The melting pots ([fig. 738.]) are made of cast iron, and hold conveniently 400 pounds of metal. They are furnished with a spout or lip for pouring out the metal, and with two ears, on which the tongs of the crane lay hold in lifting them out of the furnace. The pot rests on pedestals on the grate of the furnace, and has a ring cast on its edge to prevent the fuel falling into it. Whenever it becomes red hot, the metal properly prepared and mixed, so as to produce an alloy containing 0·915 parts of gold, is put in, and during the melting, which occupies some hours, it is occasionally stirred. The moulds are meanwhile prepared by warming them in a stove, and thereafter by rubbing their inside surfaces with a cloth dipped in oil, by which means the ingots cast in them get a better surface. [Fig. 739.] represents a side view of the carriage, charged with its moulds. When the proper number of moulds is introduced, the screws at the end, represented at t T, are screwed fast, to fix them all tight.
The pot of fused metal is lifted out of the furnace by the crane ([fig. 740.]), then swung round, and lowered down into the cradle l, m, n, o of the pouring machine, until the ring on the edge of it rests on the iron hoop n, which, being screwed tight up, holds it secure, and the crane-tongs are removed. One of the assistants now takes the winch handle s in one hand, and y in the other. By turning y he moves the carriage forward, so as to bring the first mould beneath the lip of the melting pot; and by turning s, he inclines the pot, and pours the metal into the mould. He then fills the other moulds in succession. The first portion of liquid metal is received in a small iron spoon, and is reserved for the assay-master; a second sample is taken from the centre of the pot, and a third from the bottom part. Each of these is examined as to its quality.
The ingots, which are about 10 inches long, 7 broad, and 6 tenths of an inch thick, are now carried to the rolling mill.
[Fig. 741.], where A represents a large spur wheel, fixed on the extremity of a long horizontal shaft B B, extending beneath the whole mill. This wheel and shaft are driven by a smaller wheel, fixed on the main or fly-wheel shaft of a steam engine of 36-horse power. The main shaft B of the rolling mill has wheels C, D, E fixed upon it, to give motion to the respective rollers, which are mounted at F and G, in strong iron frames, bolted to the iron sills a a, which extend through the whole length of the mill, and rest upon the masonry, in which the wheels are concealed. The two large wheels C and E give motion to the wheels H, I, which are supported on bearings between two standards b, b, bolted down to the ground sills. On the ends of the axes of these wheels are heads for the reception of coupling boxes d, d, which unite them to short connecting shafts K L; and these again, by means of coupling boxes, convey motion to the upper rollers e, e, of each pair, at F and G. The middle wheel D upon the-main shaft B gives motion to the lower rollers in a similar manner. Thus both the rollers e, f of each frame receive their motion from the main shaft with equal velocity, by means of wheels of large radius, which act with much more certainty than the small pinions usually employed in rolling mills to connect the upper and lower rollers, and cause them to move together.
The rolling mill contains four pairs of rollers, each driven by its train of wheel work; the mill, therefore, consists of two such sets of wheels and rollers as are represented in our figure. The two shafts are situated parallel to each other, and receive their motion from the same steam engine. This admirable rolling mill was erected by John Rennie, Esq.
The ingots are heated to redness in a furnace before they are rolled. The two furnaces for this purpose are situated before two pairs of rollers, which, from being used to consolidate the metal by rolling whilst hot, are termed breaking-down rollers. Two men are employed in this operation; one taking the metal from the furnace with a pair of tongs, introduces it between the rollers; and the other, catching it as it comes through, lifts it over the top roller, and returns it to his fellow, who puts it through again, having previously approximated the rollers a little by their adjusting screws. After having been rolled in this manner four or five times, they are reduced to nearly two-tenths of an inch thick, and increased lengthwise to about four times the breadth of the ingot. These plates, while still warm, are rubbed over with a dilute acid or pickle, to remove the colour produced by the heat, and are then cut up into narrow slips across the breadth of the plate, by means of the circular shears [fig. 742.]