In the Freiberg process the first step is to roast the (ground) ore with common salt, which converts the sulphide of silver into chloride (Ag 2 S + 2NaCl + 4O from the air = 2AgCl + Na 2 SO 4 ). The mass, along with certain proportions of water, scrap-iron, and mercury, is placed in barrels, which are then made to rotate about their axes so that the several ingredients are forced into constantly varying contact with one another. The salt solution takes up a small proportion of chloride, which in this (dissolved) form is quickly reduced by the iron to the metallic state (2AgCl + Fe = FeCl 2 + 2Ag), so that there is, so to say, room made in the brine for another instalment of chloride of silver, which is reduced in its turn, and so on to the end,—the metal formed uniting with the mercury into a semi-fluid amalgam. Of this the bulk at least readily unites into larger continuous masses, which, on account of their high specific gravity, are easily separated from the dross mechanically. The amalgam is pressed in linen bags to eliminate a quantity of relatively silver-free liquid mercury (this of course is utilized as such in subsequent operations), and the remaining solid amalgam is subjected to distillation from iron retorts, whereby its mercury is recovered as a distillate while a more or less impure silver remains in the retort. This process, after having been long wrought in Freiberg with great success, is now superseded there by the Augustin method (see below), but it survives in some other places, as, for example, the Washoe or Comstock district in the Sierra Nevada (United States). It is not used in Chili, Peru, and Mexico because of the scarcity of fuel.

The Mexican process, though far less perfect than that of Freiberg, evades this difficulty. It was tried for the first time, if not actually invented, by Bartolomeo de Medina in 1557. It was adopted in Mexico in 1566 and in Peru in 1574, and is in use in both countries and in Chili to this day. The stamped ore is ground into a fine paste with water; this paste, after having been allowed to dry up a little in air, is placed on a stone floor along with a quantity of salt, and the two are trodden together by mules. On the following day there are added certain proportions of “magistral” (a kind of crude sulphate of copper made by roasting copper pyrites) and of mercury, and the mules are kept going until the silver is as far as possible converted into amalgam, which takes from fifteen to forty-five days. The rationale of the process is not quite understood. According to Boussingault, the cupric chloride (formed by the salt from the sulphate) chlorinates part of the sulphide of silver, thus—

2 CuCl 2 + Ag 2 S=2 AgCl + S + Cu 2 Cl 2 , and the cuprous chloride formed acts upon another portion of sulphide of silver, thus—

Cu 2 Cl 2 + Ag 2 S = 2 AgCl + Cu 2 S, and in this way all the sulphide of silver is gradually converted into chloride. The chloride is reduced to the metallic state by the mercury (AgCl + Hg=HgCl +Ag) with formation of calomel, the metallic silver uniting with the surplus mercury into amalgam. The calomel is allowed to go to waste.

The Augustin process of silver extraction is only a peculiar mode of metallifying and collecting the silver of an ore after it has been by some preliminary operation converted into chloride or sulphate. Either salt is brought into solution—the chloride by means of hot brine, the sulphate by means of hot water, acidified with oil of vitriol; the solution is separated from the insolubles, and made to filter through a bed of precipitated copper. The copper reduces the silver to metal, which remains on the bed as a spongy mass, while an equivalent quantity of copper chloride (or sulphate) passes through as a solution. The silver sponge is collected, freed from adhering copper by muriatic acid in contact with air, and then sent to the furnace. From the copper liquor that metal is precipitated in its original form by means of iron.

The silver furnished by any of these methods is never pure, even in the commercial sense. A general method for its purification is to fuse it up with lead and subject the alloy to cupellation (see Lead, vol. xiv. p. 376). Cupel-silver is apt to contain small quantities of lead (chiefly), bismuth, antimony, copper, and more or less of gold, of which metals, however, only the first three are reckoned “contaminations” by the metallurgist. They can be removed by a supplementary cupellation, without added lead, at a high temperature. Addition of lead would remove the copper likewise, but it is usually allowed to remain and the alloy sent out as cupriferous silver, to be alloyed with more copper and thus converted into some kind of commercial “silver” (see below). If gold is present to the extent of 0∙1 per cent. or more, it is recovered by treatment of the metal with nitric acid or boiling vitriol. The gold in either case remains as such; the silver becomes nitrate or sulphate, and from the solution of either salt is recovered by precipitation with metallic copper. Although nitric acid is the more expensive of the two parting agents, it is often now preferred because photography has created a large demand for nitrate of silver. Compare Gold, vol. x. p. 749.

For the “incidental” extraction of silver from essentially base-metallic ores the method in the case of all lead ores' is simply to proceed as if only lead were present, and from the argentiferous lead produced to extract the noble metal by one of the processes described under Lead (vol. xiv. p. 376-7), while for the treatment of sulphureous copper ores one method is so to smelt the ore (with, if necessary, an addition of galena or some form of oxide of lead) as to produce a regulus of lead and a “mat” of sulphide of copper, (Cu 2 S), which latter should contain as little lead as possible. The silver follows chiefly the lead, and is extracted from it by cupellation; but some silver remains in general even with a lead-free mat. Compare account of the Lautenbach process under Lead.

A modern mode of extracting the silver from a copper mat is to roast it at a very low temperature, so as to produce a relatively large proportion of metallic sulphate, and then to destroy the bulk of the sulphate of copper by a judiciously-regulated higher temperature. The silver all remains as sulphate, which is extracted by hot dilute sulphuric acid and wrought by the Augustin method.

Very interesting is the process which was patented by Claudet for the remunerative extraction of the few hundredths of a per cent. of silver contained in that kind of cupriferous iron pyrites which is now used, almost exclusively, for the making of vitriol. The “cinders,” as returned by the vitriol maker, are habitually worked up for copper by roasting them with salt and lixiviating the roasted mass with water, when the copper dissolves as chloride, Cu 2 Cl 2 and CuCl 2 . The silver goes with it, but for its precipitation no method was known until Field found that silver dissolved as AgCl in a chloride solution can be precipitated exhaustively by addition of the calculated proportion of a soluble iodide, as AgI. Claudet’s process is only an adaptation of Field’s discovery. After having diluted the copper liquor with a certain proportion of water· he adds the weight of iodine, calculated from the assay, as solution of iodide of zinc, which produces a very impure precipitate of iodide of silver. From it he re-extracts the iodine, by treatment with zinc and dilute sulphuric acid, as iodide of zinc, which is used over again. The “silver precipitate,” which now contains its silver as metal mixed with a large quantity of (chiefly) sulphate of lead, goes to the metal-refiner, who treats it as a lead ore.

Chemically Pure Silver.— Even the best “fine” silver of commerce contains a few thousandth-parts of copper or other base metal. To produce perfectly pure metal the most popular method is to first prepare pure chloride (by applying the method given below under “Chloride” to a nitric solution of any kind of ordinary “silver”), and then to reduce the chloride to metal, which can be done in a great variety of ways. One way is to mix the dry chloride intimately with one-fifth of its weight of pure quicklime or one-third of its weight of dry carbonate of soda, and to fuse down the mixture in a fire-clay crucible at a bright red heat. In either case we obtain a regulus of silver lying under a fused slag of chloride—2AgCl + (CaO or Na 2 CO 3 ) = 2Ag + (CaCl 2 + O or 2NaCl + CO 2 + O). The fused metal is best granulated by pouring it from a sufficient height, and as a thin stream, into a mass of cold water. A convenient wet-way method for small quantities is to boil the recently precipitated chloride (which must have been produced and washed in the cοld) with caustic soda-ley and just enough of sugar to take away the. oxygen of the Ag 2 O transitorily produced. The silver in this case is obtained as a yellowish-grey heavy powder, which is easily washed by decantation; but it tends to retain unreduced chloride, which can be removed only by fusion with carbonate of soda.

Stas recommends the following process as yielding a metal which comes nearer ideal purity. Slightly cupriferous silver is made into dry nitrate and the latter fused to reduce any platinum nitrate that may be present to metal. The fused mass is taken up in dilute ammonia and diluted to about fifty times the weight of the silver it contains. The filtered (blue) solution is now mixed with an excess of solution of sulphite of ammonia, SO 3 (NH 4 ) 2 , and allowed to stand. After twenty-four hours about one-half of the silver has separated out in crystals; from the mother-liquor the rest comes down promptly on application of a water-bath heat. The rationale of the process is that the sulphite hardly acts upon the dissolved oxide of silver, but it reduces some of the oxide of copper, 2CuO, to Cu 2 O, with formation of sulphate SO 4 (NH 4 ) 2 . This Cu 2 O deoxidizes its equivalent of Ag 2 O, forming Ag+Cu 2 O 2 , which latter is reduced by the stock of sulphite and reconverted into Cu 2 O which now acts upon a fresh equivalent of Ag 2 O; and so on to the end. Pure silver (ingot) has a beautiful white colour and lustre; it is almost as plastic as pure gold, and, like it, very soft. It does not tarnish in natural air; but in air contaminated with ever so little sulphuretted hydrogen it gradually draws a black film of sulphide. The specific gravity of the frozen metal is 10∙42 to 10∙51, rising to 10∙57 after compression under a die. It is the best conductor of heat and electricity. The expansion of unit length from 0° to 100° C. is 0∙001936 (Fizeau). The specific heat is 0∙0570 (Regnault), 0∙0559 (Bunsen). It fuses at 954° C. (Violle)— i.e., far below the fusing point of copper or gold—without oxidation, unless it be in contact with a surface of silicate (porcelain glaze, &c.), when a trace of silicate of Ag 2 O is produced. It volatilizes appreciably at a full red heat; in the oxyhydrogen flame it boils, with formation of a blue vapour. The fused metal readily absorbs oxygen gas (under fused nitre as much as twenty times its volume—Gay-Lussac). When the oxygenated metal freezes the absorbed gas goes off suddenly at the temperature of solidification, and, by forcing its way through the solid crust produces volcanic eruptions of metal which are sometimes very beautiful. The presence of even very little base metal in the silver prevents this “spitting,” the base metal combining with the oxygen faster than it can be reabsorbed. Pure silver retains a trace of the absorbed oxygen permanently, and Dumas in an experiment on one kilogramme of metal extracted from it 82 milligrammes of oxygen in an absolute vacuum at 400°–500° C. Water, and ordinary non-oxidizing aqueous acids generally, do not attack silver in the least, hydrochloric acid excepted,—which, in the presence of air, dissolves the metal very slowly as chloride. A solution of common salt acts similarly, the liberated sodium becoming NaOH. Aqueous hydriodic acid, even in the absence of air, dissolves silver perceptibly, with evolution of hydrogen (Deville). Aqueous nitric acid dissolves the metal readily as nitrate; hot vitriol converts it into a magma of crystalline sulphate, with evolution of sulphurous acid. Silver is absolutely proof against the action of caustic alkali leys, and almost so against that of fused caustic alkalies even in the presence of air. It ranks in this respect next to gold, and is much used to make vessels for chemical operations involving the use of fused caustic potash or soda. The ordinary “fine” metal is good enough for this purpose.

Silver Alloys. —Pure silver is too soft to make durable coins or vessels combining lightness with stability of form. This defect can be cured by alloying it with a little copper. All ordinary “silver” articles consist of such alloys. The proportion of silver in these (their “fineness”) is habitually stated in parts of real silver per 1000 parts of alloy. In Great Britain all silver coins are made of “standard silver,” the fineness of which, by legal definition, is 925. The toleration is 4 units (of pure silver in 1000 of alloy), i.e., a specimen passes as long as its fineness lies between 925 and 921 (compare Mint, vol. xvi. p. 483). As regards silver-plate the “Hall” in London refuses to stamp any poorer alloy. In Germany and in the United States all silver coins, in France and Austria the major silver coins, are of the fineness 900, with a toleration of 3 units. The minor coins of Austria are of the fineness 375 to 520; in France all silver coins under one franc contain 835 of silver, 93 of copper, and 72 of zinc in 1000 parts. The fineness prescribed by law or custom for “silver” articles is 950 or 800 (±5) in France, 750 in North Germany, 812∙5 in South Germany, and 820 in Austria. All these alloys at least are liable to “liquation,” which means that, although they are perfectly homogeneous in the crucible, they freeze into layers of not absolutely the same composition. According to Levai, passing from the skin to the core of an ingot of 900 per mille silver the difference may amount to 3 units. Of all the alloys tried by that chemist only that composed according to the formula Ag 3 Cu 2 , corresponding to 719 per mille of silver, remained perfectly homogeneous on freezing. He therefore recommends this alloy for coinage; unfortunately, however, any silver-copper alloy which contains less than about 750 per mille of noble metal tarnishes very perceptibly in the air. British standard silver is quite free of this defect, but it is inconveniently soft, far softer than the “900” alloy.

The extent to which the properties of silver are modified by addition of copper depends on the fineness of the alloy produced. The addition of even three parts of copper to one of silver does not quite obliterate the whiteness of the noble metal. According to Kamarsch the relative abrasion suffered by silver coins of the degrees of fineness named is as follows :— [table]

The same observer established the following relation between fineness p and specific gravity in coins containing from 375 to 875 of silver per 1000 :—sp. gr. = 0∙001647 p + 8∙833.

The fusing points of all copper-silver alloys lies below that of pure copper; that of British standard silver is lower than even that of pure silver. For the alloys of silver with other metals than copper, see Gold, Platinum, and Nickel. The present writer has introduced an alloy of 91 of silver, 7 of gold, and 2 of nickel as a material far superior, on account of its higher rigidity, to fine silver for the making of alkali-proof vessels.

“Oxidized” silver is ordinary cupriferous silver superficially modified by immersion into sulphide of sodium solution (which produces a dark film of sulphide), or otherwise.

Silvering.— For the production of a silver coating on a base-metallic object we have chiefly two methods. One of these is to dissolve silver in mercury and to apply this amalgam to the (carefully cleaned) surface of the object by means of a brush. The mercury then is driven away by heat, when a coherent film of silver remains, which adheres very firmly, is quite continuous, and needs not be thick to stand polishing and other surface treatment. This very old method is to this day the best for producing a strong coating, but it is dangerous to the health of the workmen, expensive, and troublesome, and has been almost superseded by the modern process of electroplating (see Electro-Metallurgy, vol. viii. p. 116). Objects made of iron or steel must first be coated over with copper, and then treated as if they consisted of that metal.

For Glass-Silvering, see Mirror, vol. xvi. p. 500.

Inscriptions on linen, consisting of black metallic silver and consequently proof against all ordinary processes of washing, can be produced by using suitably-contrived silver solutions as inks. A mere solution of nitrate of silver (1 to 8 of water) will do, if the surface to which it is applied has been prepared by impregnation with a solution of 6 parts of soda crystals and 17 of gum arabic in 30 of water, and subsequent ironing. The ink must be applied with a quill or gold pen (compare vol. xiii. p. 81).

Silver Compounds. —(1) Nitrate of Silver ( AgNO 3 ) is made by dissolving fine silver in a moderate excess of nitric acid of 1∙2 sp. gr., applying heat at the end. The solution on cooling deposits crystals—very readily if somewhat strongly acid. Even a slightly cupriferous solution deposits pure or almost pure crystals. Any admixture of copper in these can be removed by fusing the dry crystals, when the copper salt only is reduced to black oxide of copper insoluble in water and thus removable, or by boiling the solution with a little pure oxide of silver (Ag 2 O), which precipitates the CuO and takes its place. Nitrate of silver forms colourless transparent sonorous plates, which, if free of organic matter, remain unchanged in the light,—which agent readily produces black metallic silver if organic matter be in contact with the salt or its solution. One hundred parts of water dissolve, of nitrate of silver— [table]

The solution is neutral to litmus. The salt dissolves in 4 parts of cold alcohol. Nitrate of silver fuses at 198° C. into a thin colourless liquid, which stands even higher temperatures without decomposition. At a red heat it is reduced to metal. The fused salt, cast into the form of quill-sized sticks, is used in surgery as a cauterizing agent...