REFINING or METALS. The last operation connected with the smelting of copper, tin, lead, and some other metals, is usually called the refiniim. process. With copper, for example. the impure or " blister" copper, containing from 95to 98 per cent of the metal, alloyed usually with small quantities of iron, tin, antimony, etc., is melted in a refining furnace, and exposed to the oxidizing influence of the air. By this means, the foreign metals present become oxidized, and rise to the surface as slag, which is skimmed off ; the oxide of copper, formed dining the process, being afterward reduced by throw ing coal on the surface of the melted metal, and stirring with a pole of green wood. The disengagement of gases from the wood during the "poling" causes the metal to splash about, and so expose every portion of it to the reducing action of the coal; thus the oxide of copper is deprived of its oxygen, and the copper rendered nearly pure.
Tin is also refined by throwing; billets of green wood into it while in a melted state, which has the effect of bringing impurities to the surface its froth, in a somewhat similar way to the oxidizing of foreign metals in copper. See Trx.
Lead is purified from antimony and tin by an analogous mode of oxidation, and silver is separated from it by a special process. See LEAD.
The refining of iron is a mono to the process for partially s!parating the carbon from cast iron, and is described under luox. Of the less important metals used in the arts, zinc, antimony, and mercury do not usually undergo any special refining process; aluminimn, it is said, will not afterward purify when once to the metallic state; and nickel, of which German silver is largely composed, is refined by a process or proc esses kept strictly secret by manufacturers.
We nay state here that no metal is ever quite pure in its commercial state, even though it has gone through the usual operation of refining, but all are to a certain Extent alloyed with certain others. For the great majority of purposes, it is not necessary that metals should he chemically pure, and when it is, they can only be made so by refined chemical processes.
It will be readily understood, that it is always necessary to carry the refining of gold and silver further than the less valuable metals. To render gold sufficiently pore for manufacture into coin, an ingenious process has, within the last few years, been proposed, by which fused gold is mixed with about 10 per cent of black oxide of clipper, and then stirred so as to oxidize any foreign metals which happen to be present. Time oxide of copper does not fuse, but is disseminated through the melted metal, mid oxidizes any tin, antimony, or arsenic, and causes them to rise to the surface, so that they may he skimmed off. Perfectly pure gold is prepared by dissolving the metal in aqua-regia—a mixture of nitric and hydrochloric acids—and precipitating silver (with which it is almost always alloyed) as well as any other foreign metals by chemicals which have no action on the solution of gold. The metallic gold is afterward precipi tated as a finely divided powder, by a salt of iron, and is then fused and cast into bars.
Silver is rendered pure by dissolving it in nitric acid, filtering the solution, and then precipitating the metal with common salt as achloridc of silver. This is afterward mixed with salphuric acid, and then, by introducing bars of zinc, a chloride of zinc is formed. while the silver is reduced to the metallic state.