GOLD.— CUPELLATION.

J- 90

■will unite with the greater portion of the goldpresent, forming a heavy regulus, which will subsideto the bottom. The stratum of sulphide of iron thusformed beneath the surface of the slag and scorimproduced by the fusion of the earthy and siliceousgangue, is technically termed a matt. If this matt,containing the gold, be now roasted, or oxidized byheating it with free access of air, whereby it is deprivedof a further portion of its sulphur, and the productbe mixed with a fresh quantity of crude auriferouspyrites, and smelted again, a second matt will beobtained, which will contain the gold from the firstand second charges of the auriferous ore. This pro-cess may be repeated a third, fourth, or any numberof times, until the precious metal has been suffi-ciently concentrated in the substratum of sulphideof iron to admit of its profitable extraction. Thelast matt, containing the whole of the gold which hasaccumulated from the successive charges, and brought,therefore, to any required richness, is then fusedwith metallic lead, litharge, or with a mixture ofgalena and metallic iron. The gold is abandonedto the liberated lead, from the top of which the im-poverished sulphide may be readily skimmed off.

By this process the gold is obtained in a moreconcentrated form, or mixed with a smaller propor-tion of lead, than when the latter is used directly asa means of separating and collecting it from thesmelted ore. Still, as the lead must be used for itsfinal separation from the ore in all cases—even whenthe gold has been concentrated by washing andamalgamation—the employment of the iron pyritesin the manner above described is only attended withadvantage when it constitutes part of the ore itself,or when it can be readily and cheaply procured insufficient abundance. When it forms a principalingredient in the ore, it is manifestly highly advan-tageous, in point of economy, to employ it as avehicle for the concentration of the precious metal,while it contributes at the same time to increase theamount of the product.

Longmaid's Smelting Process. —In 1852 a patent wassecured for another smelting process, invented byMr. Longmaid, which consists in separating the goldfrom the quartz or siliceous ore by introducingplates of wrought iron into the melted mass. Thequartz is first crushed to a fine powder, and fluxedwith a mixture of lime and oxide of iron, as usual.Into the fused mass thus obtained plates of wroughtiron are from time to time introduced, and whenwithdrawn their surface is found to be coated witha film of gold. The plates are then plunged into avessel of melted lead, which dissolves off the preciousmetal, and afterwards again put into the furnace.This process is constantly repeated till the whole ofthe gold is extracted from the ore; and it is said to bethus obtained both more economically and in greaterproportion than by the usual process of washingand amalgamation. The gold is then extracted fromthis auriferous lead by cupellation, a process nowto be described.

Cupellation.— The next metallurgical operationis the extraction of the gold from the lead, and this

has been performed from the most remote antiquityby a process termed Cupellation. This process, whichis very simple in principle, is at the same time oneof the most elegant chemical operations ever invented.It depends on the circumstance that when silver andgold are exposed in a state of fusion to the action ofthe air or oxygen, they neither give off perceptiblevapours noT are sensibly oxidized—they remain, inshort, unaffected; whereas, under similar circum-stances, lead and almost all the other metals areoxidized with greater or less rapidity. There is alsoan important peculiarity connected with lead, whichrenders it the only metal, except bismuth, applicableto the process of cupellation. This peculiarity con-sists in the circumstance that the oxide of lead,known as litharge, is fusible at a bright red heat,and in this state absorbs any other metallic oxidewith which it may happen to be in contact, butwhich, without the influence of the oxide of lead,would remain uninfluenced by the heat. For ex-ample, if copper be present as an oxide, it is takenup by the melted oxide of lead; and if the lattercan be separated from the gold and silver, the oxideof copper will be expelled along with it.

This separation may be effected in various ways,but particularly by means of bone-ash, whichpossesses the peculiar property of absorbing themelted litharge even when mixed with a certainproportion of other oxides, whereas it remains imper-meable to the unoxidizable metals. Hence, if avessel be formed of pounded and compressed bone-ash, and into this vessel a mass be introduced, con-sisting of a mixture of litharge or oxide of lead,copper or oxide of copper, and native gold, whichalways contains a certain proportion of silver; andif this mixture be exposed to a considerable heat,the whole will be reduced to a state of fusion; theoxide of lead will dissolve the oxide of copper,and both will be absorbed by the bone-ash, whilethe mixture of gold and silver will remain in thevessel, melted, but otherwise unaffected. Henceit follows that gold or silver, or a mixture of thetwo, may readily be deprived of copper or any otheroxidizable metals by cupelling with oxide of lead.For this purpose it is not necessary to employ theoxide itself in the first instance. When metallic leadis heated to a temperature above its melting point,it rapidly combines with the oxygen of the air, theproduct being in the first place yellow oxide of lead,which appears as a scum on the surface ; and whenthe temperature is raised to about bright redness,this oxide is converted into litharge or the fusibleoxide, which immediately melts. Pure copper re-quires no less than sixteen or seventeen times itsweight of lead to absorb it in the manner abovementioned.

A vessel of bone-ash, formed for this purpose, istermed a cupel or test; and in using such vessels forassaying, the absorptive power of the cupel is theonly agency employed to separate the gold from thelead and other oxidizable metals, as will be explainedafterwards. It is otherwise in cupellation on thelarge scale; in this case the cupel is soon saturated,