G OL

C 524 )

GOL

are by degrees thrown out of the buddle intothe water, hardly any thing but the gold re-maining behind, which is then either amalga-mated or diredtly fufed in an earthen crucible-with the addition of a little nitre.

The gold that is found in alluvial foil or inthe funds of rivers, is obtained in precifely thefame way, except that it is not neceffary to pafsit through the ftamping mill previous to wafti-ng-

The proper auriferous ores being lighter thangold, and their contents being rarely of fufficientvalue to admit of that accuracy in walhing towhich native gold is fubjefded, are always foundafter they come off the wafhing tables to bemixed with a very confiderable proportion offtony matter. If the metallic part confifts ofpyrites, as is ufually the cafe, it is advifableprevious to fufion to give it a moderate roaftingin order to drive off' the greater part of the ful-phur. The extent to which this procefs oughtto be carried depends in fome degree on thequantity and refractorinefs of the ftony part ofthe ore, for the fulphur in the fubfequent fufionafting the part of a flux, it is obvious that thecleaner the ore is the more perfectly may it beroafted. This being finifhed, the ore is accu-rately mingled with a little quick-lime by wayof flux, and with galena proportioned to thequantity of gold and filver that the pyrites con-tains according to the refult of a previous allay.The mixture being now placed in a rever-beratory furnace is made red hot, and as foonas it begins to clot together is ftirred up fromtime to time and kept at a temperature inade-quate to its fufion till part of the fulphur isdriven off; this being effected the fire is in-creafed, the whole is brought to a ftate of thinfufion and then let out in the ufual way into amould of fand. During the fufion the iron, onaccount of its powerful affinity for fulphur,refumes the portion of which it had been de-prived by the previous roafting, by decompofingthe fulphurets of lead and copper with whichit is mixed, in confequence of which thefemetals by 'their fpecific gravity fall in dropsthrough the vitreous ferruginous fcoria, carryingwith them the gold and filver, and unite at thebottom into a denfe metallic mafs. Hence thepig that is formed in the mould of fand is foundto confift of two parts adhering to each other,but eafily feparable by a hammer; the upper-moft and by far the largeft portion is more orlefs cellular and confifts of fcoria, beneathwhich is a black heavy compact mafs contain-* Lewis, Comm, tech.

ing the gold and filver, together with lead, cop-per, fome fulphur and iron. It is now brokeninto fmall pieces and roafted and fufed once ortwice more till all the fulphur and other impu-rities are got rid of, nothing remaining but thegold, filver, lead and copper.

§ 4. Refitting. s

Although the refining of gold generally re-lates to the feparation of this metal from filverand copper, thefe being the principal fubftanceswith which it is alloyed, yet it may with muchpropriety be confidered as including all themethods of purifying gold from every othermetallic admixture. In this fenfe the term isufed on the prefent occafion, and we ffiall pro-ceed to treat of it accordingly.

Separation of Gold from Lead and other metahby Cupellation.

The moft economical method, and the onlyone habitually pradtifed, of feparating gold fromlead is by a procefs called cupellation in thefmall way, and telling when practifed on alarge feale. The particular precautions requifitein cupellation are deferibed under the articleAssay, as the method of telling is under Silver .All that is neceffary to be Hated here therefore,is merely the rationale of the procefs. Thecupel or tell is a porous infufible earthy mafs,with a ffiallow concavity at the top for the re-ception of the metal: this being placed in afurnace fo as not to be in contact with theburning fuel, and a current of air at the fametime palling over the furface of the tell, themetal is brought almoll to a ftate of ebullition.At this temperature the lead is feparated fromthe gold in the form of a vitreous oxyd, whichfinking into the pores of the tell leaves the goldbehind nearly in a ftate of purity, this lattermetal being incapable of oxydation at any tem-perature by Ample expofure to heat and air.

Not only lead may thus be feparated fromgold, but its effect as a flux in fcorifying andcarrying down moft of the imperfect metals isfo great that by means of the procefs of cupel-lation with lead, repeated according to the pro-portion of the alloy and its affinity to the gold,thefe may alfo be got rid of nearly to the laft;atom. And this method is actually pradtifedwhere the proportion of alloy is but fmall; whenhowever it exceeds a certain ratio it is more-economical to have recourfe to the other me-thods detailed in this feclion.

It is to be obferved however that when fineor pure gold is thus cupelled with lead, it al-ways retains a fmall portion of this metal, whichGellert Chcm. Metall . Fleta minor.