When there is not enough borax present the assayer will observe an infusible skin floating upon the surface; should this be the case more borax must at once be employed, in order to dissolve such impurity. When a chloride of silver ore is to be assayed, carbonate of soda must be added to the mixture to prevent sublimation.
The following method of assaying is adopted in several large Continental establishments, where the ores have, beside the usual earthy matter and the sulphides of lead, an admixture of zinc, iron, and copper. The process is precisely similar to the crucible assay, in the case of genuine silver ores, as already described—with this exception, that no more lead is added than the ores then contain—that is, if we are treating galena or silver lead; other ores require different treatment according to their known composition. In this process wrought-iron crucibles are employed having the form and shape as shown in [Fig. 5]. They are made of thick iron plate, and are rendered secure by welding the edges firmly together. Their dimensions are as follows: a depth of 4½ ins., with a thickness of iron at the bottom of 1½ in., and a ¼ of an inch in the sides; the diameter at the top of the crucible should be about 2½ ins., and at the bottom between 2 and 2¼ ins. A mechanical mixture or flux is prepared to use with the ores to which we have referred, consisting of the following chemicals, all of which should be finely powdered and well mixed with the ore to be assayed:—
| Carbonate of soda | 6 | parts. |
| Tartar | 3 | ” |
| Saltpetre | 2 | ” |
| Borax | 1 | part. |
The furnace used for this assay is the ordinary one, having rather a high chimney, to insure a perfect draught. In effecting the reduction of the silver, the crucible is first placed as before on the fire, and allowed to become hot; when this is accomplished, take
| Well powdered ore | 480 | grains. |
| Prepared flux | 500 | ” |
These ingredients should be thoroughly mixed together, and put into the red hot crucible. Fuse at a low heat for about twenty minutes, when the whole will be in a perfect state of fusion; then give about five minutes strong heat, and at the end of that time the crucible may be withdrawn, and its contents poured into an iron mould, as represented in [Fig. 4], having one or two conical holes for the reception of the fused mass. The silver and lead collect at the bottom of the mould by reason of its high specific gravity. It may be removed by reversing the position of the latter, when a gentle tap or two will deprive it of that slag or flux which is usually attached to it. A large quantity of silver can be readily collected from its ores by an alternate use of crucibles, in which case it is possible to make a regular number of fusions per hour. Wrought-iron crucibles, when strongly prepared and carefully made, will stand about thirty of these fusions, giving way in the end on account of the action of the sulphur contained in the ores.
Fig. 4. Iron Casting-moulds.
Another kind of crucible, in addition to those already mentioned, is used by the trade, and is recommended by many assayers as superior to all others. [Fig. 6] represents the form of it. It is about 4½ ins. high, and 2 ins. in its greatest interior diameter, being in the form of a skittle. The charge consists of the following in this assay: