SILVER: METALLURGY, MINING AND PRO DUCTION. Silver was discovered later than gold and copper, but has been known to man from prehistoric times. Silver orna ments and decorations have been found in the royal tombs of Chaldaea, built in the fourth millenium B.C. It was in use as money probably as early as gold, and it is recorded that Abraham paid Ephron in silver for land bought as a burial place. Silver is also mentioned as money in a Chaldaean inscription of about 4500 B.C., according to Gowland. In ancient Egypt it was scarce, and in the IV. and succeeding dynasties it -was more valuable than gold, but in the XVIII. dynasty trade with the eastern Mediterranean countries made silver more plentiful and cheaper than gold.
Silver is sometimes found in the metallic state ("native silver") but unlike gold it is gener ally combined with other elements in its ores. Horn-silver, AgC1, and embolite (a chloro-bromide), found in the oxidized por tions of lodes near the surface, have been formed by the weather ing of more complex compounds. At deeper levels, silver usually occurs as sulphides, arsenides and antimonides (compounds of silver with sulphur, arsenic and antimony respectively). Native silver occurs in dendritic and wire-like forms which are aggregates of minute crystals, usually cubes and octahedra belonging to the cubic system. It also occurs in thin sheets and sometimes in masses. At Kongsberg in Norway, where the mines have been worked for centuries, large masses of native silver have been found, one of which weighed 697 kilogrammes or nearly three quarters of a ton. Native silver has been found, associated with other silver ores, at Cobalt, Canada, at Broken Hill, N.S.W., and in many localities in the United States, Mexico and South Amer ica, occurring for example in the native copper of Lake Superior, but it is not an important source of production of silver.
Argentite, Ag2S, a soft black sulphide of silver, is one of the commonest ores of silver, but complex sulphides containing anti mony and arsenic are also of frequent occurrence. The most im portant of these as sources of silver are Stephanite, 5Ag2S,
iron black in colour; the ruby-silver ores, Pyrargyrite or dark red silver ore,
and Proustite or light red silver ore,
Dyscrasite or antimonial silver,
and Polybasite. Grey copper ore or Fahlerz, a complex mineral con taining copper, iron, zinc, antimony, arsenic and sulphur, is also an important silver ore of which no two specimens agree in com position. At Cobalt, Ontario, a large proportion of the ore taken from the mines in 1907 contained thousands of ounces of silver per ton, but such rich ores are unusual, and silver ores containing as much as 1% of silver or 150 oz. per ton are generally regarded as very rich. A large part of the world's production of silver is,
however, not obtained from true silver minerals but from lead, copper and zinc ores in which it is an accidental constituent. These are called argentiferous ores. Galena (lead sulphide), in particu lar always contains silver, in amounts ranging as a rule from 20 oz. to 200 oz. per ton, which is extracted as a by-product at a trifling cost. One result of the large yield of silver in lead and copper smelting is that its market price has not such a strongly controlling influence on the amount of production as is observable in the statistics of other metals. The amounts of lead and copper ores smelted and of silver extracted from them depend mainly on the demand for lead and copper and their market prices and not on the price of silver. About I million oz. of silver are ex tracted annually from gold bullion and the remainder is from ores obtained by mining in rock, as there is no silver obtained by washing loose alluvial deposits. For the operations of silver min ing, see MINING, METALLIFEROUS.
The treatment of silver ores was for centuries chiefly by amalgamation (q.v.) but lixivia tion processes, in which the silver is dissolved and the solutions washed out of the ore, were introduced in the middle of the 19th century and passed into wide use. In one group of these proc esses, much used for a time in the United States, the ore was roasted with common salt in rotating cylindrical or other fur naces in order to convert the silver into chloride of silver which was then dissolved by various solutions, These solutions had little or no effect on metallic silver or on silver sulphides or complex minerals containing silver. In the Augustin process the silver chloride was dissolved in hot strong brine (common salt). It was superseded by the Patera process, in which a solution of sodium thiosulphate (commonly called hyposulphite, the "hypo" of pho tography) was used. In the Kiss process the leaching solution contained calcium thiosulphate and in the Russell process a double thiosulphate of sodium and copper. These processes have been replaced by the cyanide process (q.v.), in which a preliminary roasting is not required, or by smelting. Smelting is merely the ordinary smelting of lead ores or copper ores which contain sil ver, but it is often found to be the more profitable course to add true silver ores to the smelting mixture, instead of treating them separately by the cyanide process. The increased percentage of extraction by smelting compensates for the additional cost, if ade quate supplies of lead or copper ore and fuel are at hand. For lead and copper smelting and the extraction of silver from the products, see LEAD, COPPER.