Pottery decoration under the glaze

by M. Louise McLaughlin

The first firing completed, the ware is said to be in the “biscuit,” and is then ready to be glazed. This matter of glazing is a very important one, and the success of the whole very largely depends upon the manner in which the operation is performed. To insure a successful result there must be the nicest adaptation of the materials composing the glaze to the body of the ware. The glaze for each kind of ware must be suited to its especial characteristics, and it can therefore be imagined that the number of glazes in use is very large. Each pottery has its own glaze and the variety is infinite.

The glaze used upon the finer kinds of earthenware consists of materials similar to those of which the body of the ware is made, with the addition of boracic acid (a powerful flux), which with a little lead renders the glaze fusible. In the lower grades of ware a larger amount of lead is used for a flux, and this causes the glaze to fuse at the comparatively low temperature at which these wares are fired. A glaze made of materials that fuse at an unusually low temperature is called a “soft” glaze. Some glazes are so soft that, when fired, they can be easily scratched by the point of a steel instrument. The glaze of good and durable wares is, however, so hard, that the point of the sharpest knife will make no impression on its surface.

The materials of which the glaze is made, are combined in the proper proportions and diluted to form a liquid of about the consistency and the appearance of cream. The piece of ware is then dipped carefully into the liquid, and so skillfully manipulated that it is completely covered with a coating of the glaze of the necessary thickness.

After having been covered with the glaze, which, before firing, has the appearance of an opaque white paint, the ware is ready for the second firing. This is done in what is called the “gloss” kiln, in which the heat is not brought to such a high degree as in the biscuit kiln, but is sufficient to fuse the glaze, and cause it to form a glassy, transparent surface, which should completely cover the body of the ware, and present an equally brilliant appearance in every part.

If the glaze is not suited to the body of the ware, and does not shrink equally with it, its surface will soon present a network of fine cracks. A glaze in this condition is said to be “crazed.” This, among potters, is considered a serious fault, and in ware intended for cooking or table use, is certainly very undesirable.

Old Japanese crackle-ware, in which this condition is a distinguishing feature, is, however, much esteemed, and many, indeed, the majority of the Japanese earthernwares, both ancient and modern, display a surface of fine-crackled glaze, which may not be said to detract from their value as articles of ornament.

A similar effect is also seen in old pieces of glazed ware which have seen long service as cooking utensils, in which case it has been the natural result of the usage to which they have been subjected.

In some modern wares, both French and English, decorated under the glaze, this defect is seen. In these cases it has been caused by the use of a softer glaze than the body of the ware demanded. This expedient has been resorted to in order that the brilliancy and beauty of the colors might be preserved. These articles being intended only for ornament, beauty is more of an object than durability, and this defect of glaze may be permissible as the means of obtaining more brilliant effects. In certain methods of decoration this may be a necessity, but these cases are exceptional and experience leads me to believe that it is possible to obtain beauty of coloring with a glaze which will remain intact. It is to be hoped, however, that with the improvements in making colors the time may come when it will be possible to obtain colors which will retain their beauty under the degree of heat necessary to the production of an article which a practical potter would call a perfect piece of ware.

Pottery can be decorated either under the glaze, with the glaze, as in the case of majolica, in which the color is effected by the use of colored glazes and in the decoration of soft porcelain (pâte-tendre), where the painting is executed upon the unbaked glaze, and, lastly, over the glaze. We will concern ourselves only with the first of these methods. A few words, however, may not be out of place here, upon the differences between over and under glaze painting, which are frequently compared and as frequently mistaken for each other.

Over-glaze painting, as is well known, is executed upon the glazed surface of the finished ware with vitrifiable colors having enough flux or fusible material in their composition to cause them to fuse at a comparatively low temperature, and so become attached to the glaze. Under-glaze decoration is effected by the use of colors which contain less flux, and consequently require a higher temperature for their fusion. It can be executed either upon the ware in the “green” state, or, as is more commonly the case, in the biscuit. These two methods of painting differ materially in their effects. Any one familiar with these effects can readily discover whether a piece of pottery has been decorated over or under the glaze. Those who are not practically conversant with the two methods can be easily misled, and mistakes are frequently made by those who should know better, as to the method employed in the case of a piece of ware in question.