and this defect is cause by the glaze shrinking more than the body, and so crackling.
After the biscuit has been glazed and fired,it comes from the kiln with a highly lustrous, polished, transparent surface, smooth to the touch, non-absorbent, and impervious to water ; it is, in fact, finished, and becomes the china of commerce.
If, however, while still in the biscuit, and before receiving its coating of glaze, it is decorated, underglazes colors must be used. Sometimes, and under certain conditions, the decorated biscuit is fired before glazing ; but whether it be or not, the glaze being applied after the painting is finished, and therefore, of necessity, over or on the decoration, it is obvious that the color is beneath or under the glaze, --the glaze being transparent, the painting is distinctly visible.
When the biscuit is allowed to be glazed and finished without the application of color, it is evident that the colors then required to paint on or over the glaze or overglaze colors, compared to the long list of overglaze colors, there are but a few for underglaze ; and while both are formed on mineral basis, the underglaze colors are prepared especially to resist the long and intense heat that is requisite to vitrify the glaze, as well as the colors, and fuse both to the body of the ware.
