Color in High-Fired Glazes
The use of color in porcelain and stoneware presents a challenge and an opportunity. The opportunity consists of the possibility of great subtlety, depth, variety, and surface quality which is characteristic of high-temperature glazes at their best. But there is a definite risk of achieving, instead of subtle effects, merely dull ones, and of making glazes which are dingy and lifeless.
Higher kiln temperatures have a pronounced tendency to limit glaze color and to produce colors which are relatively not as bright as those produced in glazes fired at lower temperatures. This is hardly a limitation, since a tremendous range of color is still possible in stoneware. But certain color effects are more natural to and more effective in earthenware than in stoneware, and if the potter adopts stoneware as his mode of expression he must decide to forego these effects. For example, the luminous blue-green of copper in a highly alkaline glaze is exclusively an earthenware color. The wide range of color qualities of lead glazes are also characteristic of lower firing. These qualities can be simulated at high temperatures, but the effect is never quite the same. Brilliant yellows, scintillating blues and purples, and brilliant reds are not as easy to achieve in stoneware as in earthenware.
Whatever stoneware and porcelain glazes may lack in brilliance of color they more than make up in subtlety. High-fired glazes are ideal for producing gray, or grayed, muted colors, especially in a reducing fire. Of course, much of the attraction of stoneware glazes is in their tactile quality as well as in color. This tactile quality may take the form of dense smoothness or of rough, gritty texture, but in either case the glaze of a well-made stoneware piece can be perfectly suited to the clay body of the pot.
In planning glazes for pots, the best principle to follow is to make the most of simple means. The average student or beginner in pottery engages in a seemingly endless search for colors, textures, and effects. He usually accumulates enough of these effects in a short while to keep a creative potter busy for years, but the tendency is to push on toward more colors, new textures, and new combinations rather than to use what has already been discovered. The difficulty seems to be that there are always too many colors to choose from. It is worth remembering that the achievements of the early Chinese potters were gained by the use of only two coloring oxides, iron and copper. A few colors and a few techniques of application and decoration are enough for a great body of work. This is not to say that testing is unnecessary or that knowledge of the materials and of the possibilities of the medium is not essential. But some of the greatest pottery was made with techniques which are utterly simple and involve the use of only a few materials.
White glazes are achieved by adding an opacifier, either tin oxide or zirconium