The cupelling furnace used at Stolberg is represented by Figs. 81, 82, 83, 84, and 85; a, is the grate; b, the opening for putting the fuel on to the grate; c, the fire-bridge; d, the test ring (hearth) resting upon the stones, e, and the iron beams, f, sometimes upon an iron car; g, is the flue leading into the channel, i, which communicates with the chimney. The flue is divided into two parts by the division wall, h. k, is a pan for fusing the rich lead, which is ladled into the gutter, m, and conducted by it to the hearth, d; n, is the chimney stack on the working side of the furnace; p, the tuyere for admitting blast or steam.

The mass for the hearth consists of pounded calcined bones, mixed with about 5 per cent of potash or soda, or some wood ashes. This is moistened with water, and beaten into the test ring, c, which is furnished with handles, b (Fig. 84), and then a hollow, d (Fig. 85), about 2 inches deep, is made in it for the reception of the cupelling lead; e, is the opening in the hearth for tapping off the concentrated rich lead; it is closed with bone ash during the cupelling process; ƒ, ƒ, are openings for conveying the litharge by means of gutters, g, into iron pots standing below the hearth upon an iron waggon. Two workmen are able to prepare a hearth in one shift of 12 hours. The hearth is fixed in the furnace after having been dried for 8 or 10 days.

Bellows and ventilators may be used as blast machines if

no cylinder machine is available.

Sometimes steam* of

the pressure of 1 to 2 atmospheres is employed instead of blast (Stolberg, Belgium, and the Gallongate Lead Works at St. Anthony, near Newcastle). It is said to hasten the

cupelling process and to produce superior litharge, and occasionally it is cheaper than blast. The nozzle for conducting the steam is put into a wider tuyere, so as to admit some air, and it appears that the steam has not much chemical influence, but rather occasions a draught which renders a further blast unnecessary.

Manipulations in Cupelling.-After having been formed as described, the cupel is dried for several days, and then introduced beneath the arch, and wedged firmly in its place. The furnace is then heated very carefully, lest the cupel should crack. When the temperature reaches a dark red, some lead, which is kept fused in a pot close by, is poured into the cupel, the blast is put on, and the fire increased until the lead reaches a bright red heat.

The formation of litharge takes place rapidly, and as fast as produced it is allowed to run off through a channel which is kept of a constant depth, and about two inches wide. The litharge is kept in the furnace in such a way that it forms a border 6 or 8 inches wide round the edges of the metallic bath, and the temperature and blast are maintained sufficiently high to render the litharge liquid enough to run off uniformly. As the lead is oxidised, and its level becomes lower, more molten lead is poured in, and thus about 500 or 600 lbs. of metal are constantly kept in the hearth. In some cases the lead is charged in pigs by means of an inclined iron plate, through an aperture on the side of the tuyere.

The litharge is received in large cast-iron pots, allowed to cool, and removed in a solid state from the vessels by turning them upside down. As the silver remains in the hearth, while the lead continually oxidises and flows off, the proportion of silver increases. When 4 or 5 tons have been added, and the mass in the hearth has been concentrated until it contains about 8 per cent of silver, the operation of the

furnace is arrested, and its contents run into moulds. The second shift is then commenced by adding 500 or 600 lbs. of fresh lead to the cupel, and the operation proceeds as before. Thus a single cupel will often last 48 hours, and 6 or 7 tons of lead may be oxidised upon it.

For the especial treatment of the concentrated alloy, the cupel is placed in the furnace, and brought to a dull red heat; about 600 lbs. of lead are then added after having been fused in a neighbouring pot, and a strong current of air admitted. Immediately the lead begins to oxidise, and the litharge produced flows off into a vessel provided for the purpose, while at the same time the attendant adds lead to keep the metal in the cupel always at the same level. In this manner some three tons of the alloy of lead and silver are added, and about 500 lbs. of silver are collected in the cupel. The attendant then ceases to add rich lead, and allows the silver to purify; the phenomenon of brightening is soon perceived, the fire is lowered, the blast stopped, and the cake of silver is allowed to cool slowly. The iron waggon, with its subjacent hearth, is now drawn from beneath the vault of the furnace, the silver taken from its bed in the bone ash, cleaned of its coating of impurities, and submitted to the refining process. It is either purified by melting in a plumbago crucible, or by a second oxidising fusion in a bone ash cupel.

The litharge produced in this last operation grows gradually richer in silver towards the end of the cupellation. It is all reduced to a lead usually containing from 30 to 40 ounces of silver to the ton, and, after being treated by concentration, is again treated in the cupelling furnace.

Illustrations of the English Cupelling Process.

At some lead works near Newcastle, a furnace of moderate dimensions requires 2,000 common bricks, 2,000 fire bricks, and 1 ton of fire clay. The fire-place is 22 inches by 24 inches, the fire-bridge from 14 to 18 inches broad, and

B. u. h. Ztg., 1859, p. 434; 1862, p. 300. Berggeist, 1861, p. 339. RIVOT, Traité de Métallurgie, 1860, ii., 288, 364.

the chimney 40 feet high; the hearth (test ring) for receiving the cupel consists of a strong wrought iron ring, nearly elliptical in shape, 4 feet by 2 to 3 feet, and with a framework of iron bars, 3 inches broad, to I inch thick, 4 inches space between them, across the bottom; the cupel is about 1 inch thick in its middle. One furnace cupels 5 cwts. of lead per hour, consuming 4 cwts. of coal per ton of rich lead. The cost of cupelling 1 ton of rich lead containing 400 ounces of silver, is IOS. 10d.

Four and a half tons of lead are concentrated to I cwt. in 16 or 18 hours; this is tapped off and refined when sufficient concentrated alloy has been produced.

The resulting litharge is reduced in reverberatory furnaces, the slags of the litharge reducing process are worked in a slag hearth for the production of hard lead.

At the lead works of Münsterbusch and Binsfeldhammer, near Stolberg, raw lead which has been concentrated by Pattinson's process up to 1 per cent of silver is cupelled in English cupelling furnaces by the application of steam at a pressure of 1 to 2 atmospheres, and concentrated till it contains upwards of 7 per cent of silver. In this way 10 tons are reduced to 7 cwts. in 18 hours. When this rich lead is cupelled for the production of brightened silver, wood is used for firing in the last period of the process. The brightened silver is tapped off, and afterwards refined in plumbago crucibles, 15 inches high and 10 inches in diameter on the top.

50,300 lbs. of lead containing 1 per cent of silver were concentrated in 16 shifts of 24 hours each, and the cupelling of the rich lead containing 7 per cent of silver in 3 shifts, requiring two hearths. The result was, 514 lbs., II ozs. of brightened silver; 45,000 lbs. of poor litharge, with about 7 ozs. of silver in a ton; 4,000 lbs. of rich litharge; 50 or 60 lbs. of abstrich, and some lead hearth. The consumption of fuel amounted to 6 tons of coal, 115 cwts., 20 lbs. of small coal, and 180 lbs. of small coal for heating the boiler. The wages of the chief melter are 2s. per shift of 12 hours, and the assistants receive Is. 6d.

REFINING THE BRIGhtened SILVER.

The refining process* aims at the complete removal of the impurities which the cupelling process has left in the brightened silver. These impurities are chiefly lead, bismuth, antimony, copper, &c. A small amount of lead, bismuth, antimony, and arsenic renders the silver brittle; copper does not injure the ductility of the silver, and sometimes it is purposely retained when it is to be alloyed with that metal (Freiberg).

In rare cases silver is refined in the wet way, but this is usually when it contains a fair amount of gold.

If the impurity in the silver consists only of lead, the refining process is very simple, but the reverse is the case when the silver contains much copper, arsenic, and antimony, with but little lead, as these substances are only removed by a very long reaction of the atmosphere during refining; the brightened silver requires more heat for smelting the less lead it contains. In some cases it is advantageous to give an addition of lead to the brightened silver, so as to oxidise and carry away the other base metals.

Karsten calculated that 18 parts of lead were required for I part of the contaminating base metals. When refining brightened silver containing nickel and cobalt, which cupel with difficulty and show a great inclination to a quick chilling, a little copper may be added advantageously, as it increases the heat in the metal bath. The refining process differs from the cupelling process in the circumstance that the formed litharge does not flow off from the surface of the silver, but becomes imbibed by the porous refractory test (ground bone, marl, wood ash).

The test is either prepared on a movable hearth (like the English cupelling furnace) or on the fixed bottom of a reverberatory furnace; crucibles are also used for the refining process, and the refining may be effected by the assistance of blast or under a muffle.

These operations vary in their consumption of fuel, the volatilisation of silver, and the largeness of production.

B. u. h. Ztg., 1864, p. 81.