Amalgamation.—There are a few metals, including gold, silver, copper and tin, which, with quicksilver, form a peculiar chemical union called amalgamation, a process of great importance to the gold miner. When a piece of gold or silver is placed in mercury, the latter metal gradually penetrates through it, destroys the coherence of its particles, and form with it a mass like dough. A lump of gold as large as a bean will be soaked through in three or four days; with silver and copper the process is slower, but they are affected in the same manner. Amalgamation, though a union of a solid with a liquid, differs much from a solution. In the latter the union is mechanical; in the former it is chemical. In the latter the solid is reduced to particles of impalpable fineness; in the former it is not. An ounce of salt will be dissolved in, and nearly equally diffused through, a pint of water; but if an ounce of gold be thrown into a pint of quicksilver, it will, after forming an amalgam with the quicksilver, remain at the bottom. We have no texture so fine that it will strain salt out of water; but the particles of gold are so coarse in amalgam that they can easily be strained out by means of buckskin or tight cloths. However, a little gold will remain in the quicksilver—about the fiftieth part of an ounce of gold in every pound of quicksilver; and the only method of obtaining this gold is by retorting.
Quicksilver is used in gold mining for catching the small particles of metal; the large ones are caught by their weight. But many of the particles are so small that they are almost invisible to the naked eye, and when in moving water they float. Miners frequently show visitors the fineness of their gold by putting some of the dust in a vial with water, and upon shaking, the particles of metal can be seen floating about in the clear water. Riffles, and all the devices to get the benefit of specific gravity, are of little use to arrest this "float-gold," so amalgamation is employed. If a bit of quicksilver is put in the way of the fine gold, the two metals unite at once and make a larger bulk, which can be caught.
There is no such attraction between gold and quicksilver as there is between the magnet and iron; but when the two former metals once touch, an amalgam is immediately formed, and if the proportions of the metals be about even, they in time make a hard mass. Some gold does not amalgamate readily; in various diggings of Siskiyou county, the gold has a reddish coating, which prevents amalgamation. Grease or resin in the water used for washing, is also unfavorable. So is cold. Heat is favorable, and therefore less gold is lost in summer than in winter. Quicksilver that has been once used is considered better than that fresh from the flask.
No tinned iron or copper vessel should be used for holding or panning out amalgam, or dirt containing amalgam; since quicksilver forms an amalgam with tin and copper, and will stick to the sides of a tinned or copper pan.
In most sluices, the quicksilver is put in above the riffle-bars at various places along in the boxes, with a confidence that the great specific gravity of the metal will prevent it from being lost. The greater the quantity and proportion of fine gold, the greater the importance of the quicksilver.
The best method of catching very fine gold by amalgamation is to cover a large copper plate with mercury, and let the dirt and water, in a thickness of not more than a quarter of an inch, pass over it slowly. There are various methods of covering copper plates with quicksilver. The first thing, in every case, is to wash the copper with diluted nitric acid, so as to remove all dirt and grease. The quicksilver may then be rubbed on with a rag; or, still better, it may be dissolved in nitric acid, and the liquid nitrate of quicksilver may be applied with a rag. The nitric acid will attack the copper, and leave the quicksilver as an amalgam on the surface of the copper. This is the most common process, but the nitrate of copper continues for a long time to come up through the quicksilver and interfere with the catching of the gold. When the nitrate of copper appears—it is a green slime—it should be scraped off and the place rubbed over with quicksilver. When a plate is once covered with mercury, the operation need never be repeated; but more mercury must be sprinkled on as the gold collects and forms a solid amalgam. The plate is usually three feet wide and six feet long, and is set nearly level. In very large sluices the stream should be divided so as to run over different plates. The slowness of the current and the shallowness of the water are important, for with a swift current or deep water many of the particles of float-gold may escape without touching the quicksilver. Wherever a speck of gold has fixed itself on the plate, there others will collect about, evidently preferring to fix themselves in a neighborhood rather than in a waste place. The more gold there is on a plate, the better it is considered to be. The seasons for cleaning up are usually determined by the danger of theft. Miners do not like to leave their gold out in quantities so large as to attract thieves. The amalgam is sometimes half an inch thick, and is usually, at cleaning-up time, a hard mass, which must be loosened by heat. The plate is put on a fire, and when it gets so warm that the hand can scarcely bear it, the amalgam is softened and loosened, so that it can be scraped off readily. The plate is then sprinkled anew with quicksilver, and is ready for use again. Mercury does not amalgamate with copper so readily as with gold or silver. A copper plate, the sixteenth of an inch thick, may be used for at least five years, and perhaps for ten; whereas a gold plate of equal thickness would, if exposed to the action of quicksilver in the same manner, fall to pieces in a few weeks. After a time the quicksilver pervades the copper, and gives it a silvery whiteness all through on the under side. It is said that a solution of cyanuret or prussiate of potash, is used instead of nitric acid in applying mercury to copper plates, and that it is still better, there being then no trouble with the green spots of nitrate of copper.
A good amalgamated copper plate is considered as serviceable as a bed of quicksilver of equal size, and it is very much cheaper and more convenient to manage.
The dirt and water should be admitted to the copper plate, by falling first through a sheet-iron plate, pierced with holes half an inch long and a sixteenth of an inch wide. Some miners place this sheet-iron plate immediately over the copper.
Very soon after the water and dirt commence to run in the sluice, all the spaces between the riffle-bars are filled with sand, gravel and dirt; which, however, present many little inequalities of surface, sufficient to catch all the particles of gold larger than a pin-head. The largest gold is caught near the head of the sluice; and the farther down the sluice, the finer the gold. In some sluices, where the pay-dirt contains much coarse gold, the quicksilver is introduced from thirty to sixty yards below the head, so as to catch only the fine particles of metal.