During the operation of smelting, a considerable quantity of lead is volatilized, and carried off as fume or smoke, which, when allowed to escape into the atmosphere, not only involves a loss of lead, but destroys all vegetation for some distance around the works, and poisons cattle and other animals feeding near them. Much attention has of .ate been paid to the obviating of these evils, and several plans are in use for the purpose. Where it can be done, no method is more effective than simply conducting the smoke from the furnaces through a long horizontal flue—say a mile in length—to a vertical stack. 'The fume condenses on the sides, certain openings being left for the purpose of collecting it. About 83 per cent of the fume thus.recovered consists of metallic lead.
When lead contains antimony and tin as impurities, they are separated by fusing the metal in shallow pans. and allowing it to oxidize at the surface. lu this way, the anti mony and tin form oxides, and as such are skimmed off. Lead reduced from galena always contains a little silver of which 8 or 10 ounces to the ton is a very common pro portion, although it often exists in much larger quantity. The separation of this silver is now greatly facilitated by means of a desilverizing process patented by the late Mr. H. Pattinson of Newcastle-on-Tyne. It consists in melting the lead, and allowing it to cool slowly, at the same time briskly stirring the melted mass. A portion of the lead is thus made to crystallize in small grains, which, as pure lead solidifies at a lower tempera ture than when alloyed with silver, leaves the uncrystallized portion richer in silver. In this operation, avow of, say, nine cast-iron pots are used. They are usually about 6 ft.. in diameter, and each heated with a fire below. The lead from the smelting furnace is treated as above in the middle pot, from which the poorer crystallized portion is ladled with a strainer into the first pot do the right, and the richer portion, which remains' liquid, is removed to the first pot on the left. With both kinds the process is several times one becoming poorer and the other richer in silver every time, till the lead in the pot on the extreme right has had its silver almost entirely removed, and that in the pot on the extreme left contains about 300 ounces of silver to the ton. The silver is then obtained from this rich lead by melting it on a flat bone-ash cupel, placed in a reverberatory furnace, and exposing it to a current of air which reduces the lead to the oxide or litharge of commerce, leaving the silver on the cupel. Fully 600,000 ounces
of silver are in this way annually separated from British lead, the latter at the same time being improved in quality.
.Lead is an important metal in the arts. Rolled out into sheets, it is largely used for rooting houses, for water-cisterns, and for water-pipes. It is also of great service in the construction of large chambers for the manufacture of sulphuric acid. Its value for the manufacture of shot is well known. Alloyed with antimony, etc., it is largely con sumed for type-metal. and with tin for solder. Much lead is also required for the mauu facture of pewter, Britannia metal, etc. See ALLOY.
Of the compounds of lead other than alloys which occur largely in commerce, the following are the principal: White Lead or Carbonate of Lead, a substance very extensively used as white paint, and also to form a body for other colors in painting. As ninth as 16,000 tons of it are annually made in England. White lead is still largely made by the old Dutch process, which consists in treating metallic lead, cast in the form of stars or gratings, in such a way as to facilitate the absorption of carbonic acid. These stars of lead placed in earthenware vessels, like flower-pots, containing a little weak acetic acid, are built up in tiers in the form of a stack, and surrounded with spent tan or horse-dung. The heat given out from the dung volatilizes the acid, which, along with the air, oxidizes the lead. The acetic acid changes the oxide into the acetate of lead, and this is, in turn, converted into the carbonate by the carbonic acid given off from the hotbed. By this process. metallic lead requires from 6 to 8 weeks for its conversion into white lead. Several less tedious processes for ft.-. manufacture of a white paint from lead. have been tried at various times, but the only one now practiced is that for the production of an oxychloride of lend, by acting on raw galena with hydrochloric acid.
Minium, Red Lead, or Red aside of Lead, is much consumed in the manufacture of flint-glass and porcelain, and to some extent as a pigment. It requires to be made of very pure lead, as a slight trace of copper would impart a color to glass. Minium is prepared by heating massieat or protoxide of lead to a temperature of 600° F. in iron trays, in a reverberatory furnace, carefully avoiding fusion. More oxygen is thus gradually absorbed; and a compound of the protoxide and the peroxide of lead is formed, having a bright red color, which is the red lead of commerce.—Litharge has been already noticed.