[Fig. 1007.] represents the apparatus for distilling the amalgam in the Halsbrücke works, marked m in [fig. 1006.] a is the wooden drawer, sliding in grooves upon the basis q; B is an open basin or box of cast iron, laid in the wooden drawer; y is a kind of iron candelabra, supported upon four feet, and set in the basin B; under d are five dishes or plates, of wrought iron, with a hole in the centre of each, whereby they are fitted upon the stem of the candelabra, 3 inches apart, each plate being successively smaller than the one below it. 3 indicates a cast-iron bell, furnished with a wrought-iron frame and hook, for raising it by means of a pulley and cord. s is a sheet-iron door for closing the stove, whenever the bell has been set in its place.
The box a, and the basin B, above it, are filled with water, which must be continually renewed, through a pipe in the side of the wooden box, so that the iron basin may be kept always submersed and cool. The drawer a, being properly placed, and the plates under d being charged with balls of amalgam (weighing altogether 3 cwts.), the bell 3 is to be let down into the water, as at y, and rested upon the lower part of the candelabra. Upon the ledge 1, which defines the bottom of the fireplace, a circular plate of iron is laid, having a hole in its middle for the bell to pass through. Upon this plate chips of fir-wood are kindled, then the door s, which is lined with clay, is closed and luted tight. The fuel is now placed in the vacant space k, round the upper part of the bell. The fire must be fed in most gradually, first with turf, then with charcoal; whenever the bell gets red, the mercury volatilizes, and condenses in globules into the bottom of the basin B. At the end of 8 hours, should no more drops of mercury be heard to fall into the water, the fire is stopped. When the bell has become cool, it is lifted off; the plates are removed from the candelabra d; and this being taken out, the drawer a is slid away from the furnace. The mercury is drained, dried, and sent again into the amalgamation works. The silver is fused and refined by cupellation.
The solid amalgam which is distilled in the above apparatus, would be distilled more profitably out of iron trays set in the mercurial retorts described and figured in [pages 809], [810].
From 3 cwts. of amalgam, distilled under the bell, from 95 to 100 marcs (1⁄2 lbs.) of teller silver (dish silver) are procured, containing from 10 to 131⁄2 parts of fine silver out of 16; one-fifth part of the metal being copper. The teller silver is refined in quantities of 160 or 170 marcs, in black-lead crucibles filled within two inches of their brims, and submitted to brisk ignition. The molten mass exhales some vapours, and throws up a liquid slag, which being skimmed off, the surface is to be strewed over with charcoal powder, and covered with a lid. The heat having been briskly urged for a short time, the charcoal is then removed along with any fresh slag that may have risen, in order to observe whether the vapours have ceased. If not, fresh charcoal must be again applied, the crucible must be covered, and the heat increased, till fumes are no longer produced, and the surface of the silver becomes tranquil. Finally, the alloy, which contains a little gold, and much copper, being now from 11 to 13 löthig (that is, holding from 11 to 13 parts of fine silver in 16 parts), is cast into iron moulds, in ingots of 60 marcs. The loss of weight by evaporation and skimming of the slag amounts to 2 per cent.; the loss in silver is quite inconsiderable.
The dust from the furnace (tiegelöfen) is collected in a large condensation chamber of the chimney, and affords from 40 to 50 marcs of silver per cwt. The slags and old crucibles are ground and sent to the small amalgamation mill.
The earthy residuum of the amalgamation casks being submitted to a second amalgamation, affords out of 100 cwts. about 2 lbs. of coarse silver. This is first fused along with three or four per cent. of a mixture of potashes and calcined quicksalz, (impure sulphate of soda), and then refined. The supernatant liquor that is drawn out of the tanks in which the contents of the casks are allowed to settle, consists chiefly of sulphate of soda, along with some common salt, sulphates of iron and manganese, and a little phosphate, arseniate, and fluate of soda. The earthy deposit contains from 1⁄4 to 9⁄32 of a loth of silver per cwt., but no economical method of extracting this small quantity has yet been contrived.
The argentiferous or rich lead is treated in Germany by the cupellation furnace represented in [figs. 1008], [1009], [1010], and [1011.] These figures exhibit the cupellation furnace of the principal smelting work in the Hartz, where the following parts must be distinguished; ([fig. 1010.]) 1, masonry of the foundation; 2, flues for the escape of moisture; 3, stone covers of the flues; 4, bed of hard rammed scoriæ; 5, bricks set on edge, to form the permanent area of the furnace; 6, the sole, formed of wood ashes, washed, dried, and beaten down; k, dome of iron plate, movable by a crane, and susceptible of being lined two inches thick with loam; n, n, tuyères for two bellows s; having valves suspended before their orifices to break and spread the blast; q, door for introducing into the furnace the charge of lead, equal to 84 quintals at a time; s, [fig. 1011.], two bellows, like those of a smith’s forge; y, door of the fireplace, through which billets of wood are thrown on the grate; x, small aperture or door, for giving issue to the frothy scum of the cupellation, and the litharge; z, basin of safety, usually covered with a stone slab, over which the litharge falls: in case of accident the basin is laid open to admit the rich lead.