The first of these operations is technically known as "jigging" the ore in a cistern of water. Formerly this was universally done by filling a quantity of ore into a copper-bottomed sieve, and then taking it by the handles, plunging it into the cistern, giving it a jerking, and at the same time a semi-rotatory motion. The act of forcing it quickly into the water causes a momentary suspension of the lighter pieces; after a few repetitions of the process, these are found ranged at the top, while the largest fragments of ore sink to. the bottom of the sieve, nearly in the order of their specific gravities. A thin stratum is now scraped off the top and thrown aside, a fresh quantity of ore is added to that already in the sieve, and the process is repeated. This is continued until the contents of the sieve consist almost exclusively of ore of average richness, when it is delivered to "pile." In the bottom of the cistern there will have been collected the fine particles of ore which passed through the meshes of the sieve: the water is drained off, and this is also carried to "pile." The very finest ore held in suspension by the water is collected and cleaned.
The hand process of jigging is applicable only to small quantities of ore; when large quantities are to be washed, machinery driven by steam or water power is substituted for the hand-sieve, and the manual labour is confined to filling and skimming. Machine-wrought jiggers work on the same principle as the hand apparatus. The wooden cisterns are larger, being 6 ft. long, 4 ft. wide and deep, arranged endwise along a narrow shed. At each end is a wooden framework 6 ft. high, supporting a wooden frame lever, the short forked end of which projects over the cistern, and is connected by iron suspension straps to a square sieve. To the other and longer end, the common proportions of which range from 18 in. to 11 ft., an iron rod is attached in connection with a small crank on the body of a longitudinal shaft, which is driven at a quick speed. A second lever, with a rod reaching to the attendant, serves to lift the sieve through the slotted suspension straps whenever a cessation of the motion is required. The sieves measure 4 ft. by 2 ft. wide, and 9 in. deep, strengthened by iron bands and numerous laths across the bottom to support the wire-work. Iron sieves are rarely admissible, owing to the destructive action of the mineralized water; and brass lasts a shorter period than might be imagined.
The size of the mesh is regulated by the meah of the revolving riddle of the crushing-mill; if the latter is 4 to the lineal in., the sieve will contain 5 or 6. Many novel forms of jigging machines will be found described and illustrated in Andrews " Mining Machinery."
The ore carried away by the water is partly collected by passing the current to a circular buddle. At first it passes into a wooden cistern, in which revolves a short cylindrical block, having on its periphery a number of stout projecting spokes. In its revolutions, the spiked cylinder agitates the liquid, which escapes by a short trough into a second cistern, containing a similar revolving block, armed with a number of projecting paddles. This apparatus still further agitates the passing mixture. From the second cistern it flows by a short trough into the upper end of a cylindrical riddle, revolving in an inclined direction, and discharging at the lower end any pebbly matter. The fine particles fall into the cistern under the riddle, and are conveyed by the current to the centre of the buddle. This consists, in general terms, of an excavation 18 to 24 ft. diameter, and 2 or 3 ft. deep, with a floor rising 8 or 9 in. to the centre, where is fixed a conical wooden block. A vertical spindle carrying a funnel-shaped hopper, with two projecting arms, rests on the central block, and is driven by bevel gearing at the top. To each of the projecting arms is attached, by cords running over pulleys, a board, fitted on the lower face through its length with a brush.
The weight of the board is balanced to an extent by small blocks attached to the suspending cords. On one side of the excavation is a small sluice-gate through which the water is permitted to escape as the excavation fills with matter. The ore and water enter through the funnel, and striking against the apex of the conical block, are distributed radially over the bottom. Motion being communicated to the central spindle, the hanging boards and brushes are drawn lightly over the surface of the accumulating ore. By attention to the balanc-ing weights attached to the cords, the force with which the brushes press on the mass, during their passage, may be regulated to the requirements of the ore. The value of the deposit in the excavation is determined in the same manner as with the simplest kind of buddle; namely, the richest portions at the head or centre, near the entrance of the liquid, gradually diminishing in value, to the edge of the pit, where a broad ring of " tailings," or worthless matter, is taken out.
The central portion is carefully taken out as clean ore, while the portion between this and the tailings is subjected to a repetition of the agitating and buddling process.
The water from the buddle holds in suspension a quantity of fine ore, which separates, to a great extent, by allowing the water to remain for a period in large pits; it is then drained off, and the sediment deposited on the bottom is removed to undergo a cleansing process. This may consist of a revolving " trunk-ing" apparatus. A revolving spindle carries two broad paddles; these work in a small cistern, into which the fine ore is placed, and a stream of water enters. In front of a small cistern is a longer cistern, of the same width, but slightly declining to the outlet. With each revolution of the paddles, a small quantity of the mixture passes into the long cistern, and flowing slowly over the smooth surface, the solid matter suspended in the water is again deposited, but this time in bands of varying richness, the heaviest and most valuable particles being at the upper end. A portion of the remainder is placed aside for re-operating on; but the extreme portion is not of much value if the operation has been carefully conducted.