SILVER WORKING. Silver-ores are found chiefly in veins which traverse the primary and the older of the secondary stratified rocks, but especially the former ; and also the unstratified rocks, such as granite and porphyry, which are associated with the above. Some of the richest mines in South America are situated in primary strata ; also in limestone and in graue-acke, and still more in secondary strata. In some of the mines of Pen:, and in those of Kongsberg in Norway and Freiburg in Saxony, silver has been discovered in masses weighing from 100 to 800 lbs. In the mines of Europe the veins are numerous and slender ; in some of the mines in the Barz Mountains and in the Hungarian mines the veins occur in a small number of spots, and are of considerable dimensions. In three of the richest districts of Mexico there is only one principal vein, which is worked in different placers. One of these veins, in the district of Guanaxuato, is from 130 to 148 feet wide, and it has been traced and worked to an extent of nearly eight miles.
The average richness of all the ores iu Mexico is from 3 to 4 ounces per quintal of 102 lbs. In one of the Mexican mines a working of 100 feet in length yielded in six months 432,274 lbs. troy of silver, equal in value to upwards of 1,000,000/. In Chili some of the mines yield only 8 oz. in 5000 lbs. of ore ; but in the rich mine of Copiapo, discovered in 1832, the ore frequently contains 60 or 70 per cent. of silver. The average produce of the mines in Saxony is from 3 to 4 ounces in the quintaL The lead mines of Craven in Yorkshire contained 230 ounces of silver per ton ; and those of Cardiganshire, worked in the reign of Charles I., yielded 80 ounces. The average proportion of the lead-mines of the north of England is 12 ounces per ton ; but even proportion of silver is much less than this, it has been found profitable to separate it. The pure metal is separated from the ore by various processes ; by mechanical division, roastinge to separate the sulphur and other volatile matter, and melting at different stages of purification, with the addition of fluxes of various sorts. Refining is performed by amalgamation with quicksilver, the two metals being afterwards separated by distilling off the quicksilver. [AmatoAx ; LEAD 3IANUYACTERE, COL 149.]
When silver is issued for coin, it is always alloyed with copper ; the maximum of hardness is produced by one-fifth of copper. One lb. of standard silver of the English coinage contains 11 oz. 2 dwts. of pure silver and 18 dwts. alloy, or 925 parts of pure silver in 1000 parts of standard silver. [Mosey.] For purposes connected with the manufacture of various articles of use and ornament the alloy is greater. At Birmingham rolled sheets are made which do not contain more than 3 or 4 dwts. of silver to each lb. of the inferior metal. Silver forms by far the largest proportion of the value of domestic utensils in which either of the two precious metals is used. It lies also been suggested that electro-plating with silver might be applied to the interior of leaden pipes, when used for domestic purposes.
The rolling of silver in contact with the inferior metals is performed by powerful flatting-mills. A bar of copper is made quite smooth and clean on one of its surfaces, it is then sprinkled over with glass of borax ; a plate of fine silver is laid upon it, and the two are carefully bound together by wire. The mass is then exposed to a full red beat, which melts the borax and Causes the silver to adhere to the copper. The ingot is now pissed through a rolling-press and formed into a plate, both the silver and copper extending uniformly during the whole process, at the conclusion of which they are inseparably joined.
Mr. M'Culloch, on the authority of Mr. Birkmyre, gives an estimate of the quantity of silver produced in the two years next before, and the two years next after, the discovery of gold in California. According to this authority, in the year 1846 there was procured in America silver to the value of 5,261,6191., and in the other parts of the world 1,254,306/.; while in 1850 these values were raised to 7,259,824/. and 1,528,592L, respectively. It is interesting to note that, in 1857, more than 1,300,000 ounces of silver were obtained by refining lead in the lead-smelting works of the United Kingdom, of which about half a million ounces were from British lead.
It may suffice here to refer to several foregoing articles, for further information concerning the working of silver ; such as BULLION ; GOLD LAM MANUFACTURE ; MINT ; MONEY ; PLATE AND PLATING.