The foundation-walls are made of gneiss; the arch, the fire-bridge, and the chimney, of fire-bricks. The hearth, a, is formed of a dense mixture of coal-dust, upon a bottom of well-beat clay b, which reposes upon a bed of brickwork c. Beneath this there is a slag bottom d; e is the upper, and f the under discharge hole. The hearth is egg-shaped; the longer axis being 8 feet, the shorter 61⁄2 feet: in the middle it is 10 inches deep, and furnished with the outlets g g, which lead to each of the Spleiss-hearths h h, [fig. 308.] These outlets are contracted with fire-bricks i i, till the proper period of the discharge. The two hearths are placed in communication by a canal h; they are 31⁄2 feet in diameter, 16 inches deep; are floored with well-beat coal ashes, and receive about 27 cwt. for a charge.
l is the grate; m, the fire-bridge; n, the boshes in which the tuyères lie; o, the chimney; p, the working door through which the slags may be drawn off. Above this is a small chimney, to carry off the flame and smoke whenever the door is opened.
The smelting post or charge, to be purified at once, consists of 60 cwt. of black copper, to which a little granular copper and copper of cementation is added; the consumption of pit-coal amounts to 36 cwt. As soon as the copper is melted, the bellows are set a-going, and the surface of the metal gets soon covered with a moderately thick layer of cinder, which is drawn off. This is the first skimming or decrassage. By and by, a second layer of cinder forms, which is in like manner removed; and this skimming is repeated, to allow the blast to act upon fresh metallic surfaces. After 4 or 5 hours, no more slag appears, and then the fire is increased. The melted mass now begins to boil or work (travailler), and continues so to do, for about 3⁄4 of an hour, or an hour, after which the motion ceases, though the fire be kept up. The gahrproof is now taken; but the metal is seldom fine in less than 3⁄4 of an hour after the boil is over. Whenever the metal is run off by the tap-hole into the two basins i i, called SPLIT-HEARTHS, a reddish vapour or mist rises from its surface, composed of an infinite number of minute globules, which revolve with astonishing velocity upon their axes, constituting what the Germans called spratzen (crackling) of the copper. They are composed of a nucleus of metal, covered with a film of protoxide, and are used as sand for strewing upon manuscript. The copper is separated, as usual, by sprinkling water upon the surface of the melted metal, in the state of rosettes, which are immediately immersed in a stream of water. This refining process lasts about 16 or 17 hours; the skimmings weigh about 50 cwt.; the refuse is from 15 to 17 per cent.; the loss from 2 to 3 per cent. The gahrslag amounts to 11 cwt.
The refining of the eliquated copper (called darrlinge) from which the silver has been sweated out by the intervention of lead, can be performed only in small hearths. The following is the representation of such a furnace, called, in German, Kupfergahrheerd. [Fig. 309.] is the section lengthwise; [fig. 310.] is the section across; and [fig. 311.] is the ground plan, in which a is the hearth-hollow; b, a massive wall; c, the mass out of which the hearth is formed; d, cast-iron plates covering the hearth; e, opening for running off the liquid slag; f, a small wall; g, iron curb for keeping the coals together.
The hearth being heated with a bed of charcoal, 3⁄4 cwt. of darrlinge are laid over it, and covered with more fuel: whenever this charge is melted, another layer of the coal and darrlinge is introduced, and thus in succession till the hearth become full, or contain from 21⁄4 to 21⁄2 cwt. In Neustadt 71⁄2 cwt. of darrlinge have been refined in one furnace, from which 5 cwt. of gahrcopper has been obtained. The blast oxidizes the foreign metals, namely, the lead, nickel, cobalt, and iron, with a little copper, forming the gahrslag; which is, at first, rich in lead oxide, and poor in copper oxide; but, at the end, this order is reversed. The slag, at first blackish, assumes progressively a copper red tint. The slag flows off spontaneously along the channel e, from the surface of the hearth. The gahre is tested by means of a proof rod of iron, called gahr-eisen, thrust through the tuyère into the melted copper, then drawn out and plunged in cold water. As soon as the gahrspan (scale of copper) appears brownish red on the outside, and copper red within, so thin that it seems like a net-work, and so deficient in tenacity that it cannot be bent without breaking, the refining is finished. The blast is then stopped; the coals covering the surface, as also the cinders must be raked off the copper, after being left to cool a little; the surface is now cooled by sprinkling water upon it, and the thick cake of congealed metal (rondelle) is lifted off with tongs, a process called schleissen (slicing), or sheibenreissen (shaving), which is continued till the last convex cake at the bottom of the furnace, styled the kingspiece, is withdrawn. These rondelles are immediately immersed in cold water, to prevent the oxidation of the copper; whereupon the metal becomes of a cochineal red colour, and gets covered with a thin film of protoxide. Its under surface is studded over with points and hooks, the result of tearing the congealed disc from the liquid metal. Such cakes are called rosette copper. When the metal is very pure and free from protoxide, these cakes may be obtained very thin, one 24th of an inch for example.
The refining of two cwts. and a half of darrlinge takes three quarters of an hour, and yields one cwt. and a half of gahr copper in 36 rosettes, as also some gahrslag. Gahr copper generally contains from 11⁄2 to 21⁄2 per cent. of lead, along with a little nickel, silver, iron, and aluminum.
Smelting of the Mansfeldt copper schist, or bituminous Mergelschiefer.—The cupreous ore is first roasted in large heaps, of 2000 cwts., interstratified with brush-wood, and with some slates rich in bituminous matter, mixed with the others. These heaps are 3 ells high, and go on burning 15 weeks in fair and 20 in rainy weather. The bitumen is decomposed; the sulphur is dissipated chiefly in the form of sulphurous acid; the metal gets partially oxidized, particularly the iron, which is a very desirable circumstance towards the production of a good smelting slag. The calcined ore is diminished one-tenth in bulk, and one-eighth in weight; becoming of a friable texture and a dirty yellow gray colour. The smelting furnaces are cupolas (schachtofen), 14 to 18 feet high; the fuel is partly wood charcoal, partly coke from the Berlin gas-works, and Silesia. The blast is given by cylinder bellows, recently substituted for the old barbarous Blasebälgen, or wooden bellows of the household form.