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How to Test Clay Bodies

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How do you find out what the characteristics are of a given clay body? There are several tests you can run on clay bodies to find out their suitability to your work.

Working Properties

One of the first things you will want to know about a clay is how well it will work for the processes you have in mind. To quickly test for plasticity, roll a coil of moist clay out then wrap it around your finger. If it bends easily, it is a plastic clay. If the clay cracks, it is not very plastic, or it may not have enough water mixed in it. If it breaks, it is a non-plastic clay, or it may have too many inclusions.

Make a series of pots with your test clay, pushing the limits of the form and the process of making it. This is way you discover the clay's working properties.

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Maturation Temperature

You may not be sure what temperature this clay needs in order to mature. Make your best guess, then fire some of your test pieces two or three cones above and below the estimated temperature, as well as at that temperature. It is a good idea to fire the test pieces on a bisqued plate or disk of clay, just in case the clay should unexpectedly vitrify too much.

Maturity is determined by the temperature where the clay reaches its densest and hardest without any deformation. For more information, see How Temperature Changes Clay

Absorption Rate

Once you know a clay body's maturation temperature, you can test it to find out how much water the mature clay will absorb. Using an unglazed test piece that has been fired to maturity, weigh the pot as accurately as possible with a triple beam scale or sensitive digital scale. Write this number down, then boil the pot in water for two hours. Remove the pot, dry it with a towel, then re-weigh it.

To find the absorption rate, subtract the saturated weight from the dry weight. Divide the difference by the dry weight. For example, let's say a pot weighed 0.75 pounds after it was fired to maturity. After boiling, it weighed 0.8 pounds. The difference is 0.05. Dividing 0.05 by 0.75, we get 0.067, or an absorption rate of 6.7%.

Shrinkage

To determine the clay body's shrinkage, roll out a coil of the clay to about six inches. Trim the coil to exactly five inches, cutting off both ends to make them flat and perpendicular to the table. Dry the coil, then fire it to the clay's maturation temperature.

Re-measure the coil once it has come out of the kiln; be sure to use a ruler that measures in decimal fractions of an inch. Subtract the coil's new length from the original five inches. Divide the difference by five (the original length). The answer is the shrinkage rate.

For example, let's say the coil measures four and a half inches after firing. The difference is 0.5. Dividing by 5, we get 0.10, or a shrinkage rate of 10%.

For more information, see Why Clay Bodies Shrink.

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