The precipitate of silver, called by the English, water-silver, and by the French, chaux d’argent, is drained, then strongly squeezed in a square box of cast iron, by the action of a hydraulic press; in which 60 pounds of silver are operated upon at once.
The silver lumps are dried, melted in black lead crucibles, in a furnace built near the silver end of the room, where the superintendent sits in his bureau—a closet enclosed by glazed casements, like a green-house. The whole course of the operations is so planned, that they are made to commence near the centre with the mixed metals, and progressively approach towards the office end of the apartment as the parting processes advance. Here the raw material, after being granulated and weighed, was given out, and here the pure gold and silver are finally eliminated in a separate state.
In the other half of the hall, the solutions of sulphate of copper are evaporated in large shallow leaden pans, placed over a range of furnaces; from which, at the proper degree of concentration, they are run off by syphons into crystallizing pans of the same metal. From the mother-waters, duly evaporated, a second crop of crystals is obtained; and also a third, the last being anhydrous, from the great affinity for water possessed by the strong sulphuric acid with which they are now surrounded. The acid in this way parts with almost the whole of the cupreous oxide, and is then transferred into a large alembic of platinum (value 1000l.), to be rendered fit, by re-concentration, for acting upon fresh portions of granulated silver. The capital of that alembic is connected with a leaden-worm, which traverses an oblong vessel, through which a stream of cold water flows.
The crystallized sulphate of copper fetched, two years ago, 30l. a ton. It is almost all sold to the grocers in the towns of the agricultural districts of France. In the above establishment of M. Poizat, silver to the value of 10,000l. can be operated upon daily.
There is a steam engine of 6-horse power placed in a small glazed chamber at one side of the parting hall, which serves to work all his leaden pumps for lifting the dilute sulphuric acid and acidulous solutions of copper into their appropriate cisterns of concentration, as also to grind his old crucibles, and drive his amalgamation mill, consisting of a pair of vertical round-edged wheels, working upon one shaft, in a groove formed round a central hemisphere—of cast iron. After the mercury has dissolved out of the ground crucibles all the particles of silver which it can find, the residuary earthy matter is sold to the sweep-washers. The floor of the hall around the alembics, pots, and cisterns, is covered with an iron grating, made of bars having one of their angles uppermost, to act as scrapers upon the shoes of the operatives. The dust collects in a vacant space left beneath the grating, whence it is taken to the amalgamation mill. The processes are so well arranged and conducted by M. Poizat, that he can execute as much business in his establishment with 10 workmen as is elsewhere done with from 40 to 50; and with less than 3 grains of gold, in one Paris pound or 7561 grains of silver, he can defray the whole expenses of the parting or refining.
Since 26 parts of copper afford 100 of the crystallized sulphate, the tenth of copper present in the dollars, and most foreign coins, will yield nearly four times its weight of blue vitriol; a subsidiary product of considerable value to the refiner.
The works of M. Poizat are so judiciously fitted up as to be quite salubrious, and have not those “very mischievous effects upon the trachea,” which Mr. Matheson states as being common in his refinery works in the Royal Mint.[48] But, in fact, as refining by sulphuric acid is always a nuisance to a neighbourhood, it is not suffered in the Monnaie Royale of Paris; but is best and most economically performed by private enterprise and fair competition, which is impossible in London, on account of the anomalous privilege, worth at least 2000l. a year, possessed by Mr. Matheson, who works most extensively for private profit on a public plant, fitted up with a lofty chimney, platinum vessels to the value of 3000l., and other apparatus, at the cost of the government. His charge to the crown for refining gold per lb. troy, is 6s. 6d.; that of the refiners in London, who are obliged, for fear of prosecution, to employ the more expensive, but more condensable, nitric acid, is only 4s. That of the Parisian refiners is regulated as follows. For the dealers in the precious metals:—
[48] Report of Committee of House of Commons on the Mint, in 1837, p. 91.
For gold bullion containing silver, and more than 100⁄1000 of gold, 6 fr. 12 c. per kilogramme, = 2 fr. 29 c. per lb. troy.
For silver bullion, containing from 1⁄1000 to 100⁄1000 of gold (called dorés), 3 fr. 27 c. per kilogramme, = 1 fr. 22 c. per lb. troy.