the tile is 4.2%.
Appropriate Absorption Rates: While the absorption rate of a clay body is an objective measurement, subjective judgment plays a very important part, as you must decide what is appropriate for your work. There is no single appropriate absorption rate for all clay bodies or for all types of ceramic work. If you work in porcelain, you will probably want a very low absorption rate. If you are working at low-fire temperatures, you cannot hold your clay bodies to such exacting standards.
The intended purpose of the work also plays an important part. If you make utilitarian pieces, you may want a low absorption rate. However, if you are making sculptural or decorative pieces, a low absorption rate might not be appropriate. Below are listed various clay body types and a judgment of their appropriate absorption rates.
Clay bodies shrink in both drying and firing. As is the case with absorption, appropriate shrinkage rates vary a great deal. No ceramic artist likes a high rate of shrinkage, but if you are working with a fine-grained clay body with a low absorption rate (such as a fine-grained white body), you may have to accept a high rate of shrinkage as inevitable.
- Make a test tile measuring14×4×0.7 centimeters using a clay body with water content that’s typical of your clay body.
- Draw a line along the length of the tile and mark off 10 centimeters (2, 3).
- Let the tile dry and fire it to the desired temperature.
- Measure the new length of the line (4).
- Subtract the length of the new line from 10 centimeters (the original length of the line).
- Divide this figure by 0.1 to find the percentage of shrinkage.
Appropriate Shrinkage Rates: As is the case with absorption, the ceramic artist must decide what shrinkage rate is appropriate. Appropriate shrinkage rates can and should vary a great deal. A dense porcelain is likely to have a very high shrinkage rate, while a sculpture body fired to a less dense state will have a low shrinkage rate.
The intended purpose of the work also plays an important part. The ceramic artist who works with very large pieces that cannot stand the strain of a high rate of shrinkage will have to work with coarse, highly absorptive bodies that don’t shrink very much. The ceramic artist who creates utilitarian ware may well decide that a low absorption rate is necessary and will put up with a high shrinkage rate.
Below are various clay body types and a judgment regarding their appropriate shrinkage rates.
Slumping and Warping Test
Bodies that are mature or overfired often distort or slump during the firing. If the body has a very low absorption rate and it slumps a great deal, the body may be overfired. In a mature clay body a small amount (as much as ¼ inch) of slumping is inevitable, but if the clay slumps and warps more than that, you may want to find a clay body that is more appropriate for your firing temperature.
This is an important test and a very simple one.
- Make a test tile 4 inches long and let it dry completely.
- Place refractory supports (such as kiln posts) under each end of the tile so it is free to slump in the middle (5).
- Fire the tile to the desired temperature (the tile need not be bisque fired).
- Measure its rate of slumping in the middle (see 6). If the amount of slumping is over a ¼ inch, forms made with this body are likely to slump or warp.
Excerpted from Electric Kiln Ceramics: A Guide to Clays, Glazes, and Electric Kilns by Richard Zakin and Frederick Bartolovic, published by The American Ceramic Society and available at https://mycan.ceramicartsnetwork.org/s/product-details?id=a1B3u000009udqTEAQ.