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.
An _engobe_ must, of course, be put upon the unburned or green clay ware and this should be leather hard, not dry. The body with the _engobe_ may be burned before glazing or the glaze may be put upon the unburned ware and the whole subjected to one fire only.
The ingredients in alkaline glazes are soda-ash, whiting, feldspar, flint and oxide of tin. The following is an example of a fritted glaze:
Na_{ 2}O .60 } K_{ 2}O .10 } Al_{ 2}O_{ 3} .10 SiO_{ 2} 1.30 CaO .30 }
Soda Ash 64 Whiting 30 Feldspar 56 Flint 42
The entire batch is fritted and ground in a ball mill with the usual amount of water for fritt grinding, adding a tablespoonful of gum tragacanth mucilage to the batch after it is sieved. The glaze should be the consistency of heavy cream when used.