PART VI
Alkaline Glazes
The glory of the Persian and Egyptian blue is too alluring for potters to withstand. Though the pursuit of this glory leads one into all kinds of disasters and failures, the avenues of research that it opens add unending fascination to the study. Even one beautiful glowing pot out of twenty or more efforts is a stimulating achievement though it should not be thought that this is the usual proportion.
It is a continual source of astonishment that with a slight variation of glaze formula a positive green will swim into a vibrating blue. The addition or substitution of one substance or another in the glaze mix may be the key to an unexpected transformation and may give the potter a new palette of color.
The clay body has a very positive effect on alkaline glaze both in its composition and its color. This is especially true under a transparent glaze where the effect is considerable since the color of the glaze would be modified by the red or buff clay showing through.
If, therefore, the object of the potter is to obtain a brilliant "Persian" blue, a white clay body must be composed or a white engobe applied over the buff or red clay to hide the color.
The Persians and Egyptians used a coarse, sandy body high in silica and covered the roughness of the clay with a fine white engobe on which they painted their decorations in various colors. The whole was finally covered with the transparent alkaline glaze.
While the effect of colored clay under opaque glaze is less pronounced, it still makes sufficient difference to be considered.
The word engobe is French and refers to a thin coating of clay, also called a slip, laid over a colored body to change the color or over a coarse body to give a finer texture.
The engobe is usually composed of china clay, flint, and feldspar much as a white earthenware body is constituted but with a larger content of flint. Ball clay may also be used but the color is not so white.
The mixture of porcelain given on page forty will make an engobe suitable for many clay bodies. If it should crack on drying more flint should be added.