GROUP I  . Elements.—All the black pigments in ordinary use—ivory black, lamp black, charcoal black, Indian ink, and graphite, less correctly termed black-lead and plumbago—consist of or contain carbon, an element not liable to change . The metallic pigments, gold, silver, aluminium and platinum, belong here; of these, silver alone is easily susceptible of change, tarnishing by combination with sulphur . Gaoue II . Oxides.—The oxides have generally been formed at a high temperature ana are not easily amenable to physical or chemical change; they are, moreover, not liable to affect other pigments, being practically inert, red lead only being an exception . The oxides include zinc white, green chromium oxide, burnt umber (a mixture of iron and manganese oxide), cobalt green (CoO,nZnO), cobalt blue (CoO,nAl2O3), coeruleum (CoO,nSnO2), Venetian red, light red, Indian red and burnt sienna (all chiefly composed of ferric oxide), and red lead (Pb204) .