Both operate in a tradition in which the epitome of beauty is recognized to have evolved in a wood fire, a thousand years ago, in Song Dynasty China. The first glazes were developed some five thousand years ago in Egypt. There desert salts incorporated into a clay body migrated to the surface to form a glaze. Sodium and potassium will flux clay to form glaze, as sodium does in a salt kiln, and as the sodium and potassium laden wood ash fluxes clay in a wood kiln, or in ash glazes. When copper bearing minerals were additionally added, a bright blue or turquoise glaze resulted.
Glazes are essentially glasses, but glasses with a very special quality.
A fired clay object is substantially smaller than it was when it went into the glazing kiln. This is due to about six per cent shrinkage as the clay cools from its hottest temperature. If the glaze shrinks less, then it stands off the body and shivers, if more, then the glaze develops small cracks, crackle, if they are desired, crazing if they are not. The color of glazes comes from metallic salts. Few of these metal colorants can withstand the white heat of the stoneware kiln.