Types of High-Fired Glazes
All high-fired glazes are made up of a small number of ingredients which in combination have been found to melt to glass in the range of temperature of cone 9 to cone 14. However, in spite of the fact that not many different materials are involved, there are several distinct types of stoneware and porcelain glazes, each with its distinguishing appearance and "feel," and each with a somewhat differnet response to coloring oxides. The following classification of glazes refers to the base glaze alone, that is, to the glaze which is not colored by the addition of any coloring oxide or opacified by the addition of tin oxide or zirconium oxide.
Since most high-fired glazes contain considerable feldspar, usually about 40 to 50%, one could think of all high-fired glazes as being feldspathic. As we have seen, feldspar functions as the principal glaze flux at high temperatures. Glazes can be made which contain up to 90% of feldspar. Such glazes have their own particular character. For one thing, the highly feldspathic glaze crazes over most bodies, but this crazing can be made an asset rather than a liability. The so-called "fish scale" glaze is made by applying a feldspathic glaze very thickly over a flat surface, such as the inside of a plate or bowl, so that the craze lines are seen in the thick, milky glaze, giving a feeling of depth through a maze of cracks. In stoneware, where the body itself is vitreous, or nearly so, crazing is more of an aesthetic problem than a practical one, since the well-fired stoneware pot will not leak water in any case. The tendency of high feldspar glazes to craze is caused by the relatively high content of alkali in the glaze, either soda or potash.
In appearance, the high feldspar glaze is usually milky and either opaque or semiopaque. Some of the most beautiful of the old Chinese glazes were no doubt made mostly from feldspar, and very similar-appearing glazes can be made by using a fusible feldspar with small additions of ash or lime, or of both. The cloudy depth of such glazes is exceptionally beautiful over dark clay, and their veiled opacity cannot quite be duplicated with glazes opacified with tin or with low-fired glazes. The best results are obtained from cone 10 or 11 firings. The following examples are given as suggestions for glaze compositions using a great preponderance of feldspar.
This glaze will craze over most bodies, but it gives a smooth, semiopaque glaze of considerable depth.
The bone ash in this formula acts as a flux. The glaze has a buttery, opaque quality and a smooth surface.
Interesting and varied glazes can be made by adding fluxes to feldspar, such