ASSAYING. The difference between as saying and chemical analysis may be thus stated; that when an analysis is performed, the nature and proportions of all the ingre dients of a substance are determined ; but in assaying, the quantity of any particular metal only which the ore or mixture under examina tion may contain is ascertained, without re reference to the substances with which it is mixed or alloyed. Assaying is sometimes conducted in what is called the dry way, or by heat; at other times in the moist way, or by acids and other re-agents ; and in some cases both methods are necessarily resorted to in assaying the same ore or mixture of metals.
The assaying of silver and gold is effected by a process called cupellation. Cupels are small flat crucibles made by pressing bone ash, moistened with water, into circular steel moulds, and they are dried by exposure to the air. The principle upon which the operation depends is, that all metals with which gold and silver are usually alloyed, are convertible into oxides by exposure to atmospheric air at a high temperature, whereas the precious metals remain unacted upon.
To assay silver by cupellation, the silver is flattened, and wrapped up in an envelope of lead. A muffle or oven is heated in an assay furnace, and the two metals put into it. The metals melt, and the lead becomes converted into an oxide, which, as well as any baser metals before combined with the silver, is ab sorbed by the substance of the cupel, until at length the silver is left absolutely pure.
The assaying of gold is performed, to a cer tain extent, exactly in the same way as that of silver ; and if the gold were alloyed only with copper, the process would be as simple as that of silver assaying. Usually, however, gold contains silver, and this cannot be got rid of by cupellation the parling process is therefore had recourse to ; this consists in dissolving the silver by dilute nitric acid, which leaves the gold perfectly pure.
Iron ores are assayed by separating the oxygen from the iron, by the greater affinity of charcoal for that element at high tempera tures. The ore, some charcoal, and an alka line flux, are heated in a crucible; and the result is that all the impurities in the ore are made to leave the iron, so that the latter is presented in a purely metallic form.
Copper ores for the most part contain sul phur ; and in order to assay them, a flux is prepared of fluor spar, borax, slaked lime, argol, and nitre. The ore is pounded, calcined in a crucible at a red heat; then cooled ; then heated again with some of the flux until it is brought to a liquid state. The liquid metal is poured into a mould, and quenched when solid but yet hot. There is then found a por tion of metal underneath a layer of coarse slag. The metal is separated from the slag, reduced to powder, and again heated until the sulphur is driven off from it.
Lead.—The principal ore of lead is the sul phuret, commonly called galena ; but the car bonate, or white lead ore, is sometimes found in considerable quantity. The former of these is assayed by being put into a crucible with iron and flux, all in small grains ; and after being covered with a layer of salt, they are heated until the lead becomes separated from all impurities. The second kind of ore is assayed in the same way, but with a different flux.
Tin.—The ores of tin are principally of two kinds, the oxide and the sulphuret. The oxide is assayed by simple fusion with a flux, which removes the oxygen. The sulphuret is assayed by being first pounded and calcined, to drive off any sulphur or arsenic ; and then melted again with a flux of alkalies, fluor spar, and lime, by which the tin becomes separated from all the other impurities.
Zinc—The ores of zinc are of two kinds, the carbonate, or calamine, and tho sulphuret or blende. The carbonate is assayed by being broken into small pieces, brought to a red heat, cooled, reduced to a fine powder, mixed with powdered charcoal, and melted in a cru cible, under such conditions that the zinc may leave the ore, and combine with a thin layer of granulated copper so as to produce brass ; and the quantity of the brass so pro duced tests the richness of the ore in zinc. The sulphuret, or blende, is assayed nearly in the same way.