One of the best features of high firing is the ease with which various materials melt to form glazes. In low-fired pottery, the glaze maker must always use one of the active fluxes, such as lead, sodium, potassium, or boron to achieve a glassy melt. But at temperatures in excess of cone 8, many natural minerals will melt. This opens up the possibility of making glazes from natural materials rather than from prepared materials. Historically, potters have, of course, had to depend on glaze materials which were readily obtained rather than those which were the result of involved chemical or mechanical processing. As has already been noted in Chapter 5, feldspar alone will form an acceptable glaze, and simple combinations of feldspar, limestone, flint, and clay, all of which are available as natural minerals in most localities, will form beautiful and practical glazes. It should not be thought that there is anything superior in a hand-picked specimen of feldspar or limestone as compared to the ground and prepared feldspar or whiting which one would buy from a supplier. The commercial materials, in fact, have the definite advantage of uniformity of chemical composition and fineness of grind. However, for the potter who is interested in personal and individual achievement in ceramics, the use of selected local materials has the advantage of encouraging a reliance on essentially simple and direct means, and of bringing about a firsthand and intimate knowledge of the materials involved.
Perhaps one of the best reasons for working with natural glaze materials is that the limitations which the use of such materials impose requires sensitive and skillful potting which does not rely on the spectacular, or on virtuosity of glaze surface. Anyone who has done even a small amount of work with glazes will realize that the problem is not so much one of finding beautiful or interesting surfaces, as it is the difficulty of making choices, of using surface color and texture in ways which are appropriate to the form of the pots, and in avoiding complexities and involvements of surface which tend to become ends in themselves rather than part of a unified expression. At any firing temperature, thousands of colors and textures are possible, in fact, are very easily achieved. The temptation is always to try to use too many different glazes at once, and to inflict too much of surface interest on each defenseless pot. Most pots of great beauty, interest, or utility will be found to be quite simple, in the sense that all parts work together in creating a unity.
Slip glazes are glazes composed largely or entirely of clay. Most common clays which contain considerable iron and other impurities, mature to a tight and hard body at about cone 04 or less, and firing in excess of about cone 4 causes such clays to vitrify and finally to melt. Relatively few clays will withstand temperatures of cone 9 or more, only, in fact,