Porcelain and Stoneware Glazes
One of the joys of working with stoneware and porcelain is the extreme beauty of high-fired glazes. In texture, in color, in tactile qualities, and in relation to the clay, high-fired glazes have a strong appeal. This is not to imply an inferiority of low-fired effects in ceramics, because the low-fired glazes also have their appeal--a beauty which can take the form of scintillating, melting color, or the sparkle and glint of polished glass. The high-fired glazes are more quiet, perhaps more dignified, and they seem suited to ceramic expressions in which form is the essential value.
And from a practical point of view, high-fired glazes have certain virtues not found in lower-fired glazes. Most important is an extreme hardness. Glazes differ greatly in hardness, depending on their composition and on the temperature at which they were fired. A very low-fired lead glaze composed largely of lead oxide is easily scratched, and a plate, for example, which has such a glaze will, after a period of use, develop numerous scratches just from the abrasion of silverware. High-fired glazes are scratchproof except from the abrasion of substances harder than the glaze. These would include only such substances as quartz, diamonds, and other materials which would never be used in connection with pottery. Along with hardness, high-fired glazes have the desirable property of resistance to the attack of acids. While this is of great importance in chemical porcelains and stoneware, it is of less importance in wares for everyday use. However, over a long period of time, centuries or millenniums rather than decades, the high-fired glazes have proved durable because of their resistance to corrosion. The most glorious periods of Persian pottery are now known to us mostly from specimens which are not in a good state of preservation because of the deterioration of the lead and soda glazes over the ages. The stonewares and porcelains of the Orient, on the other hand, are frequently found in a splendid state of preservation, even though they may have been buried for a thousand years or have been in continuous use for several hundred years.
More important than practicality to most potters is the beauty of high-fired glazes. This beauty arises from three distinct factors: the close relationship which exists between glaze and the clay body, the actual condition or quality of the surface of the glaze, and the quality of the color.
In earthenware the glaze tends to exist as a distinct and separate coating on the clay body. While it may be firmly stuck to the clay, it does not relate to it except as a covering. In high-fired pottery, on the other hand, the glazes tend to be part of the clay or an extension of the clay. In stoneware, the surface of the clay and the glaze are fused together, and the color of the clay has a very strong influence on the color and quality of the glaze. In porcelain the glaze and the body