The A B C of Mining: A Handbook for Prospectors

by Charles A. Bramble

The cradle must be placed on an inclination while being worked, and under the influence of the continued side-to-side rocking the dirt is quickly disintegrated, passes through the riddle and falls on the apron. From the apron it is conveyed to the inner end of the cradle floor, from which it flows over the riffles, or bars, and out at the mouth. The difference in level of the floor is generally about 2½ inches, but this may be varied according to the nature of the dirt treated. Large stones in the riddle or hopper must be thrown out, but smaller ones assist in breaking up the lumps of dirt. Every little while the pebbles are turned out and looked over for nuggets. Clean-ups are necessary two or three times a day. The hopper is taken off first, then the apron is slid out, and washed in a bucket or tub containing clean water, and finally the gold and amalgam are collected in an iron spoon from behind the riffle bars, and panned out. Gravel requires at least three times its own weight of water to wash it. The most convenient way is to lead the water from a stream through a pipe discharging directly over the hopper, but this is, of course, impracticable in some places. More often the water is led to a little pit on the right hand side of the operator, from which he ladles it up as required. One man can wash from one to three cubic yards daily according to the character of the dirt, but every time he stops the machine to feed it with gravel or to empty the riddle, the sand will pack, and must be removed before washing can go on. Two men can wash nearly three times as much dirt in a day as one man. But in any case, the rocker is only a primitive machine, having a capacity but one-fifth as great as that of the Long Tom, and but one-tenth that of a very poor sluice, but as it is cheap, requires but little water, and saves a high percentage of coarse gold, the rocker will continue to be used in many districts.

The Long Tom was invented many years ago by Georgia miners.

LONG TOM.

It is a trough 12 feet by 15 to 20 inches at the upper end, and 30 inches at the lower, and 8 inches deep. The grade is usually 1 in 12. A sheet iron plate forms the lower end of the trough. These figures refer to the upper trough. The lower or riffle-box is 12 feet long by 3 feet wide, with a fall equal to that of the trough and a sufficient depth to keep the material and water from spilling over the sides. It should have four riffles. For this means of saving the gold, to work satisfactorily, the metal must be coarse and the water plentiful.

SLUICE BOXES.

Every sluice is an inclined channel through which flows a stream of water, carrying away all the lighter matter thrown into it, and separating it from the heavy. When the operations would not be permanent enough, or sometimes for other reasons, a ground sluice is preferred to the ordinary box sluice made of boards. Ground sluicing requires, however, six times as much water as does a box sluice to do the same amount of work. It is simply a gutter in the bed rock, and if the bottom is hard and uneven its inequalities will arrest the gold; if not, a number of boulders too heavy to be moved by the stream are put into the sluice to act as riffles. No mercury is used. The water is turned off and the collected coarse gold washed in the pan.

Sluice boxes may be any length, from 30 to 5,000 feet. They vary in width from 1 to 5 feet, though generally 16 or 18 inches. The grade is proportioned to the fineness of the gold, varying from 8 inches to 2 feet to the 12-foot box or length. The bottom should be of 1½ inch plank, and the sides of 1-inch boards. The boxes are made 4 inches wider at the upper end than at the lower, so as to telescope.

The best method found yet for arresting fine gold is the copper plate amalgamated with mercury on its face. These plates are never used at the head of a sluice or other situation where there is much coarse gold, as they would be superfluous in such a situation, but are placed some distance down the sluice and are most efficacious in arresting the "flour," or excessively fine gold. Plates are always of copper above 1/16 inch thick, and may be 6 feet or more long, and of a width suited to the capacity of the sluice. When treated with quicksilver, they become as brittle as glass, and must be handled with care. The copper plate is first washed with a weak solution of nitric acid, and then mercury that has been treated with a weak nitric acid solution is rubbed on the plate. As this surface of quicksilver wears off, it may be replaced by a little fresh mercury. Any green slime on a plate is an evidence of copper salts in the water. It must be scraped off and the spot rubbed with fresh quicksilver. Gold attracts gold, therefore the plates should not be cleaned up too often.