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How to Use Local Clays in Your Pottery

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Have you ever wanted to dig your own clay? Local clays can make wonderful slips and (when fired higher than the clays' actual maturity temperature) slip glazes. They can also be very interesting clays with which to create pottery. Does clay that is dug straight from the ground need processing? How do you know what temperature to fire it at?

How to Find Local Clay

Local clays can often be found exposed in creek banks, road cuts, and construction sites.
Photo © 2008 Beth E Peterson

Approximately 80% of the earth's land surface contains clay. There is a good chance that there are local earthenware clays near where you live. How do you find it?

Some of the best places to look for clay include:

  • river banks
  • stream beds
  • road cuts
  • naturally exposed earth such as in canyons or gullies, and
  • construction sites
Always make certain to get written permission from the land's owner before digging any clay. It is especially important to get permission when the clay is in a construction area. Follow safe practices, such as staying clear of dangerous terrain or machinery. Never dig clay from National or State Parks or Monuments; disturbing the natural habitat is often illegal and can carry some serious consequences.

Processing Your Clay

Clay straight from the ground does need to be processed. The clay will need to be sieved to remove unwanted material such as rocks, twigs, and roots. Sieving can be done either of two ways. The clay can be pulverized when dry and then sieved, or (my preferred method) dried, slaked down in water, then sieved.

The finer the mesh used, the fewer inclusions your working clay will have. I suggest using a 50-mesh screen, although a 30- or 80-mesh will do. Don't go any finer than an 80-mesh screen, as that can negatively affect the clay's usability.

Once the clay has been cleaned and slaked down, you will want to bring it back to a working consistency. To do this, follow Steps 3 and 4 of How to Recycle Clay Scraps.

Create Test Pieces

Now that your clay is workable, it is time to create several test pieces. Test pieces should have upright components, so you will be better able to detect deformation when it occurs. Small bowls are a good shape for test pieces. Mark each test piece so results can be tracked. Marking can either be done by incising designators into the clay while it is still damp, or by using oxides or stains to write them on the clay.

If you want to test your clay for its use as a slip, create test pieces either using the clay body you will be working with, or a white clay body. Either of these should have a higher maturation temperature than you believe your local clay has. Decorate the test pieces with the slip, using various techniques.

Before Firing the Test Pieces

As you work with the clay creating the test pieces, you are already conducting your first test. You will inevitably discover the clay's workability. You will also need to test for maturation range. When testing for the clay's maturation temperature, make sure you always fire your test pieces on bisqued plates which are larger in circumference than the test pieces and are made of a higher firing clay. Other attributes you may want to test for include shrinkage and absorption rate. Read How to Test Clay Bodies for more information.

Test Local Clays for Maturation

Any clay you find will almost certainly be an earthenware. Most other clays must be mined; they are not generally found at the surface. You'll need to run tests on your clay to figure out what range it has for certain, but a reasonable place to jump off of is to assume it will fall somewhere in between cone 06 and cone 02. I would begin by firing test pieces at cones 08, 06, 04, and 02.

Record all results for your trials and determine if you have discovered the maturation temperature. If not, run further tests. For example, the clay may seem to be fairly mature at cone 02, but you would like to check to see. In that case, I would run a second series of test pieces, firing them at cone 01, 1, and 2.

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