The analysis and reduction of these different ores, it is scarcely necessary to observe, must be conducted according to the nature and proportion of the ingredients which enter into the composition of the ore to be examined or reduced. Pure native silver requires no other assay than fusion, with a little potash to free it from its earthy matter. In the humid way, the silver may be dissolved in nitric acid, and precipitated by common salt; the precipitate may then be fused with soda in a crucible, by which the silver is obtained pure, and the muriate of soda sublimed. The auriferous silver ores may be treated with potash, by fusion in a crucible; the alloy of silver and gold is first obtained, and the two metals may be separated by the process of parting. Those ores which consist of silver combined with antimony, or arsenic, or both, are first roasted, to drive off the arsenic and antimony, the silver remaining pure. The process is much facilitated by the use of nitre, for the purpose of oxidating the metals to be dissipated.
The ores of silver are reduced either by fusion or amalgamation; the former method is chiefly practised on native sul-phuret of lead or galena, which commonly contains a portion of silver, and often in such quantity as to make its separation from the lead a profitable undertaking. After the lead has been extracted from the ore, the object of the refiner is to obtain the silver in a separate state, which is dispersed through the mass of lead; this is performed by the process of cupellation on a large scale, or refining, as it is usually termed. The other process of reducing silver ores by amalgamation is now pretty generally followed in different parts of Europe. The ores which are subjected to amalgamation are such as contain only a small quantity of lead or copper; but it is of some importance that there should be a certain proportion of iron pyrites; and if this proportion be not naturally mixed with the ore, it is a good practice to supply the deficiency by adding what is wanting to the dressed ore, so that the pyritical contents may, as nearly as possible, be in a certain proportion to the quantity of silver, which is to be ascertained by previously assaying a portion of the ore.
The ore being reduced to the consistence of coarse sand, is carefully mixed with common salt, in the proportion of eight or nine per cent.; when the silver in the ore amounts to eight ounces per quintal, and when the latter amounts to thirty-two ounces, or even a greater proportion, from ten to twelve per cent. of salt is to be added. The next process is roasting the ore, in which about three quintals are spread on the floor of a reverberatory surface, and subjected to a moderate red heat. During the roasting, the ore is to be turned twice or thrice, that every part of it may be equally exposed to the heat. When the whole of the ore is roasted, it is ground in a mill and passed through sieves, by which it is made as fine as meal, and is then prepared for the proper process of amalgamation; this is performed in the following manner: - A number of small barrels, which are made to revolve rapidly on their axis by means of machinery; or fixed tubs, either open or covered, having in the centre of each an instrument resembling a chocolate mill, which may be turned rapidly by similar machinery; the tubs or barrels are filled about one-third with water, and, afterwards, a sufficient quantity of roasted ore and mercury, in nearly equal proportions, is introduced, so that the whole may be of the consistence of thin mud.
The machinery is put in motion, and continued without interruption, for thirty or forty-eight hours, according to the nature of the ore, when the amalgamation is completed. About a quarter of an hour after the agitation of the matter in the barrels has ceased, the greater part of it falls to the bottom, and is withdrawn by opening a hole made for the purpose; the earthy residue is carefully washed by small portions at a time, and thus a good deal of the amalgam, which, from being very minutely divided, could not sink through and mix with the rest, is recovered. The earth, however, if originally rich in silver, still retains a small proportion; it is, therefore, dried, and being mixed with about three per cent. of salt, is again roasted, but at a higher temperature than at first; and the process of amalgamation being again repeated, the whole of the silver is extracted. The fluid amalgam is strained through a closely woven bag, and is thus separated into nearly pure mercury and a stiff amalgam; and the latter being subjected to distillation, the mercury is driven over, and the silver remains behind: the copper, which is combined with the silver, is separated by cupellation.