Pottery decoration under the glaze

by M. Louise McLaughlin

Earthenware possesses less of the vitreous element, and the first firing is continued until the ware becomes so dense that it will not absorb the glaze, and the body remains opaque.

The term pottery, is more strictly applicable to the kind of ware last mentioned. It is to this that especial reference will be made in the following pages, in describing methods of manufacture and decoration.

Earthenware is of various colors, ranging from pure white, through the cream white and cream-colored wares to decided yellow, and lastly red. The clays can also be artificially colored, and changes in color are also produced by different degrees of temperature in firing. Ordinarily the process of firing produces a material change in the color of clay. Thus, a grey-tinted clay will assume a bright, yellow color, and a dull brown, green, or blue will be changed to a bright brick-red, under the action of the fire.

The heat used in firing should produce partial vitrification. If a piece of ware has not been fired sufficiently to produce that effect, it is said to be “soft-baked.”

White earthenware requires the greatest degree of heat in firing. In regard to the wares made from clays retaining the colors produced by the elements which they contain, in a natural state, it may be said that the amount of heat necessary for proper firing, decreases with the depth of color. This rule, although it may accurately indicate the temperature at which any given clay should be fired, is simply artificial, except in so far as the depth of color shows the presence of an amount of foreign matter, which by its action increases the fusibility of the clay.

Red clay, being very susceptible to heat, is commonly used as a test in the firing of white ware. The same clay will change in the process from light red to dark brown, and so enable the experienced fireman to determine the degree of heat existing in the kiln.

Clays should be prepared for the making of pottery by careful sifting, dissolving in water, and, as a final precaution, straining through sieves made of fine, silk lawn. In this way all foreign particles are removed, and the clay, having attained the consistency of dough, is ready to be molded into the shapes desired. This is done in molds made of plaster of paris, or it is “thrown” by the hands of the potter on a turning-wheel, or, again, by a combination of these processes.

Before firing, the ware is said to be in the “green state,” in which condition it passes from “wet” to “hard green,” and finally to “white,” when it is ready to be fired. If fired before it has become sufficiently dry, it will be liable to crack, or scale, from the sudden expansion of the moisture confined in the clay. When dry, it is placed in “seggars,” or boxes, made of fire-clay, which are piled upon each other in a kiln, constructed for the purpose of firing the ware.

During the process of drying and firing, the clay loses a certain portion of its bulk. The amount of this shrinkage varies in different clays, but ordinarily may be said to be about one-eighth. A piece of ware, therefore, made from clay which exhibits this degree of shrinkage, would be one-eighth smaller after firing than before. The shrinkage is also modified by the degree of heat to which the clay has been subjected in firing, and there will sometimes be a perceptible difference in the size of two pieces of ware, made from the same clay, and of equal size before firing, after having been fired at different temperatures.

The length of time consumed in firing, varies with the qualities of the different wares. In the manufacture of white earthenware, the firing lasts from thirty to forty hours, while the more common kinds of ware require less time. In kilns of ordinary size, a thousand dozen pieces of ware are frequently fired at once. Of course, considerable time is required to place the seggars containing the ware in the kiln. Some hours must elapse before the kiln and its contents will become cool enough to permit its being opened with safety, so that the firing of hard-baked pottery may require two or three days for its completion.