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.
Fourth, glazing. It is well to have ready a small supply of a standard clear glaze. Each of the test pieces should be covered with this in a rather thin coat and then they should all be fired again, this time close together so that they will receive the same heat treatment. This will enable one to determine what degree of fire for the clay will best suit the glaze.