POTTERY DECORATION UNDER THE GLAZE.
CHAPTER I.
POTTERY.
We will consider in this chapter some facts in regard to pottery, which, although of a very elementary character, are not so generally understood as may be desirable. These facts relate to the substances of which pottery is composed, the processes of its manufacture, and the methods by which it is or can be decorated.
Two chemical substances, viz., silica and alumina form the basis of all pottery clays. These substances are themselves infusible (except under the compound blow-pipe), but by admixture with other materials more susceptible to the action of heat, they are made useful to the potter’s art, and, when subjected to a high temperature, fuse and form the hard, insoluable combination with which we are all familiar.
Porcelain, and the finer kinds of earthen-ware, are made from clays artificially combined of various natural elements, in such a manner as to produce the qualities desired, while the coarser wares are generally made from clays in a natural or unmixed state. These clays agree in their essential characteristics, but contain other elements which cause certain differences of color, susceptibility to heat, plasticity, etc. The differences, therefore, which exist between the many kinds of pottery manufactured, may be said to be due, not so much to a diversity of the materials used, as to the changes produced by the combination of the same materials in varying proportions.
It is unnecessary that we should here enter into the details of the mixture of different pottery clays, for which each manufactory has its own formula. A few words, indicating, in a general way, the causes of certain easily-recognizable qualities, will suffice.
The peculiar beauty and translucency of fine porcelain, is due to an excess of silica, or the vitreous element of pottery, in its composition. This is attained by the use of kaolin, a fine, white clay, produced through the agency of natural causes in the decomposition of feldspar, which is itself a silicate of alumina, and one of the principal ingredients of granitic rocks. In porcelain, it may be said, that the proportion of silica to alumina is about three to one, and the other ingredients, such as iron, potash, chalk, and soda, exist only in such quantities as are necessary to cause the fusion of the two first-mentioned elements. It is fired but slightly the first time. This leaves the body of the ware very soft and porous, and upon the application of the glaze the latter is absorbed into the body, and by its action upon the materials composing it, produces a translucent effect.
We refer, here, to French porcelain. In the manufacture of English china, the process is radically different. The last-mentioned ware is sometime spoken of as “bone china.” This term has reference to the use of bones from which the lime required in its manufacture is obtained. This element produces a translucent quality in the body of the ware independently of the action of the glaze, which in this case forms merely a coating upon the surface of the previously hard baked body.