When the ore also contains copper, a different pro cess is followed for procuring silver from it. The ore is mixed with another ore, containing silver, iron, and sulphur, and with the scoria of a former process. '1'he mixture is fused in a blast furnace, and the sulphur unites with the silver, with the lead, and with the cop per. The matter, during cooling, separates into two parts ; that below, amounting to about one-fourth, is the silver mixed with lead, copper, iron, sulphur, and arse nic, and is roasted to expel the two last ingredients, and to oxidate the iron. It is then mixed with about one and a half of rich ore, and some scoria, and is fused. The metallic matter thus obtained is again roasted, and used with about one-half of litharge, and as much scoria. When cold it separates into three parts, 1st, lead mixed with silver ; 2d, copper and silver ; 3d, scoria, also containing copper. The second of these is melted with lead and copper scoria, and afterwards with litharge and scoria, by which the silver unites with the lead. Should the whole of the copper not be re moved, the metal must be repeatedly fused ; by which not only the copper, but the other impurities are, separa ted. The scoria of this process is kept and mixed with ore in other operations.
Eliquation.—Silver is also sometimes separated from copper by the process of eliquation. The eliquation of silver ore is performed by melting the alloy of copper and silver mixed with lead, or with litharge and char coal. For this purpose a blast furnace is employed, having its bottom lined with charcoal, beat together, and forming an inclined plane. When the furnace is kindled, it is filled with charcoal, to which some sco ria is added, which, by the heat, is fused, and adheres to the sides of the furnace. A quantity of lead and of the alloy is then thrown in, and afterwards litharge mixed with charcoal. Another quantity of the alloy and litharge is then added, and lastly some lead. By the application of a strong heat, these substances are fused, and the litharge is reduced by the charcoal; the melted matter collects at the bottom, and is drawn off into moulds lined with clay, and previously heated. While the metal in these is in a state of fusion, a bar of iron bent at the end is plunged a little way into it, by which it is removed, when cold, from the mould.
The loaves are then placed in a furnace, on bars of iron, and separated from each other by bricks. By the application of heat, the lead is malted, and runs along a groove in the lower part of the furnace, carrying with it the silver. In this part of the process, great care must be taken not to raise the heat too high, otherwise part of the copper will be melted. and run out with the lead. When the lead ceases to flow from the loaves, the fire must be stopped, after which the copper is left in the state of a reddish spongy mass, still retaining a minute quantity of silver, perhaps not exceeding half an ounce in the 10o pounds ; they are therefore exposed to heat, by which a metallic matter exudes from them, consisting of silver, lead, litharge, arsebic, and scoria, and if the heat has not been cau tiously applied, of some copper. When the metal
which exudes has a reddish colour, the process must be stopped. The metal procured by this operation is freed of some of its impurities by washing, and is then subjected to cupellation. The copper left in the fur nace is nearly pure.
In the first part of this process, the heat must be less than what is necessary to melt the copper; and by using litharge and charcoal instead of lead, the opera tion is expedited, because the litharge, before it is re duced, proves a good flux. Il lead be employed, it shoved amount, at least, to two and a half times the quantity of copper, otherwise the whole of the silver will not be extracted. On the contrary, it should not exceed four times the quantity of copper, otherwise part of that metal will be carried along with it and the silver ; the proper proportions therefore are between three and four parts ; the exact quantity will of course depend on that of the silver. It is necessary also, that the ore contain a certain quantity of silver, otherwise the process is not complete. From one to three-fourths per cent. seem the best Should, therefore, the different Ores have different quantities of this metal, they must be mixed, so as to get a collection which contains the proper proportion of it.
In Germany, where the copper alloy contains about three-fourths per cent. of silver, the proportions of the substances employed, are three of the alloy, and eleven of lead. When the alloy does not contain so much silver, about ten of lead arc used. lf, instead of lead, litharge be employed, 120 of the latter must be used for each 100 of the loaner that were requisite.
word assay, in its extended sense, means the analysis of the different metallic ores, with the view of ascertaining the quantity of valuable materials which they contain; in its more limited acceptation, however, and in which sense it is usually employed, it signifies the process f flowed for ascertaining the quantity of gold and silver, either in an ore, or in alloys of these metals. In a still more limited sense, it applies only to the analysis of the latter, as in determining the quantity of gold or silver in plate, coins, and articles of jewellery. The assay of gold and silver alloys requires to he performed with very great care, as it is done on a very small scale. Before a piece of plate can be stamped, it must be assayed. For this purpose, before it is finished, it is t iken to the assay-m ester, a person in the pay of the Company of Goldsmiths, who scrapes off a small quantity of it at different places ; these he collects together, and sets aside for the assay. The same is practised with the other alloys submitted to his examination.