I. TECHNICAL PROCESSES
IN THE MAKING OF MODERN POTTERY AND THEIR APPLICATION TO THE TECHNIQUE OF ANCIENT VASES
PREPARATION OF THE CLAY
Ingredients and properties.
The making of a pot begins in the clay bed. The clay has to be found, it has to be transported, and above all it has to be tested to see whether it is adapted to the potter’s needs. For there are many different kinds of clay and they are as individual as human beings; so that a thorough understanding of them is essential to the successful potter.
The chief ingredients of clay are silica, alumina, and water. Other possible ingredients are iron oxide, lime (calcium oxide), magnesia, and potash. To the iron compounds are due the different colors of the clay. When potters speak of the color of a clay—red, yellow, white—they refer to the color after burning, not in the raw state. The tones of the color are controlled by heat; for instance, a red clay becomes first pink, then in a higher fire a deeper red, and in a still higher fire a brownish red.
The potter demands three properties of his clay: (1) plasticity, the property which enables the clay to acquire form; (2) porosity, the property which enables the water to escape; and (3) vitrification, the property which enables the clay to be fired. These three properties are due to the three chief component parts of the clay; namely, clay base, quartz, and feldspar. It will be found that some clays are not plastic enough, others not sufficiently porous, and others again not properly vitrifiable; in such cases the addition of certain substances is necessary to make the clay usable. The actual composition of the clay, therefore, is of great importance, as no amount of skilful labor will avail if the clay itself has not the right consistency.
Washing.
When the right composition of the clay has been assured, the next step is to wash it and separate it from the many natural impurities, such as stones, sticks, etc., with which it is mixed. A clay not properly washed is a source of great vexation in the later stages of pottery making. The best method is that of “blunging,” that is, the dry clay is put into water and stirred constantly until it reaches the consistency of cream, technically called “slip,” whereupon it is poured through sieves, coarse or fine according to the desired consistency. The liquid clay or slip must then be dried sufficiently to become plastic and workable. This can be done either in filter presses in which the water is squeezed out in a comparatively short time, or in shallow receptacles in the open air where the water is allowed slowly to evaporate.