Clay shrinks when it dries, gets bisque fired, and again during the glaze firing. Whenever you make a new batch of clay or buy premixed commercial clay you should test your clay even if you are using the “same” clay or ingredients as before. Gradual changes in chemical and physical properties may or may not affect your clay. Some kaolin and ball clay mines do remain pretty pure from year to year but it is still a good idea to test your clay as changes sometimes occur. Often the changes are unnoticeable and not serious, they can just be ignored, however, some potters take precautions by doing random sample tests from their new clay shipments before they use the clay in production work. This can be with your clays whether you mix your own clay or you buy premixed commercial clay.
A great simple and easy test to do is make up a small batch of your clay body, make a few shrinkage/absorption test tiles and a few small pieces you would like to try the glazes you commonly use on the clay body.
Shrinkage and absorption is a very useful piece of information to know about your clay. If you don’t have this information, I would take your existing clay that you use and do the tests on them. Each time you mix a new batch of clay, or buy a new bag of clay, with new materials, you can do new tests to ensure nothing changed.
The average plasticity of your clay (wheel throwing) will usually have around 6% shrinkage going from plastic clay to bone dry, which is commonly called the dry shrinkage. Going from bone try to top firing temperature, is called the fired shrinkage, which is between 6-8% which means that the total shrinkage (wet to finished) is between 12 and 14%.
Most plastic and throwing clay bodies will have these kind of shrinkage rates if they are mature, dense and hard at top firing points as specified. This is a good standard even though there can be exceptions and deviations up or down from this. Average absorption for dense, mature and hard clay bodies, at any temperature, should be below 5%. Hopefully the clay body will be more along the lines of 2% or 3%. China and porcelain clays should have 0 to 1% absorption rate. Traditional earthenwares are softer and more porous clay bodies, will have 8-12% absorption. Sculptural clay bodies can have absorption rates as high as 25%.
Testing for Shrinkage and Absorption rates
Create a flat rectangular tile that is around 5″ or 6″ by 2-2.5″ test tile that is 10 cm long by and make it the average thickness of the kinds of pieces you like to make. Make the test tiles from clay that is the consistency you would use when making your own work whether that is softer for wheel throwing or a little more on the dry side for slab or handbuilding.
Make an incision that runs 10 cm down the middle of the test tile. Label the line so that you remember what they are for. Let them dry in the same manner as you would your pieces. don’t speed up the drying process of your tiles. Put the 10 cm line on the test tiles when they are at the same consistency you would use the clay at. Leave them to dry on the board you made them on as moving them around after marking the line might distort the measurement. Every millimeter less than 10 cm represents 1% shrinkage. For example if the line measures 9 cm that means that the bclay body has a total shrinkage of 10%.
To find the percentage of absorption carefully weigh the tile after it has been fired. Record the dry weight. Boil the test tile in water for 2 hours and let it soak overnight. Take the tile out, wipe it so there is no water on the tiles surface and weigh again. This is the wet weight. Subtract the wet weight from the dry weight and then divide by the dry weight, and finally multiply the result by 100 to give you the percentage of absorption of the clay body.
Fire the shrinkage tiles in different areas of the kild and take the average shrinkage of all the tiles to get a good working percentage. The same can be applied for absorption. Apply glazes to other test pieces, fire, and compare them to the results with the previous firings. Some clays have been altered in important ways, these tests will alert you to the changes. Some ceramicists will actually dry screen using 40 mesh their fire clays and earthenware clays to catch any obvious calcium or coal particles, as these are usually the culprits of the sourced clay body problems
Another challenge in clays is sulfur. a chemical analysis can give you an insight into how much sulfur is present. A slow bisque, and proper ventilation is the best way to resolve the sulfur problem.