The Photographic News. Vol. 7, No. 320. January 30, 1863. p.53
THE employment of albumen as an additional sizing for positive paper, in connection with the action it exercises upon nitrate of silver, and the modification which may result in the proofs, is a subject that very properly engages the attention of the photographer.
We have been much gratified at seeing that the researches made subsequent to those which we presented to the French Photographic Society, in December, 1859, have confirmed what we then said upon this subject. The Abbé Pujo has recently called the attention of photographers to the combination which takes place between albumen and nitrate of silver, a combination but little studied hitherto, but upon which we have made some experiments. M. Roussin has lately shown that this combination alone, and well washed, is sensitive to light; and that with it we have even been able to obtain positives on paper. He has also obtained negatives upon glass developed with gallic acid. Mr. Spiller has also confirmed this fact, which we had stated, that albumenized papers also retain silver in the parts unexposed to light, even after fixing and the moat careful washing. We should doubtless have preferred that these authors had known and mentioned our previous researches; but, as we have referred to them,. we need not enter upon further details.
The combination of albumen with nitrate of silver varies according to the strength of the silver bath. Thus, in treating 5 drachms of albumen with 5 drachms of nitrate of silver, we obtained very different precipitates, according as these 5 drachms of silver were dissolved so as to form solutions of 15, 10, 5, 2½, or 1 per cent. With the solutions of 10 and 15 per cent., the albumen is strongly coagulated; it gives a heavy, abundant precipitate, which may be collected on the filter in the form of distinctly separate pellicles.
With weak solutions of silver, the quantity of the precipitate is much less considerable. A notable quantity of albumen remains in the bath in a state of solution. The portion precipitated is soft, gelatinous, and sticky. A rapid analysis seems to prove that the compound of albumen and nitrate of silver is much poorer in albumen when it has been formed in presence of a more dilute solution of silver, which readily explains, by this cause alone, that the albumen, less strongly coagulated, is carried off by the washings.
In view of these facts, we can easily understand that a weak silver bath allows a portion of the albumen to be dissolved without coagulating; it thus becomes charged with organic matters which alter it; and the proof, much less rich in the argento-organic compound, does not acquire that brilliancy and vigour of tone which is due principally to this compound.
This combination of albumen and nitrate of silver is analogous to that which it forms with bichloride of mercury. It is insoluble in water, and sensitive to light even after perfect washing. When the light is intense, the compound rapidly assumes a red colour, and soon bronzes; but a feeble diffused light produces no perceptible colour; and this explains why papers strongly albumenized, and which are not chlorodized in proportion, yield, under the same negative, and by the same light, proofs in which the contrasts are harsh, and the details in the shadows less distinct. Under the influence of light, therefore, a decomposition takes place, nitric acid is separated, while a portion of the albumen remains combined with the silver. This latter combination is insoluble in hyposulphite of soda, and the total of these facts gives us the theory of the formation of positive or negative albumenized proofs, even in the absence of every other salt of silver; but it remains to be explained why the best reserved whites of the positive proofs, after fixing and washing, still retain silver if these proofs have been taken upon albumenized paper, while they retain none if taken on paper simply salted.
Some experiments enable us to explain this anomaly. We have found that the quantities of silver thus retained upon white sheets of paper, and well fixed, vary in considerable proportion.
Whatever care may be taken even by fixing the compound of albumen and silver immediately after sensitizing, we have always found a certain quantity of silver which the fixing agent has not removed, and which is probably owing to the formation of a little sulphide of silver. Albumen is, in fact, a sulphurized body, which most frequently, in contact with hyposulphite of soda, disengages a very strong odour. The quantity of silver which thus passes to the state of sulphide is very inconsiderable. It is this which imparts to the whites of our proofs a light tone which perhaps can only be appreciated when these whites are in direct contact with the margins. But there are other circumstances which cause the quantities of silver retained in the albumenized papers to vary; these are the preservation of the paper and the action of diffused light.
For while a whole sheet sensitized and immediately fixed gave, upon analysis, only a quantity of silver equal to .010, that which had been prepared the day before, fixed under the same conditions, gave 0.020.
Lastly, another sensitized sheet, dried, and exposed for a few moments to diffused light, although it did not become discoloured, contained, after fixing, 0.016 of silver. The experiment, several times repeated, has always given, with some variations in the proportions, analogous results.
We have therefore to conclude
Now we know that a good negative must not have absolute blacks; the light can then always influence the largest whites of the photographic proofs, all the parts of which must necessarily contain silver.
But we do not think that the presence of this minimum quantity of silver can have the least influence upon the keeping of these proofs; and we all know that a great many proofs have existed a good number of years and have not undergone any appreciable change.
In connection with this practical question, the proof of the persistence with which the e silver remains allied to the albumen throws some theoretical light on the subject.
We now understand that albumenized negatives may, up to a certain point, be developed without the addition of nitrate of silver, because the albumen has energetically retained a certain quantity, while non-albumenized and well washed negatives can be made to appear only after the addition of the necessary quantity of nitrate of silver.
And, as diffused light causes a portion of this compound of silver and albumen to pass to the insoluble state in the hyposulphite of soda, we can easily explain what appears abnormal in the experiments of Mr. Young, where the proof was first fixed in the dark in hyposulphite of soda; then, after proper washing, developed in full light by gallic acid cum nitrate of silver.
This experiment succeeds only with albumen negatives, and after a good exposure. The preceding facts give a very simple explanation, as in these circumstances an argentico-albumen compound is produced, which the hyposulphite of soda does not remove, and upon which the developing agents cannot act.--Bulletin de la Société Francais de Photographie.