Stoneware clay is usually a rather plastic clay which contains a good deal of sand, hence stoneware clays can be used for certain classes of ware without admixture. A rather high temperature is required for most of these clays, though occasionally one can be found which will become dense at the fire of a studio kiln. The clays sold by the Enfield Pottery Company and by the Western Stoneware Company are of this type.
Ground flint is a necessary ingredient in almost all pottery. It aids in the porosity of the clay and enables the mixture to be adjusted to fit a special glaze.
Ground feldspar is also necessary. Like flint it aids in the porosity of the unburned clay but unlike flint it produces density in the firing.
By a proper adjustment of these ingredients a clay can be composed which will meet the special requirements of the worker.
In order to ascertain the properties of any given clay certain simple tests may be made and every clay-worker should know how to do this because one cannot be too well informed as to the materials to be used.
First, water of plasticity. A certain portion of the clay, dried and powdered, is weighed out. It is convenient to weigh in grams and to measure in cubic centimeters because in this way calculation is easy. The scales and weights are described in the chapter on glazes. For measuring the water a glass vessel called a graduate is used. One holding a hundred cubic centimeters and graduated in centimeters and tenths can be obtained from a dealer in chemical supplies. One hundred grams of clay is weighed out and transferred to a glass slab. The graduate is filled with water to the one hundred mark. Some of this water is then poured on to the clay, adding little by little as needed until the whole can be worked into a stiff mass of the proper plasticity. The quantity of water used is then carefully noted by observing how much is left in the graduate. Suppose, for instance, 70 cubic centimeters are found remaining, the hundred grams of clay has absorbed thirty c.c. of water and as one c.c. of water weighs one gram the clay has taken just 30 per cent. This amount is important because it is one of the best indications of plasticity. A very plastic clay may need 40 per cent, a non-plastic clay may be satisfied with 25 per cent.