Stoneware Clays and Clay Bodies
Natural stoneware clays are fairly abundant, though not nearly as common as red-burning earthenware clays. Any clay might be called a stoneware clay which can be fired to a temperature of 1200°C (or about cone 7) and which, as a result of this treatment, is of a serviceable hardness and density. Stoneware clay may burn to a buff, tan, gray or light brown, or to a dark brown or brown-black, depending on the amount of iron or other impurities present, and depending also on the exact temperature and atmospheric condition of firing. Most common, surface, iron-bearing clays are unsuitable for stoneware because they will not withstand temperatures above about cone 4 without excessive softening and loss of shape. On the other hand, very pure clays, such as kaolin and many fire clays, cannot be used by themselves for stoneware because they are too refractory and do not become hard and dense unless they are fired to cone 14 or more. Stoneware clays are commonly of the sedimentary type, and are found in seams or layers, often associated with fire clays, flint clays, or coal.
Stoneware clays vary considerably, not only in fired color but also in working properties. Some may be very plastic with an attendant high drying and firing shrinkage, while others may be relatively nonplastic and have a low shrinkage. The stoneware potter will search for a clay which first of all has the right reaction to the fire and produces a fired substance of the desired color and density. Beyond that he will look for good working properties such as plasticity, good drying, and not too much drying and firing shrinkage. Not very many natural clays will be found to be satisfactory on all counts, and for this reason it is common practice to make stoneware bodies from a blend of several clays and nonplastic ingredients. However, it is always wise to use no more materials in a clay body than are necessary, and if one fine natural stoneware clay can be found which will perform well without the additions of any other material so much the better.
There is no large market for stoneware clay such as exists for kaolins and ball clays, and only a very few stoneware clays are mined commercially. This makes stoneware clay hard to obtain in many localities. Another trouble is that many fine stoneware clays are found in relatively small deposits which do not justify large-scale mining, and so are available only to local potters. I have used fine stoneware clays from Colorado, Wyoming, and Iowa which were excellent for potting just as they came from the ground, without any additions. These were all from small pits, and the clay was not being mined or used commercially. If one is interested in locating and using native clay for stoneware, the surveys of natural resources which have been made in the various states can be of real help in locating likely deposits, and the geologists who are employed by many of the states can often give valuable advice. The Midwestern States are rich in clays of all sorts, including fine stoneware clays.