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The consequence of using this kind of riffle-bar is, that though much of the water and light dirt runs straight over the bars, the heavier material runs down from side to side in a zigzag course. Near the head of the sluice is a vessel, from which quicksilver falls by drops into the box; and it follows the course of riffle-bars, overtaking the gold which takes the same route.
"You see," continued the man, "the motion and the water loosens and softens the dirt until all of it, except the larger stones, falls through the holes in the bottom of the hopper and runs down the apron to the upper end of the cradle and then down the bottom of the cradle and over the riffle-bars and out the lower end, leaving the gold and the heavier particles of sand and gravel behind the riffle-bars.
Sometimes all the riffle-bars are taken up at once, save one set in every thirty-six feet, and then the work of cleaning up is dispatched much more rapidly. The quicksilver and amalgam taken from the sluice are put into a buckskin or cloth, and pressed, so that the liquid metal passes through, and the amalgam is retained. The amalgam is then heated, to drive off the mercury.
Much study has been devoted to the subject of making these riffle-bars in such a manner that the dirt will not pack in them, but will always remain loose, and keep in constant motion under the influence of the water running over them; but the object has never been fully attained. Quicksilver is used in nearly all quartz-mill sluices.
The sluice used in quartz-mills is similar to the placer board-sluice, but the amount of matter to be washed is less, and there is no dirt to be dissolved, and there are no larger stones, and therefore the sluice is not so large, so strong, or so steep in grade, as the placer-sluice, and the riffle-bars are not so deep. In some quartz-mill sluices there are transverse riffle-bars.
In large sluices, wooden riffle-bars are worn away very rapidly the expense amounting sometimes, in very large and long sluices, to twenty or thirty dollars a day; and in this point there is an important saving by using the stone bottoms. They are used only in large sluices, and they generally have a grade of twelve or fourteen inches to the box of twelve feet. Hydraulic Mining.
"But, how do you get the gold out of the cradle?" queried Bud. "It seems to be mixed all up with a lot of heavy sand and gravel behind the riffle-bars." "We will show you, just as soon as we wash out this hopper full of dirt," replied the man. "Ay, Hank?" and he turned to his companion, the rocker.
Directly under the hopper, which was not nailed to the cradle-box, was an apron of wood, fastened to the sides of the cradle-box and sloping down from the lower end of the hopper to the upper end of the cradle-box. Two strips of wood, about an inch square, called riffle-bars, were nailed across the bottom of the cradle-box, one at the middle and the other near the lower end.
Before the work of sluicing commences, all the boxes are fitted with riffle-bars, and the bottom of the sluice is therefore full of holes from one to two inches wide, from three to seven inches deep, and six feet long. These are the places in which the gold, quicksilver, and amalgam are caught.
A set of riffle-bars is as many as fill one half of a box. They are wedged in, from an inch to two inches apart; the wedging being used, because the bars can more readily be fastened in their places, and more easily taken up, than if nails were used.
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