This metal frequently occurs in the native state, sometimes in masses of great weight (5 to 10 cwt.) but more or less contaminated with copper, gold, and mercury. The most common ores are as follows:- (1) The sulphide ("silver glance"), a combination of 87 parts silver and 13 sulphur, most abundantly found in association with the sulphides of the baser metals (lead, copper, antimony, iron, zinc) and arsenic, and recovered by processes incidental to the treatment of the ore for its other constituents; (2) the chloride ("horn silver "), containing 75 parts silver and 25 chlorine; (3) the antimonio-sulphide (" ruby silver "), consisting of 60 parts silver, 22 1/2 antimony, and 17 1/2 sulphur; (4) the cupro-sulphide ("silver-copper glance "), giving 53 parts silver, 31 1/4 copper, and 15f sulphur; (5) the arseno-sulphide (" light-red silver "), composed of 65 1/2 parts silver, 19 1/2 sulphur, and 15 arsenic; (6) "antimonial silver," yielding 78 parts silver, and 22 antimony; and (7) the cupro-antimonio-arsenical sulphide, affording about 95 Per cent. silver.
The German "buttermilk ore" is a mixture of silver chloride and clay.
The processes designed for the preparation of metallic silver from the minerals with which it is associated, may be catalogued in 3 groups, that being the number of distinct principles involved. According to the first, it is amalgamated with mercury, and the latter is then distilled off: this constitutes the " amalgamation " system. Or it may be alloyed with lead and recovered by subsequent oxidation of the lead : the "cupellation" process, which sometimes entails a preliminary step known as "eliquation." Or it may be converted into a salt, and brought into solution, and then be re-precipitated in the metallic form by means of copper: such processes are collectively denominated "wet methods."
(a) Amalgamation. (1) Patio process.-The earliest and rudest of the amalgamation methods is that known as the Mexican or patio process, which is employed in the treatment of ores containing metallic silver, sulphide, and chloride, disseminated through large masses of worthless material, situated in a locality devoid of both cheap fuel and plentiful water. The first step is the reduction of the ore to a more or less fine pulp, by a process of grinding in an arrastra, the ore being previously smashed into pieces of pretty uniform size in a set of perpendicular dry stampers. The arrastra or tabona, where the fine grinding with water is performed, consists of a circular inclo-sure of stone paving about 12 ft. across, surrounded by a wall about 2 ft. high, and provided in the centre with a post fixed in a footstep below, and in a beam above, and having attached to it a series of arms, from which depend heavy blocks of stone; these arms are made to revolve by mules harnessed to the outer end, and the ore is ground by being placed between the stone paving and circulating blocks, the latter being made of porphyry, granite, or basalt, and measuring some 16 in. thick, by a length that just allows them to revolve clear of the wall at one side and the centre post at the other.
The charging of the arrastra is performed somewhat differently according to local peculiarities of the mineral treated. When the ore contains gold and is reduced to a very fine state, some 600 to 1100 lb. of the coarse "sand" (granza) from the stampers are introduced with 10 gal. water at 4 A.M.; this is supplemented by the addition of 10 gal. more water at 9, 10 gal. more at 12, 30 gal. more at 3 p.m., and 50 gal. more at 4, and the grinding is continued for another 12 hours. In the meantime, small quantities of mercury, or silver-amalgam, or copper-amalgam, are repeatedly thrown into the arrastra, with the object of collecting any gold that may be present; this lies undisturbed for periods of 3 to 6 months in the bottom of the arrastra, and is removed at such intervals to be strained, retorted, and run into bars. After 24 hours' total grinding, the mud or "pulp" (Jama) is either baled out or tapped into barrels for removal to stone tanks (larneros) where a portion of the water may evaporate in the sunlight and leave a paste fit for the patio.
Ores containing no gold require a less degree of comminution.
The patio or amalgamation floor is a stone-paved area of some 250 to 300 sq. ft., rendered smooth and water-tight by cement, and slightly inclined to facilitate drainage. A heap (turta) of the pulp is conveyed from the lameros to a circular patch 30 to 50 ft. wide on the patio, amounting perhaps to nearly 100 tons, and forming a deposit about 1 ft. thick, retained by a temporary stone or plank wall with well-pugged joints. As soon as, by evaporation of the water, it has arrived at the consistence of thin-nish mud, 3 to 5 Per cent. of common salt is added, and the torta is stirred up by wooden shovels, and well trodden (repuso) by mules turned in for the purpose, which are blindfolded, yoked in fours, and guided by a driver in the centre, who causes them to gradually diminish the radius of the circle described. The salt is left to exercise its influence during the night,, and the next day the mass is treated with mercury and a material termed majis -tral. This latter is prepared by slowly roasting a mixture of iron- and copper-pyrites with a little salt in a reverbera-tory furnace, resulting in the formation of cupric- and ferrous sulphates with some chlorides; the cupric sulphate, constituting 20 to 40 Per cent. of the mass, is the most important ingredient, and its proportion regulates the amount of the majistral to be used, but the 6 to 12 Per cent. of iron salt is not without some value.
The torta being in a fit condition (or rendered so by adding water if necessary), about 1 Per cent. of its bulk of magistral is spread evenly over its surface from wooden shovels, and the mass is trodden for 1 hour; next the mercury is introduced by squeezing it through canvas so as to break it up into fine particles, the proportion being generally 3 1/2 to 4 lb. for every every marco (say 1/2 lb.) of silver estimated to be present, and the treading operation is repeated. Chemical action soon commences in the mass, and its progress is watched by the periodical examination of 1/2 -lb. samples taken from all parts. These are washed with water in a little bowl, by which the earthy particles are carried away, and the metallic portion is exposed to view. If the mercury exists in tiny globules without having .undergone much change of colour, the magis-tral is in insufficient quantity; but if it presents a leaden aspect, there is an excess of magistral, and a small dose of lime may be required, to limit the action on the mercury; while a light-grey appearance is indicative of good working order, and the globules in the sample taken next day will show a whitish hue and assume a scaly form on pressure, being largely impregnated with silver.