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to official assay ; and finding all materials, they receive for their labour 8 annas, or } per 100 on the value of the metal. The cake of litharge when cold is ground and sifted and vields granules of silver. The sifted powder is made into a paste with cow-dung, and the lead recovered in a furnace of particularly ingenious and effective construction-of which the following is a sufficient description.
A barrel-shaped clay cylinder is made, open at both ends, nine inches diameter below by 12 to 15 above, and usually 24 inches high. A bellows pipe of refractory clay 3 inches in diameter enters at the side about 4 inches from the top, and is led down the cylinder so that the nozzle of the pipe is within six inches of the bottom. The cylinder stands over a cup-shaped hollow made in the ground and sifted over with a little wood ashes. To use this furnace it is first half filled with charcoal and the fire kindled. The mixture of litharge and cow-dung is then introduced in balls the size of an orange, with layers of charcoal and the fire urged. The litharge is quickly reduced to the metallic state, and the lead containing any silver present in solution, collects in the cup-shaped hollow-100 lbs of litharge can be thus worked off in about + hours. This process is applied with remarkable success to the treatment of sweepings and other rubbish containing not more than 1 per 100 to 2 per 100 of silver, but in this case a small and variable quantity of borax is added to the mass of litharge, sweepings and cowdung.
The basin below the cylinder is open at one side, but during the process is kept partially closed by a heap of charcoal and a brick. This being removed occasionally, the surface of the melted lead is raked free from earthy slags by an iron rod, and the firing is continued till the balls are all consumed. The cylinder is then removed, water thrown on the lead-and this containing silver, is used for the next cupellation refinage.
The skill exhibited by the native refiners in conducting these processes is beyond all praise, and for the scale on which they have to operate, it would, I conceive, be scarcely practicable to effect any improvement on their system. But it has serious inconveniences when we attempt to follow it in large operations. Each operation is limited to about 2500 tola wt. and this may be repeated, so as to give 5000 as the day's work of 6 men.
The heat is almost intolerable, the lead fumes most dele.
terious. These objections might be obviated by the erection of suitable screens and hoods, but the refinage never proceeds so successfully as when the native operator is left to his own fashion. Superintendence and the prevention of pilfering become exceedingly difficult also when a large quantity of bullion has to be operated on, from the great number of people employed, the large space occupied by each gang, and the dense smoke and fumes which fill the refinery.
In the new mint there are three cupellation furnaces by Maudslay, constructed on the most approved plan, and in which the operation could be carried on very effectively and economically were it practicable to work the furnaces continuously, night and day ; but as all work must terminate in the mint and the fires be extinguished daily at 4 P.M., the furnaces are quite useless. At best no more than 3000 tola weight of silver can be refined in each daily, but with such wasteful expenditure of fuel as to render the operation much more costly than the charge of the native refiners.
The cursory description above given suffices to explain the object I had in view in attempting, towards the close of 1845, to effect the refinage of silver in large, indeed, I may say immense quantities, and to conduct the operation so that the mass of bullion acted upon should be brought into a malleable state, and safely stored, within a period of six or seven hours. How effectually this has been accomplished is shown in the sequel of this paper.
My process is based on the old French system of the poussée or salt. petre refinage. This I witnessed in the Laboratory of my friends, Messrs. Johnson and Cock, the eminent refiners in London, and it is minutely described in the works of Dumas and Berthier. The silver to be refined is granulated, the granules mixed with one-tenth their weight of fine salt petre, and projected gradually into a redhot EARTHEN crucible. The nitre oxydizes the base metals, having but little effect on the silverwhen the mass has become red hot the fire is urged till the silver is melted; the whole is then poured into ingot moulds; and the scoriæ, consisting of potash, oxides of copper, lead and other base metals, with granules of silver and oxide of silver in considerable quantity, are reserved for subsequent treatment by methods varying according to circumstances afterwards explained.
The practical drawback to this system as it existed previous to my experiments, was the supposed necessity of using earthen crucibles. This at once limited each batch to some 30lbs weight of metal, or about 1000 tolas, and where we had to deal with tons and lacs its adoption seemed hopeless. It occurred to me however, to make trial of the ordinary cast-iron melting pots of the mint, and I soon found to my great satisfaction that by a little management these could be used with complete success. The object in view was accordingly gained to the fullest extent required, and in September, 1846, this system of refinage was applied in one working-day (the 4th Sept.) to the very large quantity of 188,264 tola wt. of coarse silver — Troy pounds 5,883, value Co.'s Rs. 172,860 10 2, or £17,286 18. 3d. which was refined and returned to the mint in bright malleable ingots, and registered for assay in less than six hours from the commencement of the operation. I believe I am justified in asserting that in point of rapidity, economy and quantity, this day's refinage has never been equalled in any refining establishment in any part of the world. I now proceed to the detailed description of the process--its expenses and total results.
The cast-iron silver melting pots used in the mint, are of cylindrical shape, with round bottoms, 17 inches external height, 11] inches, internal diameter, l1 inch thickness of metal. The quantity of silver usually melted in each pot is 10,000 tola wt. or 312; Troy pounds. If the silver to be refined is in the state of coin the operation may be commenced at once. Itin bars or other solid masses it must be granulated. For this purpose about 8000 tola wt. are melted and poured from the pot placed on a suitable frame over a tank of water, beneath the surface of which two or three brooms are kept in constant motion. This reduces the silver to granules like small shot.
6000 tola wt. of understandard coins or granules are placed in each iron pot, and heated to low redness in the ordinary melting furnace, of which there are 16 in the mint. When at a low red heat the mass of silver is hollowed out with an iron rod with flattened end, so as to make a funnel-shaped depression of the metal in the centre. About 2 pounds weight of saltpetre are thrown into this hollow. The saltpetre rapidly melting percolates through the granules or coins, and, as it filters through parts with its oxygen to the base metals. After a few minutes the fireman with the same rod stirs up the silver from the bottom of the pot and works it in every direction, again cupping the centre as before. The heat is slightly urged and the saltpetreing is repeated in the same manner, until from 5 to 7 seers (10 to 14lbs) are used, the quantity being determined by the coarseness of the silver. In half an hour from the beginning the whole mass of metal becomes pasty, and when pressed towards the bottom of the pot coheres in a mass upon which there floats a very liquid scum, composed of melted potash and litharge with some oxide of copper and a little oxide of silver in solution. This liquid scum is skimmed off with an iron ladle, and when as much is removed as is practicable, the pot is covered and the fire run up by the register to a degree somewhat higher than that usually given in silver meltings, and which experience can alone teach.
In about half an hour the silver is found to be quite melted, its surface being covered with thick but loose and dry crusts of oxide of copper. It is now ready for pouring, and a piece of coke being placed across the lip of the pot, the refined silver is cast in ingots in the usual manner, without any of the dry scoriæ entering the moulds. The ingots when cool are perfectly clean and bright, and fit in every respect for delivery in the Bullion department, to be registered for assay.
On the 4th of September 1846, this process was, as above stated, performed on silver to the value of Co.'s Rs. 172,860 10 2,=£17,286 18. 3d. sterling. At 8} A. M. the fires were lighted in the 16 furnaces. At 9; A. M. the silver (consisting of Nanashaye rupees, average 18 worse than standard, and containing about 4 dwts. of lead per tb.) in the state of coin was charged into the pots—at 10 A. M. the saltpetreing was commenced-by 11} the first pot was poured off, and all sixteen by i past 12. The pots were replaced in the furnaces, charged once more and by 2} P. M. the refined silver again poured off. The refined bars were returned to the mint. The subsequent assays showed some of the pots to have been refined to 13 dwts. better, and the whole silver returned averaged 5" better.” All the ingots without exception were soft and malleable and fit for alligation.
When the scoria and sweepings were subsequently worked up, and the account closed, it was found to stand as follows :
Value of silver delivered to be refined Co.'s Rs....
Loss, Rs. 371 15 11
Being three annas and five pie per cent. in value, or about th per 100, which was found by experiment to be the mere loss on melting this kind of silver.
From the 9th of October 1815, to the present time, May 1847, I have refined in this manner coarse and brittle silver to the value of over ten lacs of rupees=£ 100,000; of the Jaloun silver alone there were refined in 1846 Rs. 882,510 11 8. In one operation about Rs. 50,000 worth of silver, containing over 30 per 100 of lead was thus treated, and the resulting ingots, though 40 to 50 dwt. worse than our standard, were cured of brittleness and rendered fit for alligation for coin.
From these numerous and large trials it results that when the saltpetreing is managed in the mode I have described, the iron vessel is entirely uninjured. In fact the saltpetre has become inert before it touches the side or bottom of the pot. Accordingly the same pot has in many instances been used more than six times over, and after this has borne the average number of common meltings, as shown by the official report of Mr. Casperz the melter to the mint.
Treatment of the Scoriæ. This part of the operation is done at leisure, and on its careful and precise management depends the economy of the process.
The scoriæ well mixed together may be represented as composed of fused potash, oxides of copper and base metals, granules of metallic silver with oxide of silver, and a minute quantity of chloride of silver.
The mass is first bruized in iron mortars and steeped in water for two days in a leaden tub, the water then drained off and replaced, and this repeated a second time. The potash is thus dissolved out, the mass disintegrated and rendered pulpy, and its oxide of silver reduced to the metallic state. It is now in successive portions rubbed in iron mortars, and sieved on fine cane or bamboo sieves floating on water in the leaden tub. The pulp of oxides passes through, and nearly all the silver in granules remains on the sieve. This silver only needs to be melted and returned.
The oxides, with finely divided metallic silver, metallic copper, and chloride of silver, after settling to the bottom of the tub, and the water decanted or syphoned off, are placed on dry tiles, which soon absorb the moisture; of this mass from 4 to 5 cwt. weight are placed in a reverberatory furnace and calcined at a low red heat for four hours.