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or underglaze, and they may be developed by an oxidising or a reducing atmosphere. But it is obvious that the atmosphere could not greatly influence an underglaze colour. Overglaze effects, on the other hand, are generally intended to be sensitive to the air condition of the kiln. The rouge-flambe and sang-de-boeuf ware, for example, may be produced by reducing an overglaze containing copper ;, but more permanent colours are obtained, though with greater difficulty, when the copper is reduced in the main glaze. Generally, overglaze effects obtained by reduction which include all the lustre wares, are less permanent than others. As overglaze may be made exceedingly thin, it is possible to employ stains which would otherwise be too costly. The reduction effects of platinum, iridium, and similar metals are thereby produced. Both with over- glaze and underglaze the stain is first mixed or fritted with a fusible glaze which may conveniently be 1.0 PbO- 0.5 SiO 2 , 0.5 B 2 3 . There is obviously room for a great number of combinations of clear glaze over coloured glaze,, one coloured glaze over another, glaze over enamel, or glaze over coloured body. Most of the artistic pottery in existence falls into one or other of these classes. Thus faience is a coloured body covered with a clear glaze, majolica a coloured body concealed by an opaque enamel or coloured glaze. But the chemistry of the processes is. much the same in all. 62