743 being the same as the rolling out of the assayslips, to allow every part of the metal to come in contact with the nitric acid, in which it is to be placed to extract the silver. The water is drained off, and the granulated metal is taken to the "Parting Houses." These are closets with sliding windows, which shut close in order to prevent the fumes from escaping. The bottom is a tank with steampipes around it. Here are placed large jars of porcelain, holding about twenty gallons. Into each is put 150 pounds of the metal. The nitric acid, which has been decanted from the carboys in which it is held, is poured in-about ten POURING OUT ACID. gallons, properly diluted, to each jar. Six or sev-| care must be taken in handling it, for it will en thousand carboys of acid, costing nine dollars each, are used in the course of a year. Great burn the flesh or clothes of the workmen should it touch them. It will not, however, act upon passing into the room, escape through the tall chimney seen in our first illustration. This vapor presents a brill iant appearance, and many who see it suppose that vast quantities of gold are passing off into the air. The vapor, however, is nothing more valuable than nitrous acid. Aft- INTERIOR OF VAULT.-THE CHEESES. | penetrates the mass, washing away the nitrate. In eight or ten hours it is thoroughly cleansed. The gold, which is in the form of a dark It is then taken to a hydraulic press. About a brown powder, resembling Scotch snuff, with no peck is placed in a "stave" or mould. A presslustre or indication of its metallic character, is ure of two hundred tons is applied, which forces yet almost pure-probably 993 fine. It has out the water through slits left in the stave, and been by this process purified as high as 995. compresses the powder into a solid cake, about It still contains a little nitrate of silver mixed twelve inches in diameter and three inches thick. with it, to remove which it is placed in a large The top and bottom of this cake present a dull tub, having at the bottom a strainer composed yellow "frosted" appearance; but the sides, of layers of muslin and filtering paper. A which have been exposed to friction, shine with stream of warm water is poured upon it, which metallic lustre. It looks like a fine yellow cheese, by which name it is called. One of these gold "cheeses," however, weighs about sixty pounds, and is worth eighteen or nineteen thousand dollars. Twelve of them are made from the contents of one filtertub. CASTING FINE BARS. These cheeses, after having been baked in an oven, heated by steampipes, to expel any moisture that may remain, are taken down to the "Fine Melting - Room," broken up, and melted. A little saltpetre and borax is thrown into the crucible, to extract any base metal which may have become mixed. It is turned into iron moulds, smoked with rosin and pitch to prevent adherence; oil is poured on the bar while cooling, to give it a good surface. It comes out in the shape of "fine bars," each weighing about seventeen pounds, and worth some five thousand dollars. These of the expense of coinage. It will be remembered that two parts of silver were melted with every part of gold. This, dissolved in nitric acid, constituting nitrate of silver, was left at the "Parting House." The recovery of this silver forms one of the most beautiful operations in the Assay Office. are cooled in a "pickle" of sulphuric acid di- | more convenient to pack, and there is a saving luted with water, which removes any oxydation of iron on the surface, and gives them a bright appearance. These bars are about 993 fine. When the moulding is begun a cake of five or six ounces is cast, and one notch is cut in it as a mark; when the last bar is cast another cake is made, and marked with two notches. These are called "assay pieces;" they are stamped with the number of the melt, and assayed, as before explained. Each bar is then weighed separately by the Bar Weigh-Clerk, who calculates its value from the weight and fineness, as reported by the Assayer. The number of the melt, the year of melting, the office where refined, the number of the bar, its weight, fineness, and value, are then stamped on every bar. These are now delivered to the Treasurer, who keeps a record of every one. Thus the bullion, which was delivered to the Melter and Refiner in various shapes, is now converted into "fine bars," refined, assayed, its exact value stamped on each, and is ready to be sent to the Mint at Philadelphia for coinage, or to be exported. For shipments bars are generally used in preference to coin, because they are This nitrate, in the form of a greenish fluid, is poured into an immense tank, holding about 3000 gallons, in which has been placed a strong solution of common salt. Salt is a combination of chlorine and soda-its chemical name is chloride of sodium. Four substances are contained in this tank: silver, combined with nitric acid, forming nitrate of silver, and chlorine, combined with soda, forming chloride of sodium. A double chemical action takes place. Nitric acid has a stronger affinity for soda than for silver, and chlorine a stronger affinity for silver than for soda. The consequence is that the acid leaves the silver and unites with the soda, forming nitrate of soda; while the chlorine unites with the silver, forming chloride of silver. The chloride of silver thus formed falls down, or is "precip itated" in soft downy flakes, like falling snow, leaving the nitrate of soda separate. The nitrate of soda is soluble in water, while the chloride of silver is insoluble. When the new combination is complete, the contents of the tank are drawn off into large wooden filters; a stream of hot water is poured in; the soluble nitrate of soda, which also contains all the base metals of the melt, is held to be of no value; it passes off into the sewer and is lost, leaving the insoluble chloride of silver behind in the filter. This, when "sweetened" or washed free from nitric acid, resembles thick white paint, except that the surface, which is exposed to the light, is of a delicate purple. A similar action of light upon the silver-coating of a copper-plate is the basis of the daguerreotype process. About 4000 bushels of salt are used up every year in this process. When thoroughly "sweet," the chloride of silver is transferred from the filter to large leaden vats, in which has been placed a quantity of granulated zinc. Water is added, and another chemical action takes place. The chlorine, having a stronger affinity for zinc than for silver, leaves it, and unites with the zinc, forming chloride of zinc. This being soluble in water, is washed away, not being worth preserving. A little sulphuric acid has been meantime added to destroy any surplus of zinc. The silver, thus successively freed by purely chemical processes from nitric acid and chlorine, is in the form of a dull grayish powder, looking very like a heap of ashes. It is nevertheless almost pure silver, much finer than coin. This powder is "cheesed" by the hydraulic press, like the gold powder, and is deposited in the Melter's safe, to be melted up into bars, or used again for refining gold. We have described only the processes of assaying and refining gold. Silver bullion does not, like gold, require to be refined; but after being assayed is sent to the Mint to be coined. unless it is required to be used for refining gold. The process of assay applied to silver is known among chemists as the humid assay. It is a very delicate operation, based upon the principle that a definite weight of a solution of salt, of known strength, added to a definite weight of silver dissolved in nitric acid, will precipitate a definite weight of chloride of silver. Before leaving the Assay Office we must descend to the basement, and visit the "Sweep Room"-a dirty and disagreeable apartment, but a very important one, nevertheless. Here all the rubbish of the establishmentsweepings of rooms, ashes from the furnace, old crucibles, brooms, brushes, clothes, mittens, and so forth are treated to extract the gold contained in them. These are first burned to ashes, then reduced to an almost impalpable powder in the "Grinding Mill," under heavy iron rollers weighing 2000 lbs. This powder is then placed in the "Gold Separator"-a machine invented by Mr. Kent, the Melter and Refiner. The dirt is put into the cireular vessel A, where the powder is ground still finer, by heavy iron rollers working under water, which is supplied in a continuous stream. The motion of the rollers keeps the mass in perpetual agi weighs about 104 ounces, and is worth a trifle less than $210; a cubic foot weighs about 1454 pounds, and is worth $362,600. A hundred millions in fine bars will measure about 273 tation. The coarser particles of gold sink to the bottom; while the dirt, still containing some gold, being lighter, passes off with the water, through the pipe B into the basin C. At the bottom of this basin is a quantity of quick-cubic feet, say two and one-seventh cords. If silver. The surface of this is kept bright and clean by a couple of paddle-wheels, which just touch it, and also keep the water in motion. Any particle of gold touching the quicksilver is instantly seized, while the water and dirt pass out at the top, through the waste-pipe D. For the right to use this Separator in all the United States Mints Mr. Kent received, by vote of Congress, $20,000; but as the gold saved by it, beyond what was recovered by the old process, amounts to $20,000 a year, the arrangement is a profitable one. In the processes of melting, assaying, and refining, there is of course some waste, for which the Government makes allowance; but declares that it must not exceed two parts in a thousand of the amount operated upon. That is, unless the loss exceeds this proportion, the Melter and Refiner is deemed to have returned all that he received. The actual loss has always been considerably less. It is now reduced, by careful economy, to one-thirteenth part of the legal allowance, saving to Government in five years, up to December, 1859, $191,151 55. We have said that the amount of bullion which will pass through the Assay Office during the present year is estimated at one hundred millions of dollars. Let us try to give some general idea of the weight and bulk of the gold required to make up this sum. A cubic inch of fine gold cast into a solid cube, each side would measure about six and a half feet. Our coinage is of "standard gold," the ten-dollar piece weighing 258 grains. A million dollars in coin weighs nearly 4479 pounds Troy, equivalent to about 3685 pounds avoirdupois, or more than one and three-quarter tons. A keg 13 inches high, 10 inches at the heads, and 11 at the bilge, will hold about $50,000 in gold coin. Two thousand such kegs would be required for $100,000,000. Gold in grains occupies about twice the space that it does when cast into bars. It is said that in California gold dust was for a while sold by the pint, this measure holding 142 ounces, worth $2500. A hundred millions in California dust would measure 40,000 pints, or 625 bushels. After all, the coin which any depositor would be likely to carry with him makes no very great bulk. Into a box measuring on the inside 10 inches long, 8 wide, and 5 high, $36,000 in gold coin can be packed. A bag 6 inches by 9 will hold $5000, leaving room to tie. Silver coin occupies a little more than twenty-five times the bulk of gold coin of the same value. We have now followed the bullion from the time of its reception in the Weigh-Room until it has been converted into "fine bars." Next month we propose to follow some of these bars to the Mint in Philadelphia, and see them converted into coin of the United States. |