"We will now, as briefly as possible, describe the sluice-boxes which carry the water and earth away from the mine, catch the gold, and conduct the tailings out of the miner’s way. A sufficient fall from the head to the bottom of the sluice-box is an all-important consideration. This grade varies from three inches, in twelve feet, where an abundant and cheap supply of water is at hand, to nine inches, in the same length of sluice-box, when the supply of water is limited. These grades may be considered as extremes. From four and a half to six inches fall in twelve feet is about the normal grade.
In arranging the grade of a sluice-box many important questions are involved. One of the greatest being the establishment of “under-currents.” These are large, flat boxes, generally varying from thirty to fifty feet in length, and from ten to twenty feet in width, and often containing a surface of from 500 to 1,000 square feet. These boxes are provided through their whole extent with riffles, as we have seen in the rocker, which catch the gold, or amalgam. The “under-current” is beside and a little below the main sluice-box. An opening from fifteen to eighteen inches in width is cut in the bottom of the sluice-box. Over this a frame-work of steel bars, about one inch square, and an inch apart, allows the fine particles of sand and metal to drop into a sloping box. This box has a pitch of about one inch in a foot. Through this box the finer materials reach the broad area of riffles, while the coarser gravel and cobble-stones are carried down in the main sluice-box. The water which flows into the “under-current” afterward unites with that in the sluice farther down from the mine. The cobbles and small boulders are next disposed of by “a grizzly,” or a grate of parallel bars which permits the finer particles to pass down through the sluice, while all stones are thrown out of the sluice altogether. If a “grizzly” is used, a drop of a few feet is necessary, as the material which passes through the grating must drop into a series of sluice-boxes underneath, and is thus carried farther from the mining ground. Should a precipice be along the line of sluice-boxes, a “grizzly” may be most profitably constructed, but otherwise the accumulation of refuse thus thrown out of the sluice soon prevents farther deposits. When a “grizzly” is made, and a precipice is at hand, a great amount of wear on the sluice-boxes below is prevented. Then a frame of iron bars, which may be condemned iron rails, is made, extending beneath the end of the higher sluice-box and above the drop-box, into which the finer matter falls. The bars of the frame-work are placed parallel to each other, towards the cliff, and about six inches apart. The whole frame slopes at an angle of about 30 towards the cliff, over which stones of more than six inches in diameter may be rolled, while strong wooden sides prevent their escape in any other direction.
A sluice is constructed of boards resting on strong sills, laid four feet apart. These sills are usually four inches by six, and for a double sluice, about fifteen feet long. The advantage of a double sluice is that the mining can be prosecuted at the same time that one of the sluice-boxes is “cleaned up.” The sluice-box is from four to six feet wide, and for the latter width, about three feet deep. The sides are supported by posts, four by five inches, and strongly braced. The floor and sides of the sluice-box are made of plank one and a half inches thick.
The paving of the sluice next requires attention. This is sometimes done with hard, flat rocks, standing on their edges, and so placed as to be least effected by the flow of water. These rocky pavements are from ten to twelve inches thick. Separate compartments, formed by pieces of stout plank permanently fixed across the bed of the sluice-box, from six to eight feet apart, prevent a great displacement of the rocky pavement, should any stone give way. Above the stone pavement the sides of the sluice-box are lined to the height of about a foot with two-inch plank, to save the real sluice from the wear of the gravel. Square blocks, about ten inches deep, are also used for pavement, especially where the sluice is in a tunnel, as the work of taking out the pavement, for the purpose of obtaining the gold there retained, is, in this way, greatly facilitated. These blocks are made of wood and fastened together in sections, with a space of an inch and a half, extending across the sluice, between the rows of blocks".
This type of mining is still being used in areas where the gold is mainly in nugget form. |
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