REFINING OF GOLD AND SIL VER; called also Parting. For several uses in the arts, these precious metals are required in an absolutely pare state, in which alone they possess their malleabil ity and peculiar properties in the most eminent degree. Thus, for example, neither gold nor silver leaf can be made of the requisite fineness, if the metals contain the smallest portion of copper al loy. Till within these ten or twelve years, the parting of silver from gold was effected everywhere by nitric acid.
2. On parting by the nitric acid. The principle on which this process is found ed, is the fact of silver 'being soluble in nitric acid, while gold is insoluble in that menstruum. If the proportion of gold to that of silver be greater than one to two, then the particles of the former metal so protect or envelope those of the latter, that the nitric acid, even at a boiling heat, remains quite inactive on the alloy. It is indispensable, therefore, that the weight of the silver be at least double that of the gold. 100 pounds of silver take 88 pounds of nitric acid, of specific gravity 1.320, for oxidizement, and 111 for solu tion of the oxide ; being together 149• but the refiner often consumes, in acid of the above strength, more than double the weight of silver, which shows great waste, owing to the imperfect means of condensation employed for recovering the vapors of the boiling and very volatile acid.
100 pounds of copper require 130 pounds of the above acid for oxidizement; and 890 for solution of the oxide ; being 520 pounds in whole, of which less than ith part could be recovered by the above apparatus. It is therefore manifest that it is desirable to employ silver pretty well freed from copper by a previous pro cess; and always, if practicable, a silver containing some gold.
In parting by nitric acid, the is gen erally retains a little silver • fis proved by the cloud of chloride of silver which it affords, at the end of some hours, when dissolved in aqua regia. And on the other hand, the silver retains a little gold. These facts induced M. Dize, when he was inspector of the French mint, to adopt some other process, which would give more accurate analytical results ; and after numerous experiments, he ascertain ed that sulphuric acid presented great ad vantages in this point of view, since with it he succeeded in detecting, In silver, quantites of gold which had eluded the other plan of parting. The suggestion of M. Dize has been since universally adopt ed in France. M. Costell, about nine or ten years ago, erected in Pomeroy street, Old Kent road, a laboratory upon the French plan, for parting by sulphuric acid ; but he was not successful in his en terprise; and since he relinquished the business, Mr. Matheson introduced the same system into our Royal Mint, under the management of M. Costell's French operatives. In the Parisian refineries, gold, to the amount of one thousandth ' part of the weight, has been extracted from all the silver which had been pre viously parted by the nitric acid process ; being 8,500 francs in value upon every thousand killogrammes of silver.
The most suitable alloy for refining gold, by the sulphuric acid process, is the compound ofgold, silver, and copper, having a standard quality, by the cupel, of from 900 to 950 milliemes, and contain ing one fifth of its weight of gold. The
best proportions of the three metals are the following :—silver, 725; gold, 200 copper, 75 ;=1000. It has been found that alloys which contain more copper afford solutions that hold some anhydrous sulphate of that metal in solution, which prevents the gold from being readily se parated ; and that alloys containing more gold are not acted on easily by the sul phuric acid. The refiner ought, therefore, when at all convenient, to reduce the al loys that he has to treat to the above stated proportions. He may effect this purpose either by fusing the coarser al loys with nitre in a crucible, or by add ing finer alloys, or even fine silver, or finally, by subjecting the coarser alloys to a previous cupellation with lead on the great scale. As to gold or silver bullion, which contains lead and other easily ox idizable metals besides copper, the refiner ought always to avoid treating them by sulphuric acid ; and should separate, first of all, these foreign metals by the agency of nitre, if they exist in minute quantity ; but if in larger, he should have recourse to the cope]. Great advantage will there fore be derived from the judicious pre paration of the alloy to be refined.
For an alloy of the above description, the principal Parisian refiners are in the habit of employing thrice its weight of sulphuric acid, in order to obtain a clear solution of sulphate of silver, which does not too suddenly concrete on cooling, so as to obstruct its discharge from the alembic by decantation. A small increase in the quantity of copper calls for a con siderable increase in the quantity of acid. Generally speaking, one half of the sul phuric acid strictly required for convert ing the silver and copper into sulphates, is decomposed into sulphurous acid, which is lost to the manufacturer, unless he has recourse to the agency of nitrous acid.
The Parisian refiners restore to the owners the whole of the gold and silver contained in the ingots, reserving to them selves the copper which formed the alloy, and charging only the sum of 51 francs per killogramme (2.68 lbs. troy) for the expense of the parting of the metals.
If they are employed to refine an ingot of silver containing less than one tenth of gold, they retain for themselves a two thousandth part of the gold, and all the copper, existing in the alloy ; return all the rest of the gold, with the whole of the silver, in the ingot ; and give, besides, to the owners a premium or bonus, which amounted lately to Iths of a franc on the kilogramme of metal. Should the owner desire to have the whole of the gold and silver contained in his ingot, the refiner then demands from him '2 francs and 68 centimes per kilogramme, retaining the copper of the alloy. As to silver ingots of low standard, the perfection of the re fining process is such, that the mere cop per contained in them pays all the costs ; for in this case, the refiner restores to the proprietor of the ingot as much fine silver as the assay indicated to exist in the in got, contenting himself with the copper if the alloy.