[Fig. 159.] represents the furnace as it was formerly worked there with charcoal; a, the laboratory in which the crucibles were placed. It was walled with fire bricks. The foundations and the filling-in walls were formed of stone rubbish, as being bad conductors of heat; sand and ashes may be also used; b, cast-iron circular grating plates pierced with 12 holes (see [fig. 160.]), over them a sole of loam, c, is beat down, and perforated with holes corresponding to those in the iron discs; d, the ash-pit; e, the bock, a draught flue which conducts the air requisite to the combustion, from a sunk tunnel, in communication with several melting furnaces. The terrace or crown of the furnace, f, lies on a level with the foundry floor, h h, and is shut with a tile of fire-clay, g, which may be moved in any direction by means of hooks and eyes in its binding iron ring. [Fig. 161.] the tongs for putting in and taking out the charges, as viewed from above and from the side.
[Figs. 162], [163.] represent the furnaces constructed more recently for the use of pitcoal fuel; [fig. 162.] being an upright section, and [fig. 163.] the ground plan. In this furnace the crucibles are not surrounded with the fuel, but they receive the requisite melting heat from the flame proceeding from the grate upon which it is burned. The crucibles stand upon 7 binding arches, a, which unite in the middle at the key-stone b, [fig. 163.] Between the arches are spaces through which the flame rises from the grate c. d is the fire-door; e, a sliding tile or damper for regulating or shutting off the air-draught; f an inclined plane, for carrying off the cinders that fall through the grate, along the draught tunnel g, so that the air in entering below may not be heated by them.
The crucibles are 16 inches deep, 91⁄2 wide at the mouth, 61⁄2 at the bottom; with a thickness in the sides of 1 inch and 11⁄2 below; they stand from 40 to 50 meltings. The old brass, which fills their whole capacity, is first put in and melted down; the crucibles are now taken out, and are charged with the half of the zinc in pieces of from 1 to 3 cubic inches in size, covered over with coal ashes; then one half of the copper charge is introduced, again coal-dust; and thus the layers of zinc and copper are distributed alternately with coal-ashes betwixt them, till the whole charge gets finally fused. Over all, a thicker layer of carbonaceous matter is laid, to prevent oxidizement of the brass. Eight crucibles filled in this way are put into the furnace between the 11 holes of the grate shelf; and over them two empty crucibles are laid to be heated for the casting operation. In from 31⁄2 to 4 hours the brass is ready to be poured out. Fifteen English bushels of coals are consumed in one operation; of which six are used at the introduction of the crucibles, and four gradually afterwards.
When sheet brass is to be made the following process is pursued:—
An empty crucible, called a caster (giesser), is taken out of the furnace through the crown with a pair of tongs, and is kept red hot by placing it in a hollow hearth (mundal), surrounded with burning coals; into this crucible the contents of four of the melting pots are poured; the dross being raked out with an iron scraper. As soon as the melting pot is emptied, it is immediately re-charged in the manner above described, and replaced in the furnace. The surface of the melted brass in the caster is swept with the stump of a broom, then stirred about with the iron rake, to bring up any light foreign matter to the surface, which is then skimmed with a little scraper; the crucible is now seized with the casting tongs, and emptied in the following way:—
The mould or form for casting sheet brass consists of two slabs of granite, a a, [figs. 164], [165.] They are 51⁄2 feet long; 3 feet broad, 1 foot thick, and, for greater security, girt with iron bands, b b, 2 inches broad, 11⁄2 thick, and joined at the four corners with bolts and nuts. The mould rests upon an oaken block, c, 31⁄2 feet long, 21⁄6 broad, and 11⁄4 thick, which is suspended at each end upon gudgeons, in bearing blocks, placed under the foundery floor, d d, in the casting pit, e e. This is lined with bricks; and is 63⁄4 feet long, 51⁄2 broad, and 2 deep; upon the two long side walls of the pit, the bearing blocks are laid, which support the gudgeons. The swing-blocks are 10 inches long, 18 inches broad, 15 inches thick, and are somewhat rounded upon their back edge, so that the casting frame may slope a little to the horizon. To these blocks two cross wooden arms, f f, are mortised, upon which the under slab rests, freely, but so as to project about 5 inches backwards over the block, to secure an equipoise in the act of casting. g g are bars, placed at both of the long sides, and one of the ends, between the slabs, to determine the thickness of the brass-plate. Upon the other slab the gate h is fastened, a sheet of iron 6 inches broad, which has nearly the shape of a parallel trapezium (lozenge), and slopes a little towards the horizon. It serves for setting the casting pot upon in the act of pouring out, and renders its emptying more convenient. That gate (steinmaul) is coated with a mixture of loam and hair. The upper slab is secured to the under one in its slanting position by an armour or binding. This consists of the tension bars of wood, i k l m, of the iron bars n, (3 to 31⁄2 inches broad, 11⁄2 inch thick, see the top view, [fig. 165.]) of a rod with holes and pins at its upper end, and of the iron screw spindle o. The mode in which these parts act may be understood from inspection of the figure. In order to lift the upper slab from the under one, which is effected by turning it round its edge, a chain is employed, suspending two others, connected with the slab. The former passes over a pulley, and may be pulled up and down by means of a wheel and axle, or with the aid of a counterweight. Upon each of the two long sides of the slab there are two iron rings, to which the ends of the chains may be hooked. The casting faces of the slab must be coated with a layer of finely ground loam; the thinner the better.