English refining furnaces.—The refining of lead is well performed in some works in the neighbourhood of Alston-moor, in reverberatory furnaces, [figs. 1026.] and [1027.], whose fireplace is 22 inches square, and is separated from the sole by a fire-bridge, 14 inches in breadth. The flame, after having passed over the surface of the lead in the cupel, enters two flues e, e, on the opposite side of the furnace, which terminate in a chimney i, i, i, i, 40 feet high. At the bottom of the chimney are openings f, f, for taking out the metallic dust deposited within. These openings are shut during the process.
The cupel or test, which constitutes, in fact, the sole of the hearth in which the operation takes place, is movable. It consists of a vertical elliptical ring of iron, A, B, C, D, [figs. 1028.] and [1029.], 33⁄4 inches high, the greatest diameter of the ellipse being 4 feet, and the smallest 21⁄2. Four iron bars (A D, m, m′, B C, n, n′,) are fixed across its bottom, which are also 33⁄4 inches broad, and an inch thick. The first of these bars is placed 9 inches from the end of the elliptic ring nearest the fireplace, and the three others are equally distributed between this bar and the back end.
In forming the cupel, several layers of a mixture of moistened bone ashes, and fern ashes, in very fine powder, are put into the test-frame. The bone ash constitutes from 1⁄8 to 1⁄16 of the bulk of the mixture, according to the purity of the fern ashes employed, estimated by the proportion of potash they contain, which has the property of semi-vitrifying the powder of burnt bones, of thus removing its friability, and rendering it more durable. The layers of ashes are strongly beat down, till the frame is entirely filled. The mass thus formed is then hollowed out by means of a little spade, made on purpose, till it is only three quarters of an inch thick above the iron bars near the centre of the bottom. A flange, 2 inches broad, is made at the upper part, and 21⁄2 inches at the lower part, except on the front or breast, which is 5 inches thick. In this anterior part, there is hollowed out an opening of an inch and a quarter broad, and 6 inches long, with which the outlet or gateway of the litharge communicates.
The cupel thus prepared is placed in the refining furnace. It rests in an iron ring built into the brickwork. The arched roof of the furnace is 12 inches above the cupel near the fire-bridge, and 9 inches near the flue at the other end.
The tuyère is placed in the back of the furnace, opposite to the side at which the litharge is allowed to overflow.
Openings g, g, are left at the sides of each cupel, either for running off or for introducing melted lead.
Refining of lead to extract its silver.—This operation, which the lead of Derbyshire cannot be submitted to with advantage, is performed in a certain number of the smelting-houses at Alston-moor, and always upon leads reduced in the Scotch furnace.