A certain delicacy of tint and firmness of outline characterizes overglaze painting, which is easily recognized by experts, but those who can not distinguish between the two methods by this means may ascertain to which class the decoration belongs by looking aslant the surface. The glaze upon the painted portions of a piece of ware decorated over the glaze, will not, even after the most perfect firing, equal the glaze upon the uncovered portions in brilliancy, and there will probably be certain inequalities of surface between the painted and unpainted parts which will be revealed to the touch.
If the decoration has been executed under the glaze the surface will be uniformly covered with a brilliant glaze. Of the two, underglaze painting is probably the most difficult, as the colors are more liable to change under the action of the great heat to which the ware must be subjected, and the final results being, therefore, somewhat uncertain, can not be accurately counted upon, until experience has been gained from repeated failures. On the other hand, overglaze painting, while not subject to the changes produced by the fire, to so great an extent, is more difficult as regards the manipulation of the painting upon the glazed surface. The facilities for the practice of overglaze painting are greater than those afforded for painting under the glaze, which last, requires not only the handling of an artist who has acquired facile use of the brush and some experience of pottery clays and colors, but also the assistance of an intelligent and skillful potter.
Let us not, however, laud one method of painting at the expense of the other. Both are good in their way, and confined within their proper limits, have beauties peculiarly their own. Overglaze painting possesses a delicacy of effect and a variety of color which the underglaze decoration can not rival. The latter is, however, the most artistic as well as the most effective, and lends itself more readily to the uses of decorative art and to the modern taste in color.
It is this latter method of decorating pottery which we will now consider in some of its various forms.
CHAPTER II.
COLORS FOR PAINTING UNDER THE GLAZE.
Colors for painting under the glaze are specially prepared for the purpose from various metallic oxyds. As mentioned in the previous chapter, the colors used under the glaze have less flux or fusible matter in their composition than those used for painting overglaze: this is necessary, because in the former the colors are expected to bear a much greater degree of heat in firing than in the latter. They are so prepared as not to fuse except at the temperature required in the manufacture of the pottery to which they are applied. The variety of underglaze colors is less than that to be found in overglaze colors, as the oxyds from which colors may be made which will stand so great a degree of heat, are few in number.
The oxyds from which the colors are prepared are called the bases of these colors. The colors used in underglaze painting are made from the following bases:
Blue from the oxyd of cobalt. Co. O.
Green from the sesquioxyd of chromium. Cr2. O3.