A large proportion of the silver of commerce is extracted from ores (which are too poor to allow of their being smelted or fused) by a process called amalgamation. Founded on the ready solubility of silver, &c., in metallic mercury, the ore is first crushed to powder, then mixed with common salt, and afterwards roasted. By the adoption of this plan the silver is reduced to a state of chloride. The roasting is done in a reverberatory furnace, in which the heat is very gradually raised, the ore being constantly stirred; the heat is then increased sufficiently to raise the ore to a good red heat. It is then put into wooden barrels, revolving on iron axles attached to the ends, and scraps of iron are then added to it; both are then agitated together by rotary motion, the effect of which is to reduce the chloride of silver to a metallic state. When this is effected, it is again agitated with mercury, and a fluid amalgam is formed with the metal, together with any other metallic ingredient that may happen to be present in the roasted ore. Subsequently, to recover the silver, the mercury is driven off by heat, and the silver is thus left behind in an impure state.
There are three ways of assaying silver ores; they are in the test assay as follows:—
1. Melting in a crucible.
2. Scorification.
3. Cupellation.
In the crucible assay the ore is commonly run down with a suitable flux, those most frequently employed being litharge, carbonate of soda, borax, and charcoal. These four substances are all that are required by the practical assayer in the treatment of the regular ores of silver.
Fig. 2. Fire-clay Crucible.
The assaying of the genuine ores is performed in the following manner; that is, if they contain but little earthy matter. They may then be conveniently treated by fusing with carbonate of soda, on account of its cheapness, and borax, in a fire-clay crucible ([Fig. 2]). The dimensions of the crucible should be as follows: 4½ inches in height, and 2½ inches in its greatest diameter, which should be at the top. A quantity of litharge (a semi-vitrious substance, oxide of lead), more than is actually necessary to take up the whole of the silver in the ore, should be added, so as to promote fusion, and collect the ingredients into one mass at the bottom of the crucible. In preparing the ore for the crucible, it must be well pounded, and intimately mixed with the undermentioned chemicals:—
| Pounded silver ore | 240 | grains. |
| Litharge | 800 | ” |
| Carbonate of soda | 700 | ” |
| Borax | 300 | ” |
| Charcoal | 50 | ” |
Place two crucibles to warm during the time occupied in the preparation of the mixture, then put it into the warm crucible; take 100 grains more of litharge, and powder it over the contents in the vessel. Prepare in this manner a second mixture for the other crucible, place them both in the furnace, and put plenty of coke round them. The mixtures may be melted in an ordinary wind or melting furnace, such as is used by jewellers in the preparation of their material for art working. The fusion should take place very gradually at first, as silver in combination with lead is sensibly volatile at a high temperature: it may then be continued at a low heat for twenty-five minutes, and finally the operation may be completed with a full red heat for five minutes longer.