Clay-Making Project ~ Part 1
After many years of purchasing our clay from regional and national suppliers, we are looking into mixing our own clay blends for the studio. Our first step is formulating and testing clay recipes to see if they will suit our production needs better than the clays we can purchase.
Why do you need a special clay blend, you ask? Different areas of the world have different types of clay. Kentucky and Tennessee have deposits of clay that have a very fine particle size, referred to as ball clays. Florida offers a very white clay called kaolin. North Carolina has stoneware clays, red earthenware clays, kaolin, and feldspar, which is a material used to help the clay blend vitrify at the proper kiln temperature. The differences in clays come from how they have weathered and combined with other materials differently over time. By blending different clays (and their distinct qualities), potters can produce a clay that has qualities best suited to their production method... in our case, for throwing on the potter's wheel and doing slab work.
The qualities that we are looking for from our new clay blends are low shrinkage, good vitrification at our maturing temperature (cone 10, or 2340° F), resistance to thermal shock, and good standing strength when the clay is still wet. Our initial tests have yielded quite good results. The clay shrinkage of the tests is down to 8.8% from the 14.5% of the clay we've been purchasing, and the throwing properties are excellent.
Here you see us performing the first part of an absorption test, which helps determine if the clay is sufficiently vitrified after firing. The test sample gets weighed while dry, then the sample is soaked in water (or boiled) for a long period of time to allow as much water as possible to absorb into the clay structure. After soaking, the sample is weighed again, and the ratio between the dry weight and the soaked weight is expressed as a percentage. For stoneware, anything below 3% absorption suggests sufficient vitrification of the clay body. Anything over 3% suggests that the clay may leak or seep moisture over time. When the test is complete, we'll try to be sure to share our results.