Gold exists in nature only in the metallic state; but it is scarcely ever found perfectly pure, for it is alloyed in different proportions with silver, copper, tellurium, and some other metals. When it is alloyed with silver or copper, or even with both, the gold retains its ductility; but when combined with tellurium, its distinctive characters entirely disappear. The presence of gold may easily be detected by treating the mineral supposed to contain it with nitro-muriatic acid, and dropping muriate of tin into the solution. If the solution contains any gold, a purple precipitate immediately appears. Native gold ought to be dissolved in nitro-muriatic acid; the silver, if any is present, falls to the bottom in the state of muriate, and may be separated by filtration, and weighed. Pour sulphate of iron into the solution, and the gold is precipitated in the metallic state. The copper, if any is present, may be precipitated by means of a plate of iron; the presence of iron may be ascertained by dropping tincture of nutgalls into a portion of the solution. The auriferous pyrites may be treated with diluted nitrous acid, which dissolves the iron, and separates the sulphur; the gold remains insoluble, and is found in the state of small grains.
In the Hungarian gold mines, which are the richest yet known in the old continent, the attention of the miner is not merely limited to the strings of ore, but to the whole contents of the vein, which are usually extracted, and raised to the surface in large masses. These masses are distributed to the workmen, who break them down, first with large hammers, and afterwards with smaller ones, till they are reduced to pieces of the size of a walnut; the native gold, with the matrix attached to it, is again to be broken by hand into still smaller pieces, by which means other impurities and stony matters are separated. The ore is then introduced into a wooden box, floored with cast-iron plates, and, by the action of two or more heavy spars of oak, which are shod with iron, and alternately worked like the common stamping mill, it is reduced to a fine powder; this powder, which is called flour, is then removed into a vessel like a large basin, and mixed with such a quantity of salt and water as will render it damp; the workman then takes a thin, porous leather bag, introduces a quantity of mercury into it, and, by regular and continued pressure, forces the mercury, in very minute drops, through the leather.
In this divided state it falls upon the pulverized ore, and is immediately kneaded up with it, till the requisite quantity, which depends on the proportion of gold, has been added. After completing this part of the process, the next object is to incorporate the mercury and gold: this is effected by rubbing the mixture together for some time by means of a wooden pestle; the mixture is then heated in a proper vessel, and subjected, for three or four days, to the temperature of boiling water; and, lastly, the mixture is to be carefully washed by small parcels at a time, so that the earthy particles may be carried off by the water; the mercury, combined with the gold, only remains behind in the form of amalgam. A portion of this mercury is then separated by pressure, in a leathern bag, and the remainder is driven off by distillation, leaving behind the gold and silver, with which it may be alloyed. When this metal is found in other ores, they are first roasted, to disperse the volatile principles, and to oxidize the other metals. The gold, which is but little subject to oxidation, is extracted by amalgamation or by cupellation, or either methods, adapted to each ore, according to its properties or constituent parts.
The metal obtained in these ways is always more or less alloyed, particularly with silver and copper. The first step in its purification is the process of cupellation. (See Cupellation.) The gold, after it has been submitted to this process, is often alloyed with silver, which, being nearly as difficult of oxidation, is not removed by the action of lead; and hence the necessity of the operation denominated parting, for which see Parting.