The agents take these to the Cornish assayers, a set of men, who (strange to relate,) are destitute of the most distant notion of the theories of chemistry or metallurgy, but who nevertheless can practically determine, with great accuracy, the value of each sample of ore. As soon as the agents have been informed of the assay, they determine how much a ton they will offer for each heap of ore at the weekly ticketing. At this meeting, all the mine-agents, as well as the agents for the several copper companies, attend, and it is singular to see the whole of the ores, amounting to several thousand tons, sold without the utterance of a single word. The agents for the copper companies, seated at a long table, hand up individually to the chairman, a ticket or tender, stating what sum per ton they offer for each heap. As soon as every man has delivered his ticket, they are all ordered to be printed together, in a tabular form. The largest sum offered for each heap, is distinguished by a line drawn under it in the table, and the agent who has made this offer is the purchaser.
In order to prepare copper ores for market, the first process is to throw aside the rubbish, with which they are unavoidably mixed; this task is performed by children. The largest fragments of ore are then cobbed, or broken into small pieces, by women, and after being again picked, they are given to what the Cornish miners term maidens, that is, young girls. These maidens buck the ores, that is, with a bucking iron, or flat hammer, they break them into pieces not exceeding half an inch in size. The richer parts of the ore, which are more easily broken, are now crushed smaller in a kind of mill, the principle of the construction of which is shown in the diagram on p. 162; where a represents a weighted lever, by the depression of which the ore between it and the roller b becomes crushed; and on the raising of the lever, the crushed ore falls away, and a fresh portion of ore is thrown into a position to receive the pressure upon the succeeding depression of the lever. The coarser portions, which are the hardest, are bruised in a stamping mill, in which heavy weights or hammers are lifted by cams on a revolving shaft, and allowed to fall upon the ore, a stream of water constantly passing through the mass, and washing away the portion which is sufficiently reduced to pass through the holes made in an iron plate, which forms one side of the box in which the stampers work.
The next operation is that of jigging; this used to be performed entirely by boys, and consists in shaking a quantity of bruised ore in a kind of sieve, with an iron bottom to it, while under water. This occasions the heavier parts, which consist almost entirely of metal, to sink to the bottom; while the earthy matter is washed away, and the small fragments of stone, being lighter than the metal, and containing little or no ore, are left on the surface in the sieve; these are carefully skimmed off with the hand, and the remainder is piled up in heaps for sale. This process has been recently considerably improved by Mr. Thomas Petherick, a mine-agent, of Penpellick, who took out a patent in 1830, "for machinery for separating copper, lead, and other ores from earths and other substances with which they are and may be mixed, and is more particularly intended to supersede the operation now practised for that purpose, commonly called jigging." This machinery is thus composed; namely, a large vat or tub, with a fixed cover, in which cover are apertures and receptacles adapted to the form and size of a number of sieves, such as are used in the operation of separating copper, lead, and other ores, from the substances with which they are usually mixed.
The vat is filled with water, and the sieves with the minerals in them are placed in their receptacles, so as to be immersed in the water contained in the vat; the interior capacity of which communicates with the interior capacity of a hollow cylinder; into this a plunger or piston is fitted, which is moved alternately up and down within it, so as alternately to displace water therefrom, and force the same into the vat, and then withdraw water from the vat into the hollow cylinder; thus causing a sudden flux and reflux of the water through the sieves, which is continued until the required degree of separation of the earths from the ores is effected.
In the specification of a second patent, granted in 1832, to Mr. Petherick, in conjunction with Mr. Kingston, of Islington, in Devonshire, for improvements in the patent machinery just described, it is directed that the aforesaid cylinder is to be provided with a bottom plate and foot valves, opening outwards to allow the escape of the water into the vat, but not to permit its return; and the piston is furnished with valves opening downwards to allow the water to pass through it in that direction, so that the motion of the piston shall cause the water to pass through the cylinder the same as in a common lifting water-pump. By this improvement, the water instead of being made to pass up and down through the sieves, containing the minerals, as in the previous plan, is forced through the sieves by a series of impulses varying in extent and intensity with the proportion of the area of the piston to the areas of the sieves, and the extent and rapidity of the motion communicated to the piston. The first mover of this machinery may be steam, or water, or horse, or man power, as circumstances may demand.
It is proposed by the patentees as one modification of their plans, to carry a shaft from a first mover over a series of separating vats placed in a row, and made to actuate each piston, by means of a piston rod and crank connected with the main shaft. It is also proposed by the patentees in the specification of" this second invention, to admit the water from an elevated reservoir into the sieve vat, instead of forcing it in by a pump, as in the first part. If there be a sufficient supply of running water, the elevated reservoir is to be kept constantly filled therefrom, and it is to be admitted into the vat and forced through the sieves, by means of a stop-cock or valve, in a series of impulses, actuated by an hydraulic pressure proportionate to the altitude of the reservoir. Where there is not a running stream for the supply of the elevated reservoir, the water is to be pumped up again for that purpose, after it has passed through the sieves. The stop-cocks or valves for the admission of the water from the reservoir to the vat, are to be opened and closed to produce the impulses, either by a boy operating with a lever, or by being connected with one of the pumps or water wheels, when such are used.