THE PHOTOGRAPHIC NEWS. VOL. X. No. 406.--June 15, 1866. p.276
WE have to bring before photographers another aid to permanency, which has, within the last few days, been brought under our attention by Mr. Spiller. Perhaps the most abiding photographic care which rests on the mind of every one of our readers, professional or amateur, is that arising from misgivings as to the permanency of his pictures. Hence everything which tends to extend their tenure of stability is listened to with eager interest. The recent pro position of Dr. Angus Smith, to employ peroxide of hydrogen for the elimination of the final traces of hyposulphites from prints, is one of the suggestions which, possessing undoubted value, and coming from a high source, is seized with laudable avidity, and will, if found as efficient as some anticipate, be generally adopted by conscientious photographers.
In order, however, to avoid the formation of exaggerated hopes, or the indulgence of unfounded notions of security, it may be well to enquire to what extent the elimination of hyposulphites from prints will increase their permanency, and, before referring to Mr. Spiller's suggestion, again to glance at the causes of fading.
The familiar impression on the minds of many photographers is, that fading is due, in some way, to the action of the hyposulphite, and that its imperfect removal from the print is the chief cause of the evil. This is, however, by no means the only source of fading, nor, in our conviction, by any means the most prevalent. The chief sources of fading inherent in the constitution of albumenized silver prints appear to be four in number; they are :--
1st. The formation of sulphide of silver whilst the print is in the fixing bath.
2nd. Imperfect fixation.
3rd. Imperfect washing.
4th. The existence of a compound of albumen and silver in the whites of the print, which is not removed by the fixing bath.
First.--By far the most common cause of fading is, we believe, the formation of a minute portion of sulphide of silver in the process of fixing; or, in other words, a slight amount of sulphur toning. Any acid left in the print during toning will readily bring about the decomposition of the hyposulphite, by which this will be effected; the action of light and heat upon a bath, in any degree approaching saturation with hyposulphite of silver, and some other causes, will also effect a similar end, in which the hyposulphite is decomposed, throwing down sulphur, and leaving sulphurous acid in solution, the result on the print being the formation of sulphide of silver. The tendency to this decomposition is so great, that nothing but the most watchful and zealous attention will serve to prevent it. The fact that, when this takes place in slight degree, the print, when fixed, does not appear to have suffered, but rather to have gained in beauty, having a richer and intenser black or purple tone, leads the photographer to suspect no evil. This black sulphide of silver, which communicates the additional depth, has been shown by Messrs. Davanne and Girard to have a strong tendency to pass into the yellow sulphide of silver, communicating to the blacks of the print a disagreeable olive green tint, and to the whites and delicate half-tones the unpleasant cheesy hue so well known in faded prints. When once formed, there is no method, that we know of, which will re store the print. The means of preventing the formation of sulphide of silver during fixing consist in thorough washing of the print before fixing, using the hypo solution fresh and strong, shielding it from the action of light and heat, and in the addition of an alkali, to which we shall presently advert.
Second .--Imperfect Fixing is a source less of fading than of the formation of yellow spots, made apparent very,, soon after the finishing of the print. If the hyposulphite solution be weak or exhausted; if, with the ordinary strength, the temperature be very low; if the prints are stuck together; only partially immersed; not kept moving; too soon re moved; or if air-bubbles form and protect small portions of the surface, imperfect fixation is apt to result. To fix the print properly, there must be sufficient hyposulphite present to convert all the chloride of silver into hyposulphite . of silver, and to furnish two equivalents of hyposulphite of soda besides, to form the soluble double salt. .If only sufficient hyposulphite of soda were present to convert the silver salt into hyposulphite of silver, it would decompose almost immediately, and pass into sulphide of silver. Theoretically, three equivalents of hyposulphite of soda are sufficient to form the soluble double salt, necessary in perfect fixation, with one equivalent of chloride of silver; but it would scarcely be safe in practice to work to this standard. It is difficult to state, with any definiteness, the proportion of hyposulphite of soda which should be used for fixing a given number of prints, inasmuch as the quantity of chloride of silver present varies with the proportion of salt used in the preparation of the paper It has been assumed that one ounce of hyposulphite of soda dissolved in five ounces of water would fix about three sheets of paper; but in practice it would be wiser never to exceed two sheets of paper with that quantity, observing, all other proper precautions as to perfect immersion, &c. Imperfectly fixed prints, however, generally show spots or stains arising' from the decomposition of the hyposulphite of silver, either in the washing water or shortly after they are mounted; and these defects ought not to be regarded as constituting fading.
Third.--Imperfect Washing will of course prove a source of fading. When a print is not perfectly washed after fixing, it contains traces of the double hyposulphite of soda and silver, and of excess of hyposulphite of soda, and these, remaining in the print in contact with silver, are constantly liable to decomposition, from which sulphide of silver would result. It is here that the modes of eliminating the hyposulphites, recently proposed, by converting them into sulphates, become available. Dr. Angus Smith proposes to submit the washed print to dilute peroxide of hydrogen, which readily gives up its excess of oxygen to the hyposulphites, and changes them into sulphates, which are less liable to decomposition. Mr. Hart proposed, a couple of years ago, to effect the same change by other and cheaper means. So far as we see at present, there is no reason why the use of chlorine water or hypochlorite of soda should not answer well. This result, so far as it goes, is a good one, as it undoubtedly removes one of the sources of fading; but it does not remove all. It leaves sulphate of silver in minute quantity still in the print, and it will not remedy cases of imperfect fixation or sulphuretting in previous stages of the work.
Fourth.--The most persistent source of fading in albumenized prints has been the formation of an insoluble compound of silver and albumen which remained in the whites of the print unremoved by the fixing salt. However perfectly all the operations were performed, this defect remained. If the print were perfectly fixed, and no traces of sulphide of silver were formed in the process, and if the washing was of the most perfect kind, a very fair tenure of permanency, indeed, might be anticipated for the print; and well preserved photographs may be pointed out, which have changed little, if at all, in ten or a dozen years. We have photographs in our possession which we printed over a dozen years ago, which are apparently unchanged; but we have others, done since, with no less care, which have changed. So long as this compound of silver with albumen, a substance containing sulphur, exists unremoved, it is liable to a decomposition, which will set up the fading action, which consists in the whites being degraded by turning yellow, and the blacks losing force by turning yellow. To Mr. Spiller is due the discovery of this existence of silver in the whites of the albumenized prints, and to him photographers are indebted for the only systematic attempts to find a solvent for the compound. His former attempts will be found duly recorded in our pages.
We are happy to announce that his recent efforts have met with higher promise of success than he has hitherto attained. At the last meeting of the North London Photographic Association, Mr. Spiller read a paper the whole of which we commend to the careful consideration of our readers. But it is especially to this branch of his subject we now call attention. Be exhibited portions of a fixed print treated with peroxide of hydrogen, the whites of which were then brought into contact with sulphide of ammonium. This substance practically anticipated the decomposition which time, and other agents to which the print might be submitted, could bring about, and a marked brownish yellow patch of sulphide of silver was the result, showing that the print was by no means removed from the risk of fading. He also showed a portion of the same print, which, after fixation, had been submitted to a second bath of hyposulphite of soda and carbonate of ammonia, and then treated with sulphide of ammonium. In this case the amount of silver present was perceptibly smaller, and the discolouration produced much slighter, a slight yellowness, not deeper than the tint on India paper, being the result.
Since then we learn that Mr. Spiller has carried his experiments further with very marked success. He finds that, on adding one pint of a cold saturated solution of carbonate of ammonia to each pound of hyposulphite used in the fixing ,solution, and immersing the prints for the usual time, almost all the silver is removed from the whites of the picture, a very slight discolouration indeed being apparent on the application of sulphide of ammonium. This is not the only advantage gained. When carbonate of ammonia is added to the fixing bath, the risk is materially decreased, if not entirely removed, of the decomposition arising to which we have referred, by which sulphur toning is set up, and thus two of the chief sources of fading are largely removed by the operation.
Mr. Spiller is still pursuing the experiments. The minimum proportion of carbonate of ammonia which will effect the purpose remains to be ascertained. A quarter of a pint of saturated solution to a pound of hypo did not answer so well as a pint of the same solution, but a pint and a half did the work no more effectually than a pint. It is probable, Mr. Spiller thinks, that about half a pint of the saturated solution to a pound of hyposulphite of soda may answer the purpose. A saturated solution of carbonate of ammonia contains about thirty per cent. of the salt, and its price is, we believe, about double that of hyposulphite of soda.* It will be seen, therefore, that the increased cost of this additional security against fading would be equivalent, stating it in round numbers, to something like an increase of three halfpence on each pound of hyposulphite of soda used.
One word in conclusion. It has sometimes been argued that, in a finished print treated in the ordinary manner, the traces of unreduced silver of any kind left in a print must be so infinitesimally small that little injury can result from its decomposition. Let it be remembered, however, that the photographer has to consider rather potency than quantity. A very minute trace of sulphide of silver is sufficient to destroy the beauty and affect the permanency of a print. Nothing can, perhaps, secure absolute stability in a picture formed of a thin layer of a metal having such an affinity for sulphur as silver, especially in the atmosphere of cities, in which sulphurous gases are common. But it is worth while, so long as silver printing is practised, to use every appliance for removing the sources of fading from the print itself. Mr. Spiller's suggestions will materially affect this in one direction, as will also that of Dr. Smith, or of Mr. Hart--whichever may-be found efficient in practice--in another; and until carbon supersedes silver, which it is probable it shortly will, no aid to permanency should be neglected.
* Carbonate of ammonia is well known to have a variable composition, commonly supposed to be a sesqui-carbonate with water--
2NH4O, 3CO2 + 3Aq;
but when sublimed, the first and last products are never identical, So at the commercial salt will present variable degrees of solubility; these will however, its between the limits of 30 parts in 100 of water at 60° F. (Berzelius), and 33 parts (Ure). Exposed to air, it becomes changed to bicarbonate, and falls to a dry white powder. It costs eightpence per pound.