THE PHOTOGRAPHIC NEWS. Vol. 7, No. 236, March 13, 1863. p.126



Read at a meeting of the South London Photographic Society, March 12th, 1863.

THAT I may not be considered presumptuous, or actuated by motives of vanity, in addressing those who are much more practically conversant with every branch of photography than I am myself, allow me to assure you that I should not have come thus prominently forward had I not been solicited to write a paper' for one of our meetings. I have complied with that request, because I consider that every one who becomes a member of our society virtually binds himself to do all in his power to aid the others in their endeavour to acquire knowledge. however humble may be his own abilities ; for, though by reading a paper he may not himself be able to impart any information, or even to place his subject in a new point of view, he may, perchance, give rise to a discussion in which many important facts may be elicited.

The subject which was proposed to me was PRINTING; and it is, without doubt, the most important branch of the art, inasmuch as, without it, the others would be comparatively useless, for we should be unable to disseminate throughout the world the beautiful results which photography is capable of producing.

Although we are able to produce splendid prints by very different formulae, still, it is an undeniable fact, that we absolutely know nothing whatever respecting the theory of positive printing upon albumenized paper-.; even the philosophy of the action of the preliminary sensitizing bath is but little understood; and thus many erroneous notions are prevalent respecting it. Not. withstanding I am not conceited enough to imagine that I know any more of the subject than yourselves, it is nevertheless the one to which I invite your attention in order to raise a discussion upon a few points that require elucidation, believing that the illumination of even a farthing rushlight is preferable to total darkness.

Unfortunately for the progress of science, the majority of mankind have no individual opinions of their own, but accept those of others, without caring to take the trouble of ascertaining whether the opinions they adopt are facts or fallacies. It is, I believe, this universal acceptance of several fallacies as facts, that has so much retarded the advancement of our knowledge respecting the philosophy of positive printing upon albumenized paper. A fallacy is like the fabled hydra; if you cut off one head, another springs up to supply its place; and like slander, it lives long and travels far and wide. Should we flatter ourselves that we have killed it in England, lo! it makes its appearance in France; when we imagine we have destroyed it in France, we find it resuscitated in America; and when we hug ourselves with the idea that it is deprived of life in America, we are startled by its appearing again in England, endued with new life and vigour. -

On a sheet of paper, which has been albumenized for photo. graphic purposes, we have a superficial layer of dried albumen in conjunction with a salting chloride, this dried chlorided albumen being soluble in water. Notwithstanding I have written so much in the NEWS, endeavouring to expose the fallacy of the belief that dried albumen can be coagulated, ii is necessary to say something about it here, not only in order to have a comprehensive view of our subject, but also in order to clear away much error which exists amongst photographers and scientific men respecting it. Any one who is conversant with the various branches of science must be aware that there is much looseness of expression in giving a name to various phenomena.

It is from this looseness of expression that so much error has arisen from using the word "coagulation." In making use of this term, we should bear in mind that it was originally given as a name for the phenomenon which takes place upon the application of heat to albumen in its normal state; and was intended to signify the production of a semi-solid opaline mass. As other agents were afterwards found to produce the same apparent effect, and insolubility was an invariable accompaniment, the expressions "insoluble albumen" and "coagulated albumen," came to be considered as synonymous when applied to the effect produced by these various agents upon normal albumen. Upon the erroneous supposition that what would render albumen insoluble when applied to it in its normal state, would also do so in its dried state--the term "coagulation" came to be mis-applied to dried albumen; losing sight of the important fact that fluidity of the substance to be coagulated is an absolute necessity. It happens, however, that several agents which will render albumen insoluble when applied to it in its normal state, will not do so when applied to it in its dried state; heat and alcohol are the only two of them which it is necessary to mention here.

Notwithstanding the assertions and opinions of many photographers and scientific men to the contrary, I unhesitatingly affirm that these agents (heat and alcohol) cannot render dried albumen insoluble; and, moreover, we have no means whatever of rendering it so; we may form an insoluble compound by chemical combination in various ways, but it is then no longer albumen; and whatever means we employ to do so before sensitizing, will, I believe, render it unfit for photographic printing. I think there can be very little doubt that the term coagulation, as synonymous with insolubility, was applied to dried albumen by photographers before it was recognised that it entered into chemical combination with nitrate of silver; but now, that such is known to be a fact, to say that nitrate of silver coagulates the dried albumen is erroneous in every way. In the first place, as there is no fluidity, there can be no coagulation (according to the original meaning of the term); in the second place, the expression cannot with any correctness imply that the dried albumen has been rendered insoluble, for it is the new chemical compound which has been formed that is insoluble (and to which has been given the name of albuminate of silver) and not the dried albumen per se.

The coagulation of albumen by heat, and its (so called) coagulation by chemical agents, are two separate and distinct phenomena; but photographers un-scientifically apply to them the same term. In what way heat coagulates albumen chemists have not yet determined; but no additional substance enters into combination, with it; it is simply the same albumen in another state; it is not so, however, when chemical agents are employed to produce insolubility; the generality of them enter into combination with it, and an insoluble precipitate is formed, which is not albumen alone, but a compound substance, and no one has any right to mis-call it coagulated albumen. I have said the generality of these chemical agents enter into combination with albumen, because J believe that alcohol simply unites with the water of the albumen, and causes an insoluble precipitate to be formed, with which it does not enter into combination; this may, perhaps, be the case with the coagulating acids, as it is not yet satisfactorily determined whether they form any definite compound with albumen; it is, however, otherwise with such metallic salts as nitrate of silver--here a direct chemical combination takes place. I have also said, we have no means whatever of rendering dried albumen insoluble; for I maintain that when normal albumen is coagulated by any agents, without their entering into chemical combination with it, these same agents, as they cannot coagulate dried albumen, do not render insoluble, the insolubility of the normal albumen being due to its coagulation.

From what I have now stated I leave you to judge 'whether it be not want of knowledge which can alone induce any one to use the term coagulation in any way whatever when applied to dried albumen.

I need, perhaps, scarcely say to those who have paid any attention to the subject, that this fallacious notion of dried albumen being capable of coagulation, even by heat, is very prevalent, notwithstanding it is at variance with the dictates of common sense. In a pretty extensive and varied course of reading, I have never yet met with any author, be he photographer or chemist, who has ever stated the impossibility of so coagulating it; the most that any of them has advanced is, that it is not coagulated up to a certain temperature.

Mr. Hardwich, in the last edition of his Manual of Photographic Chemistry, leads his readers to suppose it to be a possibility. -At page 470 he says:--"A layer of dried albumen cannot easily be coagulated by the mere application of heat." Now, we all know that when a person says a certain thing cannot easily be done, the inference to be drawn from his statement is, that it can be done with difficulty. Again, in speaking of albumenized paper, at page 371, he says :--" Some have recommended to press it with a heated iron, in order to coagulate the layer of albumen upon the surface; but this precaution is unnecessary, since the coagulation is perfectly effected by the nitrate of silver used in sensitizing; and it is doubtful whether a layer of dry albumen admits of coagulation by the simple application of a heated iron." You will, no doubt, have noticed that he does not say the application of the hot iron is useless for coagulation; but, merely, that it is unnecessary, because it will be effected by other means, thus ignoring the beneficial effect produced by its desiccating the paper; and you must be well aware that the statement, "it is doubtful" whether a thing can be done, is, in fact, an avowed acknowledgement that it is not certain it cannot be done.

M. Gaudin, in an article lately published,* " alluding to albumenized paper, says that it will be sufficient to hang it in a cellar, or other damp locality, to coagulate the albumen by the passage of a hot iron. across its surface; and also that at the time of albumenizing if the hot iron be passed over it before it is dry enough to cease sticking to the fingers, the same effect will be produced. Now, this statement is a very great fallacy, for the following reason :-- Whatever moisture is imbibed will be evaporated by the application of the hot iron before it can arrive at sufficient temperature to coagulate the albumen. The same will also be the case with the albumen in its sticky state. -Moreover, as I stated nearly two years ago, no amount of dry heat will coagulate or render insoluble a mere film of albumen, even in its normal state--it simply dries the albumen; therefore, the albumenized surface of a sheet of paper, whether it be rendered damp after being dried, or be taken before the albumen has ceased to be sticky, cannot be rendered insoluble by the application of a hot iron. By the aid of steam and boiling water, the albumenized surface of the paper may perhaps be made to imbibe sufficient moisture for coagulation to take place, but it becomes a question whether this coagulated moist albumen would return to a sufficiently dried state to he fit for photographic printing; but, even if it did so, I believe that this very insolubility would render it almost--if not wholly--incapable of being sensitized by the nitrate of silver bath.

The belief in the fallacy that dried albumen could be coagulated, or even rendered insoluble, naturally induced means to be sought for to accomplish it ; but, its being an impossibility, the beneficial effects produced by them were attributed to a cause which had no existence ; and thus attention was diverted from the real philosophy of their action. The rapidity with which a substance, soluble in water, is dissolved in it, is dependent upon its more or less hydrated state at the time it is submitted to the action of the solvent; thus, the passing a hot iron over the surface of albumenized paper, by desiccating it, renders it more difficult of solution; and, therefore, the nitrate of silver is enabled to be forming an insoluble compound before the solvent power of the water can come much into play.

Unfortunately, in making the sensitizing bath as we do at present, we use a solvent for the nitrate of silver, which is also a solvent of dried albumen; and thus, with respect to the albumenized surface of our paper, we have two antagonistic forces in operation at the same time--one having power to dissolve, and the other to form an insoluble compound. This being the case, we can diminish the power of either of them by increasing that of the other; and therefore it is perfectly possible, by sufficiently increasing the quantity of nitrate of silver, to almost (if not entirely) neutralize the solvent power of the water. The stronger the sensitizing bath the quicker will be its action, and the more will the albumenized surface of the paper be able to maintain its brilliancy; and I believe it to be possible to make the nitrate bath strong enough to render a mere drawing of the paper across its surface quite sufficient to form the albuminate and chloride of silver with the requisite quantity of free nitrate; and, from the solution not having time to penetrate the substance of the paper, that the prints will have all the brilliancy which albumen is capable of affording; and, moreover, that less silver will be wasted than at present, and less washing required; we should also, I believe, then receive the full benefit of colouration which the nitrates of the bases of the salting chlorides would give us. As alcohol is not a solvent of dried albumen its addition to the nitrate bath, by mixing with the water it contains, mitigates its solvent power in proportion to the strength and quantity added; hence the addition of alcohol to the sensitizing bath, and the desiccation of the paper by a hot iron, are beneficial because the solvent action of the water has been decreased by their means, and the albumenized surface is enabled to retain more of its brilliancy than it can do without their employment. It is scarcely necessary, perhaps, to say that alcohol must not be added in sufficient quantity to cause precipitation of the nitrate of silver.

In printing upon albumenized paper, it is usually considered that we have present three compounds of silver; the albuminate, chloride, and nitrate ; and that the chloride is the most important of them. I do not hold this opinion myself, but believe that if the chloride be not deserving of being degraded to the lowest rank, it only holds a secondary one. That the chloride is not of that paramount importance usually attributed to it, is fully evidenced by the beautiful effects produced by the use of low salting formulae. The almost universally accepted theory respecting the production of the image, is, that the action of the light decomposes the chloride of silver, liberating the chlorine; and silver having a greater affinity for chlorine than it has for nitric acid, the silver of the free nitrate combines with this liberated chlorine, setting free in its turn the nitric acid and oxygen with which it had previously been combined; this decomposition and recomposition of the chloride of silver going on during the whole time of printing: the vigour and intensity of the print being due to this constant and continued re-com position of the chloride of silver. According to this theory, the presence of the albuminate of silver is entirely ignored, and the free nitrate--as such--plays no part whatever in the production of the image. Were this the case, we could produce the same result by giving at once the same amount of chloride of silver as this additional re-composition has afforded, and dispensing entirely with any free nitrate; but experience tells us that this will not produce the same effect. I therefore hold the opinion -that the nitrate of silver plays an important part itself in the production of the image: for surely it is too much to ask us to believe that the chloride can only produce a good effect when the nitrate is present; that is to say, that its continued re-production is necessary. The vigour of the print is not, I think, due to the chloride of silver, but to the albuminate; to which is also due the richness of tone. So little is understood of the action of a solution of nitrate of silver upon dried albumen, that it has not yet been ascertained whether that which we term albuminate of silver, be a combination of the silver salt and albumen, or, a mixture of two compounds ; one consisting of the acid of the salt and albumen, and the other of the base of the salt and albumen, Dr. Löwig says :-- If to a solution of albumen we add a solution of metallic salts, as sulphate of copper, nitrate of silver, bichloride of mercury, &c., precipitates are formed, which consist of albumen-metaloxyd and of the compound of albumen with the acids. The latter can be removed with water, whilst the albumen-metaloxyd remains undissolved."

I have said--" In printing upon albumenized paper it is usually considered that we have present three compounds of silver: the albuminate, chloride, and nitrate ;" hut I believe this idea rests only on assumption, as it has not been ascertained as a fact, whether sensitizing the dried chlorided albumen really produces two separate and distinct compounds of silver, the albuminate and chloride; or whether a double compound be not formed which, for distinction's sake, I will call a chlor-albuminate. The Abbé Pujo makes what I consider an almost astounding assertion respecting the formation of the image upon sensitized albumenized paper. He says ;** " A proof when taken from the printing Ira me, is formed of three superimposed images--the first formed by the albuminate, the second by the chloride, and the third by the nitrate of silver." Supposing the chlorided albumen to be simply a solution of the chloride in albumen, without any chemical combination, we have still an intimate commixture of the two substances; and surely the new compounds formed by sensitizing will also be intimately blended with each other; but, if they be not so, what proof can be given to warrant the Abbé's assertion, that the albuminate will form the lowest stratum.

(To be continued.)