The amount of silica required in any bisilicate glaze may be found by the following equation:
Thus if the alumina content were .25 equivalent this would be expressed:
Now in order to produce this as a mixture it would be possible to introduce the alumina in the pure state, but pure alumina is expensive and clay which contains alumina is cheap so that clay is generally used to supply the alumina. Clay, however, contains silica as well, and therefore allowance must be made for this. On referring to the formula for kaolin, the purest form of clay, Al_{ 2}O_{ 3}, 2SiO_{ 2}, 2H_{ 2}O, it will be seen that there is twice as much silica present in equivalence as there is alumina and therefore .2 kaolin will contain .2Al_{ 2}O_{ 3} and 4SiO_{ 2}. Subtracting, then, the 4SiO_{ 2} from the 1.6SiO_{ 2} needed there will be 1.2 left to be supplied in the form of flint. The mixture therefore is--
White Lead 1.0 × 258 = 258 Kaolin .2 × 258 = 51.6 Flint 1.2 × 60 = 72
This is a glaze of the same character as that first given except that it no longer flows unduly from the higher places nor will it devitrify in a long-continued fire. The alumina will have counteracted both these evils.
A glaze with only lead oxide as the base is not, however, desirable for general use. The color is yellowish and the lead oxide is apt to destroy the hue of any colors which are used with it. The available bases may be classified under three heads. 1. The metallic oxides, lead and zinc oxides. 2. The alkaline earths, the oxides of calcium and barium. 3. The alkalies, potash and soda. Barium oxide is not often used and soda cannot be used in raw glazes because there is no convenient substance which contains it. As glazes are always ground in water only insoluble ingredients can be employed without preparation. Potash is found in feldspar which is insoluble and while there is a so-called soda feldspar it can rarely be obtained of sufficient purity.
In arranging the bases with which to compose a glaze it is desirable to use one at least from each class, but it must be borne in mind that however many bases are introduced the total must always be unity. This unit is, for the sake of brevity, described as RO. For example the following groups may be set forth: