In order to mark the transition from lead oxide, PbO, into lead dioxide PbO_{2} (plumbic anhydride), it is necessary to direct our attention to the intermediate oxide, or _red lead_, Pb_{3}O_{4}.[54] In the arts it is used in considerable quantities, because it forms a very durable yellowish-red paint used for colouring the resins (shellac, colophony, &c.) composing sealing wax. It also forms a very good cheap oil paint, used especially for painting metals, more particularly because drying oils--for instance, hemp seed, linseed oils--very quickly dry with red lead and with lead salts. Red lead is prepared by slightly heating massicot, for which purpose two-storied stoves are used. In the lower story the lead is turned into massicot, and in the higher one, having the lower temperature (about 300°), the massicot is transformed into red lead. Frémy and others showed the instability of red lead prepared by various methods, and its decomposition by acids, with formation of lead dioxide, which is insoluble in acids, and a solution of the salts of lead oxide. The artificial production (synthesis) of red lead by double decomposition was most important. For this purpose Frémy mixed an alkaline solution of potassium plumbate, K_{2}PbO_{3} (prepared by dissolving the dioxide in fused potash),[54 bis] with an alkaline solution of lead oxide. In this way a yellow precipitate of minium hydrate is formed, which, when slightly heated, loses water and turns into bright red anhydrous minium Pb_{3}O_{4}. [54] If lead hydroxide be dissolved in potash and sodium hypochlorite be added to the solution, the oxygen of the latter acts on the dissolved lead oxide, and partially converts it into dioxide, so that the so-called lead sesquioxide is obtained; its empirical formula is Pb_{2}O_{3}. Probably it is nothing but a lead salt--_i.e._ is referable to the type of dioxide of lead, or its hydroxide, PbO(OH)_{2}, in which two atoms of hydrogen are replaced by lead, PbO(O_{2}Pb). The brown compound precipitated by the action of dilute acids--for example, nitric--splits up, even at the ordinary temperature, into insoluble lead dioxide and a solution of a lead salt. This compound evolves oxygen when it is heated. It dissolves in hydrochloric acid, forming a yellow liquid, which probably contains compounds of the composition PbCl_{2} and PbCl_{4}, but even at the ordinary temperature the latter soon loses the excess of chlorine, and then only lead chloride, PbCl_{2}, remains. In order to see the relation between red lead and lead sesquioxide, it must be observed that they only differ by an extra quantity of lead oxide--that is, red lead is a basic salt of the preceding compound, and if the compound Pb_{2}O_{3} may be regarded as PbO_{3}Pb, then red lead should be looked on as PbO_{3}Pb,PbO--that is, as basic lead plumbate.