The board-sluice is a long wooden trough, through which a constant stream of water runs, and into which the auriferous dirt is thrown. The water carries away the clay, sand, gravel and stones, and leaves the gold in the bottom of the sluice where it is caught by gravity and the use of quicksilver. The sluice is not less than fifty feet long, nor less than a foot wide, made of boards. The width was usually sixteen or eighteen inches ; and never exceeds five feet.
The length was ordinarily several hundred and sometimes several thousand feet. It was made in sections or ” boxes ” twelve or fourteen feet long. The boards are an inch and a half thick, the bottom boards being four inches wider at one end than the other. The narrow end of one box therefore fits in the wide end of another, and in that way the sluice is put together, a long succession of boxes, the lower end of each resting in the upper end of another, and not fastened together otherwise. These boxes stand upon trestles, with a descent varying from eight to eighteen inches in twelve feet.
The descent of a sluice is usually the same throughout its length, and is called its “grade.” If there be a fall of eight inches in twelve feet, the sluice has an “eight-inch grade.” The grade depends upon the character of the pay-dirt, the length of the sluice, and its position. The steeper the descent, the more rapidly the dirt is dissolved, but the greater the danger also that the fine particles of gold will be carried away by the water. The tougher the dirt, such as clay, the greater its resistance to the dissolving power of the water, the steeper, other things being equal, should be the sluice. A slow current does not dissolve tough clay.
The shorter the sluice, the smaller the grade should be. There is more danger that the fine particles of gold will be lost by a short sluice than by a longer one, and to diminish this danger, the rapidity of the current must be reduced by a small grade. The greater the amount of dirt to be washed, the steeper should be the grade. A swift current will wash more dirt than a slow one. In many claims the pay-dirt is full of large stones and boulders, weighing from one hundred to five hundred pounds each, all of which must be carried away through the sluice. Some are sent down whole, and others are broken into pieces with sledge hammers before they are thrown into the box.
A multitude of points should be taken into consideration in fixing the grade of a sluice; but a fall of less than eight or more than twenty inches, in a box of twelve feet, would be considered as unsuitable for the board-sluice. Sometimes the upper part of the sluice is made steeper so as to dissolve the dirt, and the lower part has a small grade to catch the gold. The clayey matter of ordinary pay-dirt is fully dissolved in a sluice two hundred feet long with a low grade, so the use of the boxes beyond that length is merely to catch the gold.
The gold is caught in the sluice-boxes by false bottoms of various kinds. The most common false bottom is the longitudinal riffle-bar, which is from two to four inches thick, from three to seven inches wide, and six feet long. Two sets of these riffle-bars go into each sluice box, the box being twice as long as the bar. They are wedged in, from an inch to two inches apart allowing for the bars to be more readily fastened in place and removed.
In all sluices, men must keep watch to see that the boxes do not choke ; that is, that the dirt and stones do not collect in one place, so as to make a dam, and cause the water to run over the sides, and thus waste the gold.
There are small sluices, from which all stones as large as a doubled fist are thrown out. For this purpose the miner uses a sluice-fork, which is like a large manure-fork or garden-fork, but has tines which are blunt and of equal width all the way down; the bluntness being intended to prevent the tines from catching in the wood, and the equality of width to prevent the stones from getting stuck in the fork.
Washing of gravel through the sluice should be carried on as continuously as possible. When processing rich gravels, watches must be set over the sluices, or gold is likely to be missed. As an extra precaution, the sluices should be run full of gravel before shutting off the water. There is no fixed custom regulating “clean ups.” Some managers do so every 20 days, others run two or three months, others again clean up but once in a season.
In the early days, large sluices were frequently paved with stone, which makes a more durable false bottom than wood, and catches fine gold better than riffle-bars. The stone bottoms have another advantage that it is not so easy for thieves to come and clean up at night, as is often done in riffle-bar sluices. But, on the other hand, cleaning up is more difficult and tedious in a rock-sluice, and so is the putting down of the false bottom after cleaning up. The stones used are cobbles, six or eight inches through at the greatest diameter, and usually flat.
The ‘long tom’ was originally a rough wooden box, about 14 ft. long, and 2 ft. wide at the upper end, and 3 ft. at the lower end. The sides were about 10 in. high and the bottom had six or more cleats or riffles. The water was fed in a continual stream and the flow of water allowed the placer gold gravels to be treated in larger quantities than with the rocker. The next step was the ‘sluice-box’, which could be lengthened indefinitely, and had a much larger capacity.
A run of sluices is composed of twenty or more boxes, one after the other. The longer the run the more gold will be saved. One end of the box is raised two, three, four and sometimes six inches higher than the other end, thereby making one end of a long run several feet higher than the other. The boxes are usually on two stakes, one on each side, nailed together by a piece of board about a foot wide.
The boxes must be perfectly tight, so that the water, gold and mercury (quicksilver) can not get out. The pay dirt is put into the boxes with a shovel. The water washes the dirt and gravel out. If there are any large stones that the water will not carry out of the box, a man walking on top of the boxes, throws them out with a long-handled fork. The tom may be used to advantage in diggings where the amount of pay-dirt is small and the gold coarse. Historically, the riffle box often contained quicksilver for amalgamation.