Ball clay is very plastic, easily vitrified, but is not white. The color varies from a cream to a gray. The use of a ball clay is therefore limited in white wares because it will spoil the color. For wares in which a light cream color is not objectionable ball clays are valuable and almost indispensable.
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.
Second, shrinkage. The mass of plastic clay is now transferred to a plaster bat and rolled or pressed out into a smooth slab about 12 centimeters long. Here again the centimeter is used in preference to the inch as being more easily calculated. A faint line is ruled on the clay slab and two fine scratches are marked exactly ten centimeters apart. The edges are trimmed and the excess clay made up into three or four small pieces which are to be fired in different parts of the kiln as tests for density. When the clay slab is dry the distance between the marks is measured and noted. The ten centimeters being divided into one hundred millimeters, each millimeter of shrinkage means one per cent. After firing, a second measurement is made and the differences are noted as dry shrinkage and fire shrinkage respectively.
Third, firing. The slab with the measurement upon it is set in the kiln in the place where the clay wares are to receive the first or biscuit fire and the small pieces are arranged in different places so as to secure as many different conditions as possible. The position of each should be carefully recorded. After firing, the marks on the slab are measured as already described and note is taken of any warping of the piece. The color is also recorded. The small pieces should be tested for porosity or absorption of water but this is rather a delicate operation and needs a particularly sensitive balance. Generally it will suffice to use a wet sponge or to dip each piece into water, removing it quickly and noting carefully the rate of speed at which the water is absorbed. If the water should be scarcely absorbed at all a line of ink may be drawn upon the pottery with a pen, the piece being perfectly dry. In a fully vitrified ware the ink can be washed off, leaving scarcely a mark but the test is quite sensitive and with a little practice will afford an excellent means of comparing the density of different clays or of the same clay at different temperatures.