The Photographic News. Vol. 7, No. 231, February 6, 1863. p.63
WITH reference to a former communication which you did me the honour to insert in the PHOTOGRAPHIC NEWS (vol. vi. p.470), I beg leave to request that you will permit me an opportunity of resuming the subject, and of describing some of the results met with in a course of experiments directed to the removal of traces of silver from the whites of albumenized prints. The difficulties to be overcome appear greater, as one chemical agent after another is tried, without accomplishing the desired result; and the object of my addressing to you the present memorandum, is rather to report progress, than to record complete success.
I have already stated, that a second immersion of the print in a fresh solution of hyposulphite of soda, is capable of removing a small proportion of the insoluble silver com pound. and that this is the case also when a treatment with iodide of potassium is resorted to. Since the date of this announcement, I have tried the action of a great number of soluble salts, particularly those of ammonia, many of which are distinguished for their power of dissolving the ordinary silver precipitates. Amongst other salts, were the carbonate, acetate, citrate, and phosphate of ammonia; the alkali itself and phosphoric acid separately; tartaric acid and Rochelle salt. None of these were capable of removing the last traces of silver, and there was so little advantage gained by their employment as auxiliaries to the fixing bath, that I have been led to try the cautious use of cyanide of potassium. By immersing the prints in very dilute solutions of the cyanide, I have succeeded certainly in removing every trace of silver from the whites, but always at the expense of the shaded portions of the picture, or so readily soluble are the metals, silver and gold, when in this extremely fine state of division, that it will, I believe, be practically impossible to limit the solvent action in the manner intended. By operating upon over-printed proofs, on albumenized paper, I obtained pictures which were presentable, but I do not consider that the details were so delicately rendered as in the case of other prints obtained from the same negative by ordinary treatment. The albumen itself, at this stage of the process, does not appear to suffer by immersion in aqueous cyanide of potassium, of the degree of concentration (one grain, or less, in the pint of water), that would be requisite or this particular application.
Terro-cyanide of potassium has the property of dissolving many insoluble compounds of silver, but, according to my experience, is of no value for the purpose at p resent in view.
In your report of the proceedings of the Marseilles Photographic Society, November 8, 1862,* allusion is made to the proposal of M. Messnier, to employ the sulpho-cyanide of ammonium as a fixing agent in photography; and your correspondent further states, that the subject has been referred to a committee, to inquire into, and report upon the chemical value of this suggestion. Pending the decision of the French investigators, it may not be out of place to describe some of the characters of this salt, which have presented themselves in the course of preparing and employing this substance as a fixing agent.
There are three processes available for the preparation of sulpho-cyanide of ammonium : -- Synthetically, by acting upon hydrocyanic acid with excess of yellow sulphide of ammonium, and evaporating to dryness over a water bath. Or, by virtue of double decomposition, on dissolving in a small bulk of water equivalent quantities of sulpho-cyanide of potassium and sulphate of ammonia, and adding alcohol or methylated spirit, when the sulphate of potash is precipitated, the sulpho-cyanide of ammonium remaining in solution. The third plan is but a modification of the second, and consists in fusing together the yellow ferro-cyanide of potassium with half its weight of sulphur, dissolving in water, and treating the crude mixture of the sulpho-cyanides of potassium and iron first with sulphide of ammonium to precipitate the iron, and then with sulphate of ammonia and alcohol as before. It is necessary to take care to exclude the presence of free sulphur and hydro-sulphuretted compounds from the products obtained by the first and last of these methods; for, unless perfectly purified, the salt is apt to discolour the whites of the photograph. The crystals obtained on evaporation are transparent and colourless, but the aqueous solution seems liable to undergo a gradual decomposition accompanied with the production of a yellow or reddish colour. The solution, when mixed with nitrate of silver, gives at the first moment of contact a bulky white precipitate of sulpho-cyanide of silver, which, left to itself, soon assumes a granular character, or becomes distinctly crystalline; this precipitate is, however, readily dissolved by an excess of the ammonium salt, and the addition of a soluble chloride to the solution yields no precipitate. Both the chloride and iodide of silver are to some extent soluble in the sulpho-cyanide of ammonium, but not nearly so freely as in cyanide of potassium or hyposulphite of soda. On testing the power of this salt in removing the traces of silver from the whites of albumen prints, I find that it succeeds perfectly; but, at the same time, it must be confessed that the sulpho-cyanide of ammonium is not likely to become a cheap commercial article, and on this account its general use, as a fixing agent must for the present be seriously impeded.
We learn from the interesting extract from the Bulletin de la Société Francaise de Photographie, which you printed last week, that the eminent French chemists, MM. Davanne and Girard, have resumed their investigation of the compounds formed by the action of nitrate of silver upon albumen. Referring to the announcement I made in your columns on the 3rd October last, they claim the prior publication, in the year 1859, of the fact of silver being contained in the unexposed parts of albumen prints; and, on looking more fully into the matter, I concede to them the merit of having first made this observation; but it must, on the other hand, be allowed that their statement refers to a mode of conducting the fixing and toning processes which is now obsolete, which seldom gave pure whites; and, according to their own description, the silver compound was faintly visible on the paper, and sometimes even was slightly affected by light.** This is certainly not the case with the photographic surfaces I have examined; and, further, I do not coincide in their opinion that the compound under discussion is probably a sulphide," since the very existence of the silver was discovered by the sulphide of ammonium test, and the production of the brown sulphide on moistening the pure whites with this re-agent must surely be the result of a chemical change. If the exposure of a sensitized sheet be delayed until it has commenced to assume a visible discolouration, or the print be fixed in hyposulphite of soda without previously removing the whole of the free nitrate of silver, we obtain a condition of surface, to which MM. Davanne and Girard's description is more nearly applicable. These gentlemen are of opinion, moreover, that the presence of the argento-albumen compound is not likely to affect the permanence of the photograph, and so have not been led to study the means of removing or decomposing the same.Royal Arsenal, Woolwich.
* Vide PHOTOGRAPHIC NEWS vol. vi., p.586.
** Vide PHOTOGRAPHIC NEWS vol. iii. p.233.