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the chief, and it is applied only to artistic decoration. The scientific principles of getting a smooth-fitting surface are the same as in iron. Bottcher employs an enamel for copper consisting of : Felspar 48, unburned gypsum 48, borax 4. The amateur in artistic enamelling is best advised to buy powdered glass as his flux. 56 CERAMIC CHEMISTRY. OPACIFIERS. The chief opacifying agent is tin oxide, but the market price rules extremely high. Arsenic, antimony oxide, or sodium metantimonate are sometimes used as substitutes, with or without the addition of zinc oxide, and zirconia and titania have also been suggested. But the present price of zirconia is twice that of tin oxide, and it requires a content of 15 per cent, to produce the requisite opacity, so that its use is not likely to extend for many years to come. Tin oxide may be rapidly tested for loss with nitric acid, and loss with hydrofluoric acid. In enamels on metal, which mature at 900 C., tin oxide is almost insoluble, not more than 0.01 equivalent going into solution. Opacity may be produced by the addition of 0.13-0.25 equivalent to the molecular formula, which means about 7-10 per cent, of the mixing. But more tin dissolves in higher fired glazes to form a silicate which is not opaque, and therefore pottery enamels in general require more opacifier than those of iron.