Tennessee Technological University - Appalachian Center for Crafts - Clay Studio

Vince Pitelka, 2007

Art 3521 - Clays and Glazes
Testing Clays and Claybodies

Clay Analysis
Even with only a very small sample of clay there are a number of simple tests which can tell you a great deal about any clay or claybody.  The following tests involve math, and you are welcome to borrow my calculator, but please return it right away so it will be available for others to use.  For each completed test, record your results in your notebook, and then transfer to the appropriate spaces on the attached chart.  In the unlabeled spaces at the bottom of the chart, write in the names of all of the claybodies you are testing.  When we are done, everyone will share their information so that you can all fill out your charts as completely as possible.

Careful Measuring and Weighing; Converting Fractions to Decimal Equivalents
When weighing and measuring your test tiles, be as accurate as possible.  Use the gram scale properly.  Read about it in the text, and ask for help if you need it.  Remember to adjust the tare beam to neutralize the weight of the weighing container, if any.  With the weighing container in place, the needle should read very close to the center of its travel. 

When measuring your test tiles, if possible use a ruler marked with decimal fractions of an inch.  Or, you may choose to do all your measurements with a metric ruler.  If you use a regular ruler, observe the measurements very carefully.  If the measurement is halfway between 1/4" (4/16) and 5/16", record it as 9/32".  Before doing the subtraction and division, you will need to convert all fractions to decimals, which is a very simple matter.  With each fraction, just divide the smaller number by the larger number to get a decimal equivalent.  For example, to convert 9/32", divide 9 by 32, giving .28".

You will each test one raw clay, plus all claybodies that you are using, and any others you would like to test.  We will assure that no two people are testing the same raw clay, in order to investigate as many clays as possible. 

1) Water of Plasticity
This term refers to the percentage water content required to bring a particular dry clay or claybody to its most plastic state.  The smaller the average particle size in the clay, the more plastic the clay will be, and the more water required for water of plasticity.  Thus a high "water of plasticity" percentage is an excellent indication of a more-plastic clay. 

To measure the water of plasticity, place a plastic weighing scoop on the gram scale, and balance the tare beam.  Move the 100-gram weight over to 100 grams, and add dry raw clay or dry powdered claybody until the scale balances.  Remove the scoop from the scale and deposit the clay on a clean area on one of the Formica tables or counters.  Begin dispensing drops of water from a clean small sponge onto the clay, mixing it in thoroughly with a modeling tool or rubber spatula until you reach the desired plasticity.  Use a rubber spatula or rib to make sure that all the dry clay from the surface and the mixing tool is worked up into the mass, remove all clay from the mixing tool, and add it to the sample.  Place the resulting lump of plastic clay back in the same scoop, weigh it again, and figure out the increase in weight.  Since the original dry sample was exactly 100 grams, the number of grams over 100 is the percentage water of plasticity.  Place the wet sample in a baggie or wrap it in plastic wrap to keep it moist for additional tests. Record the results in your notebook and repeat this test for each raw clay and claybody you are testing. 

2) Actual Plasticity
For a simple test of actual plasticity, take a small amount of each plastic clay and clay body, one at a time, and roll out a 4"-long pencil-size coil of each on a damp but not wet surface (damp plaster or canvas is ideal).  Do not roll the coil on a dry surface, because it will dry out the surface of the coil and give you an inaccurate measure of plasticity.  Keep careful track of which clay is which, and take a coil of each clay and wrap it around your finger, noting any tendency to crack or separate.  If any clays survive this test without cracking, try tying a coil in a square knot.  Note the performance of each clay claybody in your notebook.   

3) Shrinkage
You can discover a great deal about any claybody by testing the shrinkage in drying and firing.  To test shrinkage, roll out a 3/8" by 1" by 5" test tile of each clay and claybody.  Trim one long edge of tile straight, and place two incised marks exactly 4" apart along this edge.  On the surface of the tile clearly label the claybody.  Allow the tiles to dry slowly under plastic so that they do not warp upwards at the ends.  When bone dry, carefully measure between the marks, and record the measurement.  Bisque-fire the tiles, immediately weigh the bisque-fired tiles, record the weights, and measure and record lengths again.  Before you fire the tiles again, proceed with the first sage of the absorption tests explained below.

After completing the bisque absorption test, fire the tiles unglazed to the appropriate mature glaze-firing temperature (for that clay or claybody), and measure and record again.  To get the shrinkage at each stage, subtract each  measurement from the previous measurement (dry measurement from damp measurement, bisque-fired measurement from dry measurement, glaze-fired measurement from bisque-fired measurement), and in each case divide by the previous measurement.  For example, if the dry length between marks is 3 3/4", subtract from the original wet length of 4", giving 1/4" or .25".  Divide .25" by 4", giving .0625, or 6% drying shrinkage.  There will be negligible shrinkage from dry to bisque-fired.  To figure the overall percentage of firing shrinkage (bisque and glaze), measure the size after the final firing, subtract from the bone-dry size, and divide by the bone-dry size.  To determine the percentage of shrinkage overall, subtract the final fired measurement from 4", and divide by 4".  Record the shrinkage percentages at each stage and overall shrinkage for each claybody carefully in your notebook, so that we may compare everyone’s results.  Note: immediately after the tiles come out of the bisque firing and the final firing, weigh each, before it absorbs any atmospheric humidity, and record the weight. 

4) Porosity/Absorption
As mentioned above, immediately after bisque-firing the test tiles and again after the final firing, weigh the tiles immediately and record the weights.  The reason for the immediacy is to record the dry weight before the clay absorbs any atmospheric humidity.  After bisque, and again after the final firing, take the tiles home with you, place in a pan of cold water on the stove, bring it to a boil for ten minutes, and leave it on the stove to cool to room temperature overnight.  In the morning, wrap the tiles all together in a soaking-wet napkin or paper towel, seal them inside a plastic bag and bring them back to the studio.  Remove them from the bag, blot off all surface water with a dry paper towel, immediately weigh each of them carefully, and record the weights.  Subtract the dry weight from the wet weight, and divide by the dry weight to give percentage absorption after bisque firing and after glaze firing.