It is a good arrangement. The floor on each side of a furnace is paved with bricks to a width of 14 feet.

Furnace Tools

49. Furnace Tools. The tools required for such a furnace are: three or four hoes, each fourteen feet long; a couple of smaller ones about six feet; two spades not shorter than the large hoes; a poker, which, if wood is the fuel, should be made like a boat-hook. The shanks of the hoes and spades are of three-quarters or one inch gaspipe, except some three feet next the head, which is of solid round iron. The heads are of one-quarter or one-half inch boiler iron. For very heavy ores, containing arsenic and antimony, cast-iron hoe heads are used, also rakes. These tools are represented in Plate 2.

Crosby's Furnace

50. Crosby's Furnace. At Nevada City, Professor Crosby uses, in connection with a reverberatory furnace, an inclined, rotating, unlined iron cylinder, which receives the concentrated sulphides at the higher end, and delivers them at the lower end, already to a great extent oxidized, into a chamber where they are exposed to the heat from the reverberatory hearth, to which they are removed periodically, in batches, and finished under hand stirring with hoes. A fire is used under the cylinder, to commence the burning of the sulphides, after which they continue burning without such aid, combustion being supported by the air which enters through the open upper end of the inclined cylinder with the sulphides, the fumes being carried off at the other end by the draft of the furnace, with which the cylinder is in communication.

The sulphides, thus burning spontaneously, receiving an abundance of fresh air, being constantly moved by the rotation of the cylinder, and afterwards lying in the furnace, exposed to a higher heat while accumulating, require, when transferred to the finishing hearth, but a short time to complete the roasting. Thus three tons of material which requires 24 hours in an ordinary long furnace, can be turned out daily by the labor of only two men. Some power is consumed in turning the cylinder, which however, when power is not required for other purposes, might be had by applying the waste heat of the furnace to a small steam or hot air engine.

Leaching Vat

51. Leaching Vat. The leaching is done in wooden tubs, which are coated inside with a mixture of coal tar and asphalt melted together, and applied whilst hot. As the chlorination of gold is also effected in these tubs, they are provided with covers when that metal is present in the ore. If there is no gold, covers are not needed, nor is the coating of the tubs with tar so necessary, being in fact inadmissible if hot water is to be used to wash the ore.

The side of a leaching vat is either vertical, or flaired so that the top of the tub is wider than the bottom. The reverse form is not suitable, because the ore, in settling, draws away from the sides, and leaves a space, or, at least, a greater looseness, through which the chlorine can pass upward, or liquids downward, without passing through the mass. The vat represented in Plate 3, is suitable for the treatment of ore containing both gold and silver. Its capacity is two and three-quarter tons of roasted ore.

The vats have filters near the bottom, and rubber pipes, connecting under the filters, for the solution to flow through to the precipitating tubs. For the admission of chlorine a leaden nipple is inserted in the side of the vat immediately below the filter.

52. In some works the vats are suspended on iron gudgeons, attached to their sides, in order that they may be emptied quickly by dumping. It is a convenient arrangement if completed by having a stream of water in a sluice below, or a tramway, for the removal of tailings. In others the leaching vats themselves are mounted on wheels, and can be trundled to the dumping place.

53. The filter consists of a false bottom of inch boards, through which half inch holes are pierced at intervals of about four inches. The boards are laid loosely, with open spaces a quarter of an inch wide between and around them. The false bottom rests on strips of wood, by which it is raised from half an inch to an inch above the true bottom. As the vat is slightly inclined toward the discharge side, to insure complete draining, the strips are made thicker at one end than at the other, so that the false bottom is horizontal. They do not touch the sides of the vat, but leave a space for the flow of solution and diffusion of chlorine. On the false bottom is a layer of pebbles as small as may be without falling into, or through, the holes. Over the pebbles is a sheet of burlap, or a layer of old grain sacks, which are cheap and good enough, as they are soon destroyed by chlorine, on which account some operators prefer a layer of fine gravel covered with sand.