The cupellation of 84 quintals of argentiferous lead takes in general 18 or 20 hours’ working. The promptitude of the operation depends on the degree of purity of the leads employed, and on the address of the operator, with whom also lies the economy of fuel. A good workman completes the cupellation of 84 quintals with 300 billets, each equivalent to a cubic foot and eight-tenths of wood (Hartz measure); others consume 400 billets, or more. In general, the cupellation of 100 quintals of lead, executed at the rate of 84 quintal charges, occasions a consumption of 790 cubic feet of resinous wood billets.
The products of the charge are as follow:—
| 1. | Silver, holding in 100 marcs, 7 marcs and 3 loths of alloy | 24 | to | 30 | marcs. |
| 2. | Pure litharge, containing from 88 to 90 per cent. of lead | 50 | - | 60 | quintals. |
| 3. | Impure litharge, holding a little silver | 2 | - | 6 | — |
| 4. | Skimmings of the cupellation | 4 | - | 8 | — |
| 5. | Floor of the furnace impregnated with litharge | 22 | - | 30 | — |
Note.—The marc is 7 oz. 2 dwts. 4 gr. English troy; and the loth is half an ounce. 16 loths make a marc. 100 pounds Cologne are equal to 103 pounds avoirdupois; and the above quintal contains 116 Cologne pounds.
The loss of lead inevitable by this operation, is estimated at 4 parts in 100. It has been diminished as much as possible in the Frankenscharn works of the Hartz, by leading the smoke into long flues, where the lead fumes are condensed into a metallic soot. The silver cake receives a final purification at the Mint, in a cupel on a smaller scale.
From numerous experiments in the great way, it has been found that not more than 100 quintals of lead can be profitably cupelled at one operation, however large the furnace, and however powerful and multiplied the bellows and tuyères may be; for the loss on either the lead or the silver, or on both, would be increased. In one attempt, no less than 500 quintals were acted on, in a furnace with two fireplaces, and four escapes for the litharge; but the silver remained disseminated through the lead, and the lightning could not be brought on. The chief object in view was economy of fuel.
Reduction of the Litharge.—This is executed in a slag-hearth, with the aid of wood charcoal.
Such is the train of operations by which the cupriferous galena schlich, or ground ore is reduced, in the district of Clausthal, into lead, copper, and silver. The works of Frankenscharn have a front fully 400 feet long.
Silver-smelting Works of Frankenscharn, near Clausthal.