Towler, John. The Silver Sunbeam. Joseph H. Ladd, New York: 1864. Electronic edition prepared from facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson, New York. Second printing, Feb. 1974. ISBN 871000-005-9

Chapter XIII.
SILVER-SALTS OF SILVER.

Silver.

SILVER, like gold, is found in a native state; frequently too it occurs as an alloy containing gold, which is recognized, when the silver is dissolved in nitric acid, as the black sediment or oxide of gold. Arsenic and antimony are found also alloyed with it. Several of the ores of lead and copper contain silver.

AS an ore, the sulphide is the most abundant; horn silver, or the chloride, occurs native, as also the carbonate in small quantity.

Native silver, and the silver in the native sulphide, are separated in one case from the investing rocky materials, and in the other from sulphur by a process called that of amalgamation. The ores and the rocky mass are reduced to powder, and then roasted in a reverberatory furnace with about ten per cent of chloride of sodium, which converts the silver into chloride of silver. The pulverized mass is next put into barrels, hung horizontally and capable of being rotated by machinery. It is mixed with a certain quantity of water, iron and quicksilver. By being kept in continual agitation for eighteen or twenty hours, the chloride of silver becomes decomposed by the iron, whereby chloride of iron is formed, and the silver set free. Coming in contact with the mercury, an amalgam is formed, which flows off out of the barrel when the contents are made fluid by the addition of water, and by rotating the barrels very slowly. The amalgam is then subjected to pressure through chamois leather, which allows the mercury to permeate through its pores, but retains the amalgam. By distillation, the mercury can be expelled from the silver residue. Copper and lead ores, containing silver, are treated in the same way.

In certain ores of copper and lead, silver exists in small quantities, and is melted or separated by amalgamation along with them. If the quantity is sufficiently great, the silver is separated by a process called cupellation, which is practised in the mint in the assay of metals containing silver. A cupel is formed out of well-burnt and well-washed bone ashes, kneaded into a thick paste with water, and forcibly pressed in an iron ring. Cupels vary in size from one to two inches in diameter or more, and from a quarter of an inch to three fourths of an inch thick, hollowed on one side in the concave form of a watch-glass. They are afterward dried by a gentle heat, as on a stove, when they are ready for use. The metal, consisting of copper, silver and a large excess of lead, to be assayed, or the silver to be purified, is placed in the concavity of the cupel, which rests on a muffle in a furnace, over which a current of air can flow with some force. It soon melts, and by the access of the draft of air, the surface becomes covered with a film of oxide; this, as it forms, is removed. Lead oxidizes first, and finally the copper is induced to oxidize by means of the oxide of lead, and forms with it a fusible compound, which sinks into the pores of the cupel. As soon as the foreign metals are nearly removed, the silver assay assumes a rounder shape, and when the last trace of oxide disappears, there is a beautiful play of prismatic colors, and finally the silver button becomes very brilliant, and exhibits a bright flash of light, indicative of the completion of the operation.

A second process of purifying silver, and one which will be found better adapted to the wants of the photographer, consists in dissolving the silver of commerce, or of the coinage of the country, in pure nitric acid. Take one ounce and a half of silver, in thin famine, or in filings, one fluid ounce of nitric acid, and two ounces of pure rain or distilled water. Mix the acid and the water in a glazed porcelain dish, or in a glass dish; then add the silver., and place the vessel with its contents in a sand-bath, and apply a gentle heat. The silver will soon disappear in the solution. By this operation, the nitric acid is easily broken up into its combinations, one portion oxidizes the silver and liberates peroxide of nitrogen; whilst a second combines with the oxide so formed, and produces the nitrate of the oxide of silver. If the metal was impure, as is most likely, and it contained copper, the solution will be tinged blue according to the quantity of impurity. A small drop at the end of a glass stirring-rod, will give rise to a brilliant blue color, in a wine-glass full of water, made alkaline with ammonia, if there be any copper present; or a steel knitting-needle, dipped in the solution, becomes coated with a film of copper, on the same conditions.

Supposing the solution, therefore, contains copper, we may proceed as follows to separate it from the silver. Add to the solution of the nitrate, a small quantity of common salt dissolved in water, drop by drop, as long as a flocculent precipitate is formed. When flakes of the chloride of silver, thus produced by double decomposition by means of the chloride of sodium, no longer appear on the addition of the salt solution, the precipitate is allowed to subside in a dark room, or it is poured directly on a filter, and the fluid containing copper, etc., is thrown away. The precipitate is now well washed by repeatedly filtering pure hot water over it, until a drop no longer produces a blue tinge with ammonia. The chloride is now dried. Next weigh the chloride, and take twice its weight of carbonate of potassa, and fuse the latter in a crucible; when fused, add gradually to it the dry chloride of silver, which will be decomposed, as well as the carbonate of potassa. The chloride leaves the silver and gives rise to chloride of potassium, whilst the carbonic acid and oxygen escape, and the silver remains diffused through the mass. By raising the temperature, the silver sinks into a button at the bottom, and the fused chloride of potassium swims on the surface. The melted mass may now be poured out into a pail of water, or upon a hollow stone. The silver thus obtained and washed, will be quite free from copper, and all other metals, excepting lead or mercury, which might be present. If lead were present in the nitrate, the addition of sulphuric acid would produce a precipitate; and the presence of mercury is easily shown by introducing a piece of polished copper wire into a small quantity of the nitrate in solution, by which it will be covered with a film of mercury when the latter is present.

Chloride of silver may be reduced, also, by fusing it with seventy per cent of chalk, together with four or five per cent of charcoal.

A third method of reduction of the chloride, is one which is very convenient for those who do not possess a furnace, or have the convenience of fusing ores or residues. Moisten the chloride with dilute hydrochloric acid, and immerse a plate of zinc in the moistened mass for several hours. Decomposition will gradually take place, the silver being deposited, whilst the soluble chloride of zinc is formed. After the chloride has been thus completely decomposed, the remaining zinc is withdrawn, and the precipitate is washed with dilute hydrochloric acid, until there is no longer any precipitate formed in the decanted fluid by means either of ammonia or of sulphide of ammonium. The precipitate is next well washed with warm water. It is now in a condition for being dissolved in nitric acid.

Instead of precipitating the silver as chloride, in order to separate it from the copper, the solution is evaporated to dryness, and then heated nearly to redness. By this process the nitrate of silver is fused, but suffers no other change; whilst the nitrate of copper is decomposed, yielding up peroxide of nitrogen and oxygen, and leaving the insoluble black oxide of copper mixed with the fused silver salt. By dissolving a small portion of the fused mass from time to time in water, and testing the solution, after filtration, with ammonia, it can easily be ascertained whether it be free from copper or not. As soon as no copper is indicated, the fused mass is dissolved in pure water and separated from the insoluble residue, evaporated and crystallized.

The oxide of copper may be separated from the nitrate of copper in the solution by substitution of oxide of silver. This oxide of silver is obtained by precipitating a quantity of the given solution by a solution of potassa. The collected precipitates of oxide of copper and of oxide of silver, are then well washed, and afterward boiled with the remaining parts of the impure nitrate. The solution is then finally separated from the residue, evaporated and crystallized.

Finally, the mixed solution may be treated with plates of copper, whereby the silver is precipitated in a state of very fine division, which is afterward obtained on the filter, and thoroughly purified by washing. This silver is then treated with pure nitric acid until dissolved; the solution s then evaporated to dryness, redissolved, evaporated and crystallized.

In every case where the salt thus obtained is intended for photographic purposes, the crystals when thoroughly dried are dissolved in pure water, and again crystallized; or the solution of the crystals is boiled for some time in a glass flask containing fragments of pure silver, or perfectly well-washed oxide of silver, (procured as just indicated.) In this way the nitrate of silver, after evaporation and crystallization, can be had in an absolute neutral condition.

The mother-liquor remaining after the crystals have been removed, is evaporated to dryness, fused and poured into cylindrical moulds of the size of a quill. In this form it is denominated lunar caustic, and used principally by surgeons for cauterizing erysipelatous, ulcerated, etc., surfaces. From this mode of its manufacture, it can not always be relied upon by the photographer as pure. In fact it frequently blackens by exposure to light, whilst pure crystallized nitrate of silver, does not change by a similar exposure. In addition to impurities of an organic nature, it frequently contains, besides, nitrite of silver, produced by the decomposition of the nitrate by the heat of fusion.

Properties.

Nitrate of silver crystallizes in colorless square tables; it is an anhydrous salt, and neutral when carefully prepared. This salt may be fused, as before mentioned, into lunar caustic; but if the heat be too great, it is decomposed into nitrite of silver, oxygen being liberated; and by a still greater heat the nitric acid is entirely removed, and pure silver left behind. Nitrate of silver dissolves in one part of cold water, and in less of boiling water. It is soluble also in about four parts of alcohol. The oxide of the nitrate of silver, is precipitated by any of the alkalies or alkaline earths. In ammonia, added in excess, the oxide is redissolved, forming a definite compound of the formula AgO, NO₅, 2NH₃, denominated ammonio-nitrate of silver, which by evaporation is obtained in the crystalline form.

Photographic Properties of the Nitrate of Silver.

Collodion iodized with a solution of iodide of silver in iodide of potassium does not produce a picture when exposed and developed by the ordinary process; nor is a collodion film, when sensitized in the bath of nitrate of silver, and carefully washed in the dark-room after the operation of sensitizing, any longer as sensitive to the actinic influence as before; or supposing it to be so, it no longer yields a picture 'by ordinary development. It is, therefore, not the iodide of silver alone which undergoes the actinic impression, but the iodide in connection with the nitrate of silver, or the nitrate of the new base, and probably with free nitric acid, which is easily broken up or decomposed, and yields thus its oxygen to produce or induce further decompositions. Whatever the theory or the true explanation of the photographic impression on the iodides or bromides may be, whether physical, chemical, electrical, or mixed, that is, physico-chemical, etc., one thing as yet is quite certain, (and this is certainly the beginning of knowledge,) that the rationale of actinism on any substance or surface is a mystery, has not been hitherto explained on unexceptional grounds, is not satisfactorily deduced from experiments. It is useless then to give a long dissertation on a mere hypothesis. But we do know, if' not with certainty, at least nearly so, by what conditions the best results can be obtained in reference to the nitrate of silver bath in combination with the iodized or bromo-iodized collodion. For instance, collodion containing, amongst other chemical ingredients, free iodine, indicates at once that the silver-bath may be neutral, even slightly alkaline; whilst if the collodion be new, contain no free iodine or bromine, be colorless, then the bath appropriate for producing a good picture must be the very contrary of the preceding, it must be slightly acid. We know that acids retard the action of development, limit this action to the parts impressed actinically, prevent in consequence what is denominated fogging. We know, moreover, from repeated experiments, that it is immaterial whether the collodion or the silver-bath be slightly acid, the result is the same, the production of a clear picture accompanied with the disadvantage of lengthening the time of action. But we do not yet know the exact conditions of collodion and bath by which clearness and sensitiveness can be attained in a maximum degree in the shortest time without exception.

The iodide of silver, whether produced by the decomposition of iodide of cadmium, of lithium, or of any other base, is, in all probability, equally sensitive; but this sensitiveness is found to be materially changed by the presence of the other salt in the decomposition. From experiments in this direction it is known that the greatest degree of sensitiveness is arrived at when the collodion contains iodide of iron, and this probably because the proto-nitrate of iron is very unstable and easily broken up. With such an iodizer, however, the silver-bath would soon be entirely deteriorated by the continual introduction of a developing material; so that many points have to be taken into consideration before normal conditions can be isolated or legitimate deductions drawn.

Preparation of other Salts of Silver.

Other Salts of Silver--Sulphate of Silver--This salt is obtained by dissolving silver in concentrated sulphuric acid by the aid of heat; or by double decomposition of nitrate of silver with sulphate of soda. Sulphate of silver is soluble in eighty-eight times its weight of boiling water, from which it crystallizes on cooling. Like the nitrate it is anhydrous, and forms in like manner a distinct and definite combination with ammonia, whose equivalent is Ag O. SO₃ + 2 NH, in fine transparent crystals.

Hyposulphite of Silver.--This combination is obtained by the double decomposition of an alkaline hyposulphite and nitrate of silver. For instance, add a dilute solution of hyposulphite of soda to a similar one of nitrate of silver; a white precipitate is formed which is soon dissolved in the menstruum; after a while, when the hyposulphite of soda has dissolved the newly formed precipitate to saturation, a flocculent substance is formed of a dull gray color, which is permanent. This second precipitate is hyposulphite of silver in an isolated state. But the hyposulphite of soda contains a large quantity also, thus giving rise to a soluble double salt, which has a very sweet taste. Hyposulphurous acid has a very powerful affinity for silver, so that hydrochloric acid or a soluble chloride produces no precipitate in the solution of the double salt of hyposulphite of silver and of soda. In such a solution, containing a large proportion of waste silver, the best way to obtain or separate the silver is to pass a current of hydrosulphuric acid through the solution, in order that the silver may be precipitated as sulphide of silver. Hyposulphite of silver undergoes spontaneous decomposition into sulphate and sulphide of silver; on this account the fixing-bath is found to contain in general a large quantity of black sediment, which is sulphide of silver. This sulphide, when a sufficient quantity has been collected, is reduced by heat into sulphurous acid and metallic silver.

Iodide of Silver.--This salt is found native, and sometimes in the form of hexagonal prisms. It may be formed artificially by allowing the vapor of iodine to play upon polished plates of silver, as in the Daguerreotype process, or by double decomposition. When excess of nitrate of silver in solution is added to a solution of iodide of potassium or to hydriodic acid, a yellow precipitate is produced; this is iodide of silver; whereas if the iodide of potassium be in excess, the precipitate is nearly white, its soluble and yellow part having been dissolved by the alkaline iodide. The yellow precipitate is that form of the iodide which is best adapted for photographic purposes. It is insoluble in water and in dilute nitric acid; almost insoluble in ammonia; and is not so soon colored by the action of light as the chloride. It is very soluble in the alkaline iodides, in cyanide of potassium, and hyposulphite of soda, and by evaporation may be crystallized out of them as double iodides, etc. When silver is dissolved in hydriodic acid, crystals of the iodide of silver may be obtained in the solution by spontaneous evaporation. Iodide of silver may be reduced in the same way as the chloride by means of zinc. Hydrochloric acid converts it into chloride of silver. It is decomposed by both chlorine and bromine which liberate iodine. It is soluble to a small extent in solution of nitrate of silver.

Iodide of Silver for the Silver-Bath.--Add to a small quantity of iodide of potassium in solution a larger quantity of dissolved nitrate of silver; allow the canary-yellow colored precipitate to subside; decant the supernatant liquid; wash with water and again decant, and repeat the washing several times. Let this operation be performed in the dark-room. The yellow precipitate, whilst still moist, is added to the bath of nitrate of silver in proper quantity as long as it is dissolved by the same; the solution is then filtered; and as regards saturation with the iodide of silver, is ready for use.

Bromide of Silver.--This salt is found native in Mexico and in Bretagne, sometimes in an amorphous condition, and sometimes crystallized of a greenish-yellow color. It is formed artificially by exposing plates of silver to the vapor of bromine, or by decomposing nitrate of silver by an alkaline, or any other soluble bromide. The precipitate is white at first, but becomes yellow afterward. It may be fused, and when cool its color is intensely yellow. Bromide of silver is very sensitive to light, but the color when so acted upon by light is very different from that of the chloride. It is soluble in strong ammonia and in chloride of ammonium, as also in hyposulphite of soda and cyanide of potassium. The bromides are decomposed by chlorine, whereby bromine is liberated, and may be collected by ether, which, by agitation, collects the bromine and carries it to the surface, from which it may be decanted.

Chloride of Silver.--Next to the nitrate of silver, the chloride is perhaps the most important combination of this metal. It occurs native as horn-silver in translucent cubes or octohedra of a grayish-white color; its specific gravity in the native form is 5.55. Like the iodide and bromide of silver, it may be obtained by exposing plates of silver to the vapor of chlorine. The surface of the plates soon becomes covered with a chalky film, which is the chloride in question. It is obtained as an insoluble white powder by decomposing nitrate of silver, or any other solution of silver excepting the hyposulphite, by means of hydrochloric acid or a soluble chloride, by which a complete interchange takes place, and a dense curdy precipitate falls gradually to the bottom. After subsidence the liquid is poured off, and the residue is well washed in several waters. This operation must be performed in the dark-room, because the chloride of silver is very sensitive to light, and soon changes from a white to a violet color in the sun or in diffused light. This violet-colored substance is a sub-chloride or an oxy-chloride, and may be formed directly by chemical means as follows: dip a plate of polished silver into a solution of sesqui-chloride of iron, or of bichloride of mercury; the surface becomes stained black; the iron or mercury parting with a portion of its chlorine, is reduced to a lower chloride, whilst the silver film becomes converted into a sub-chloride of silver. Chloride of silver is insoluble in water; it is very soluble in ammonia, in cyanide of potassium, in hyposulphite of soda, as also in concentrated and boiling solutions of chloride of potassium, chloride of sodium, and chloride of ammonium, from which may be obtained, by evaporation in one case and by cooling in the other, crystals of double salts of chloride of silver and the other substances in the solvents. Hydrochloric acid in a very concentrated state dissolves a minute quantity of chloride of silver, which crystallizes on evaporation of the acid. It is precipitated from all solutions of silver salts, as before mentioned, except from hyposulphite of silver, by means of hydrochloric acid. At a temperature of 500° Fahr. it fuses into a transparent yellowish fluid, which when cool may be cut with a knife like a piece of horn, and has beside some other resemblance to horn; it hence received the name of horn-silver by the older pharmaceutists. Chloride of silver can not be volatilized like the protochloride of mercury. The mode of its reduction into pure silver by two or three different processes has already been given under the head of Silver. It may be reduced also by a mixture of carbonate of potassa, cane--sugar, or starch--sugar and water.

Tests: Chloride of silver is distinguished from all other precipitates, having the same color, by the property which it possesses, when exposed on a white saucer or evaporating-dish, of becoming changed into a violet-colored substance. Its insolubility in nitric acid, and solubility in ammonia, is also an excellent test when combined with the preceding.

Photographic Properties of Chloride of Silver.

There is quite an analogy in the application of iodide of silver and chloride of silver; the former being essentially in combination with a nitrate or free nitric acid, the sensitive collodion film; whilst the latter, in combination likewise with a nitrate or free nitric acid, forms the sensitive film on gelatine, albumen, arrow-root, resinized, gutta-percha, or plain paper. These papers have first imbibed, or have been invested with, certain soluble chlorides, as of ammonium, sodium, etc., by floating or otherwise, and then dried. By double decomposition afterward these chlorides are converted, by floating the papers on a solution of nitrate of silver, into chloride of silver. Organic salts of silver are formed simultaneously, such as the albuminate, etc., which assist in, or detract from, the photographic operation. Of this I shall speak more extensively when I have to discuss the theory and practice of Positive printing on paper.

Other Uses of Chloride of Silver.--The solution used in galvano-plasty, or electrolysis, for plating with silver is made by dissolving in a saturated solution of cyanide of potassium the moist and undecomposed chloride of silver to saturation, and then diluting this solution by four or five times its bulk of water.

The grayish-colored powder used for dry-plating or for silvering dial-plates, thermometer-scales, etc., consists of one part of chloride of silver, five parts of cream of tartar, and four of common salt, rubbed on with a piece of flannel or sponge dipped in solution of salt.