Copper plates may be freed from gold by heating them over a fire and causing the quicksilver to evaporate slowly. The plates, after being cooled, are rubbed with dilute muriatic acid and covered with damp cloths for one night. They are then rubbed with a solution containing salt peter and sal ammoniac, and once more heated over some hot coals, but not allowed to get red hot. Soon the gold scale rises in blisters; the plates are then removed from the fire and scraped. Those parts of the plates that have not yielded up their gold must be re-treated and fired until they do so. All these scales of gold are then collected in a porcelain dish, the base metals are dissolved out with nitric acid, and the gold is then smelted. Corrosive sublimate should be placed in the crucible as long as any blue flame is seen to come from it.
Some mill men prefer to amalgamate their copper plates with silver amalgam, claiming that silver-coated plates save a higher percentage of gold. To amalgamate in this way take some silver bullion, or silver coin, and dissolve in weak nitric acid, only just strong enough to act upon the silver. (If you use too much nitric acid you will waste mercury and make the amalgam harder than it should be for the best results.) After crystals have formed, quicksilver must be added, heating gently meanwhile, until a thick, pasty amalgam has formed. Let this new compound stand for some hours, and squeeze through chamois as usual. The proportion of silver may be about 1 ounce to the square foot of copper to be plated.
In facing new copper plates with this amalgam, they should be washed first with dilute nitric acid; then in clear water; the ball of amalgam being rubbed over their surfaces, some little force being applied. Plates should not be used for 24 hours after coating. Porous copper plates of the best quality, and not too heavily rolled, should be used. Follow the amalgam with a swab, and rub the alloy well into the plate.
Zinc amalgam (preferable when mine water containing sulphuric acid is used in the battery) is applied to the plate after it has been cleaned with a moderately dilute mixture of sulphuric acid and water. The zinc-quicksilver ball is rubbed in and applied while the plate is still wet. Zinc amalgam is prepared as follows: Cut zinc-sheet into small pieces; wash in weak sulphuric acid; and dissolve in mercury. When the quicksilver will take no more zinc, squeeze through chamois and rub in. Zinc-coated plates should stand a week before being used. Very weak sulphuric acid will always clean these plates of any scum that may form before they have received a gold coat.
Sometimes the miner will be troubled with impure gold after retorting. If the metal is very dark this shade may come from the presence of large amounts of iron. A heavy proportion of mineral salts, such as chloride of calcium (CaCl), sodium (NaCl), and magnesium (NgCl2), in the battery water sometimes accounts for this. In such cases amalgamate, retort, pulverize and roast. Then smelt with borax, the iron passing into the slag. If necessary smelt a second time, when the gold should be pure enough to dispose of. In extreme cases, the gold may weigh but one-fifth of the amalgam treated.
In districts where sufficient water for sluicing is not procurable, dry washing is resorted to. Nothing but rich, coarse gold can be worked by this method, and the dry washer rarely delves far below the surface for his gold. In the Mexican deserts the dirt is laid on raw hide, all the large pebbles picked out and the sand rubbed as fine as possible between the hands. The sand is placed in a batea and winnowed by tossing in the air, the lighter material being blown to leaward and the heavy gold falling into the batea. A form of winnowing machine has been patented, which may be driven by horse or hand-power, which is said to give satisfaction. It works by forcing a strong blast of air from a fan through a canvas screen. The inventor claims that it will do the work of three men, and work dirt for 2½ cents a cubic yard. When there is a tendency in the material to cake, dry washing is impossible.
CHAPTER VI.
CAMP LIFE.
The Indian truthfully observes: "White man make heap big fire; keep far off. Indian make little fire; get close. All same." The small fire does best in the circular tepee tent, made of canvas or leather, in use on the plains. The tepee is quite an institution, but it is generally as full of smoke as a kitchen chimney, and for that reason cannot truthfully be recommended. In theory, the smoke should all pass out of the opening in the top.
By using no second skin and carefully excluding all air from around the lower rim of the tepee, it will become an admirable place to cure hams, fish, etc., by the original smoke-dried process. The Scripture declares that he that tarrieth over the wine cup has red eyes next morning, and so has he that sleeps in a smoky tepee. Properly made, however, the tepee is the thing where wood is scarce.
Some original spirits are said to have started for Dawson City, N.W.T., a few years ago with bicycles and push carts. If these means of transport had sufficed, the world would have learnt something, as heretofore a canoe and a sturdy pair of legs were supposed to help the wayfarer in that region better than anything else. That is in summer; in winter, the dog-train is the quickest mode of travel. In the western states and in British Columbia pack horses or mules do the most of the prospector's freighting, and in the far north he either carries his outfit on his back or else transports it by canoe in summer, or by dog-train after the rivers have frozen.