The auriferous sulphurets are much more common, but much poorer than the former ores ; some contain only one 200,000th part of gold, and yet they may be worked with advantage, when treated with skill and economy.
The gold of these ores is separated by two different processes ; namely, by fu sion and amalgamation.
The auriferous metallic snlphurets are first roasted ; then melted into mattes., which are roasted anew ; next fused with lead, whence an auriferous lead is obtained, which may be refined by the process of cupellation.
When the gold ores are very rich, they are melted directly with lead, without preliminary calcination or fusion. These processes are, however, little practised, because they are less economical and cer tain than amalgamation, especially when the gold ores are very poor.
If these ores consist of copper pyrites, and if their treatment has been pushed to the point of obtaining auriferous rose copper, or even black copper including gold, the precious metal cannot be sepa rated by the process of liquation, because the gold, having more affinity for copper than for lead, can be but partially run ofi by the latter metal. For these reasons the process of amalgamation is far prefera able.
This process being the same for silver, its description is reserved for that me tal. The rich ores in which the na tive gold is apparent, and merely dissem inated in a stony gangue, are directly tri turated with quicksilver, without any preparatory operation. As to the poor ores, in which the gold seems lost amid a great mass of iron, sulphuret of copper, &c., they are subjected to a roasting be fore being amalgamated. This process seems requisite to lay bare the gold en veloped in the sulphurets. The quick silver with which the ore is now ground, seizes the whole of its cold, in however small quantity this metal may be present. The gold procured by the refining pro cess with lead, is free from copper and lead, but it may contain iron, tin, or sil ver. It cannot be separated from iron and tin without great difficulty and expense, if the proportion of gold be too small to admit of the employment of muriatie acid.
By cupellation with lead, gold may be deprived, of any antimony united with it. Tin gives gold a remarkable hardness and brittleness,; a piece of gold, exposed for some time over a bath of red-hot tin, becomes brittle. The same thing hap pens more readily over antimony, from the volatility of this metal. A 2,000th part of antimony, bismuth, or lead, de stroys the ductility of gold. The tin may be got rid of by throwing some cor rosive sublimate or nitre into a crucible containing the melted alloy. By the first agent, perchloride of tin is volatalized; by the second, stannate of potash forms, which is carried off in the resulting al kaline scorise.
Gold treated by the process of amalga mation, contains commonly nothing but a little silver. This silver is dissolved out by nitric acid, which leaves the gold untouched ; but to make this parting with success and economy on the great scale, several precautions must be ob served.
If the gold do not contain fully two thirds of its weight of silver, this metal, being thoroughly enveloped by the gold, is partially screened from the action of the acid. Whenever, therefore, it is known
by a trial on a small scale, that the silver is much below this proportion, we must bring the alloy of gold and silver to that standard by adding the requisite quanti ty of the latter metal. This process is called quartation.
This alloy is then granulated or lami nated; and from twice to thrice its weight of sulphuric or nitric acid is to be boiled upon it ; and when it is judged that the solution has been pushed as far as possible by this first acid, it is decant ed, and new acid is poured on. Lastly, after having washed the gold, some sul phuric acid is to be boiled over it, which carries off a two or three thousandth part of silver, which nitric acid alone could not dissolve. Thus perfectly pure gold is ob tained.
The alloys of gold have been examined in detail by Mr. Hatchett. Of these the most important is that used for the gold coin of England, commonly called stand ard gold, which consists of eleven parts of pure gold and one of copper ; it is ex tremely ductile and malleable, but harder than pure gold, and, therefore, better calculated to resist the wear and tear of circulation. The specific gravity of this alloy is 17.157: 20 lbs. troy of it are coined into 934 sovereigns and one half sove reign; 1 lb. troy, therefore, produces 46 29-40 sovereigns ; the same weight was formerly coined into 441 guineas. The color of this alloy is deeper yellow than that of pure gold, and verges upon orange : it frequently happens that a part of the alloy of gold coin is silver, hence the pale color of some sovereigns as com pared with others. Tho United States gold contains an alloy of one-tenth of cop per. Among the metals which destroy the color and malleability of gold, none is so remarkable as lead. It appears from Mr. Hatchett's experiments, that when lead forms about one 2000th part of the alloy, it is too brittle for rolling, and that the fumes of lead destroy the good quali ties of gold. The chemical equivalent of gold is probably about 200 and that of the protoxide 208, and of the protochlo ride 236. The peroxide is a compound of one proportional of gold and three of oxy gen, and the perchloride contains three proportionals of chlorine. When ether is agitated with solution of chloride of gold, it takes up the metal, and forms a yellow ethereal solution of gold; when polished steel instruments are dipped into this so lution, and immediately washedin water, and wiped with a piece of soft leather, they become beautifully gilt with a very thin film of gold. See GILDING.
For the separation of gold in a spongy form, Dr. C. Jackson, of Boston, adopts an economical plan. After separating the gold and silver by aqua regia, the so lution containing gold and copper is eva porated to a small bulk, and the excess of nitric acid driven of. A little oxalic acid is then added, and a solution of car bonate of potass sufficient to take up nearly all the gold as aurite of potass, is added. Then an excess of ox alic acid is added, and the whole boiled. The gold is immediately precipitated in the form of sponge : this is a suitable form for the jeweller and dentist.