The lead ore is first reduced to powder, and then digested in diluted nitric acid. To the solution, after filtration, sulphate of soda is added, which throws down the sulphate of lead : 100 gr =71 lead.
Or, the ore is first roasted, and then fused with thrice its weight of black flux, and covered with salt. The metallic button at the bottom of the vessel is the lead, containing the other metals present in the ore, which however are in very small quantity.
Should the ore contain silver, the metallic button thus obtained must be subjected to cupellatiun, or it may be dissolved in nitric acid, and muriate of soda added to the solution. The precipitate must then be digested in weak nitric acid, which will dissolve the muriate of lead, and leave the silver ; 100 of the preci pitate after this, when dried, =75.3 of silver.
Arsenic.
Arsenic very frequently exists with other metals. It is not, however, used in the metallic state in the arts. The compound of it generally employed is the white oxide, or, as it is commonly called, white arsenic, which is frequently obtained in the processes for extracting other metals from their ores.
White arsenic is obtained by subjecting the arsenic ores to heat, in large cast iron boxes, to which flues are closely luted. These are heated by flues from a fur nace. When red hot, about 15 lb. of the ore are thrown in, and when the whole of the volatile matter has sublim ed, another portion of ore is put in, and the process is continued for about 12 hours, during which time about 150 lb of the ore have been employed. What is col lected in the flues of the boxes is broken off by ham mers, and is freed from any foreign matter adhering to it.
A.,other preparation of arsenic much used in the arts, is yellow orpiment, which is procured in the same way; the ore being previously mixed with half its weight of sulphur.
Metallic arsenic is easily obtained from the white oxide, by heating it with carbonaceous matter ; the sublimed metal being condensed in cool receivers.
?lesay of .Arsenic Ore.—The most accurate method of assaying an me of arsenic, is that pointed out by Chenevix, which consists in acidifying the arsenic, and precipitating it by a salt of lead. For this purpose the ore is treated with nitric acid, so as completely to acidify the arsenic. Putassa is then added, and after
wards nitrate of lead, which throws down a precipitate, which, after exposure to a low red heat, must he weigh ed. If it is entirely soluble in nitric acid, 100 of il=22 of metallic arsenic.
If any of the precipitate is not soluble in nitric acid, it is sulphate of lead, the acid of which is formed by the action of the nitric acid on the sulphur of the ore. The weight of what is left, after the action of the nitric acid, will indicate that of the precipitate dissolved.
A mote easy way of assaying the ore, though not quite so accurate, but sufficiently so for the purposes of art, is, to dissolve the ore slowly in muriatic acid, to which a little nitric acid is added. In this way the metal is dissolved, and the sulphur is left. The quan tity of this indicates that of the arsenic ; 100 being equivalent to about 138 of metal.
Cobalt.
Cobalt is not procured in its metallic state for the purposes of art. The preparation of it in general use is the oxide. Two different kinds of this ate employed, zaffre and smalt ; the former of which is the oxide mixed with a quantity of vitrifiable earth ; the latter is the oxide which is brought to the state of glass by being exposed to heat with some lustble substance. These ale prepared in Saxony, in Bohemia, Silesia, and L•wrain. Tuat from the former is considered the best. To prepare zaffre, the cobalt ore is exposed to heat by means of the flame of wood, which is made to play on it. The vapour arising from the ore is convey ed through a long flue, in which it is condensed. Af ter the vapours have ceased to arise, the ore is removed and reduced to powder, and a second time exposed to heat. It is again reduced to fine powder, and passed through sieves; and after this is mixed with powdered flints moistened. In this state it forms zaffre. Smalt is prepared by mixing the oxide of cobalt, obtained in the above process, with about equal parts of potashes and powdered flints. The mixture is exposed to heat in large pots, and frequently stirred, when in the fused state, during 10 or 12 hours. It is then taken out in ladles, and dropped into cold water, by which it is after wards more easily reduced to powder.