THE BRITISH JOURNAL OF PHOTOGRAPHY. June 15, 1884, p.207.
[SILLIMAN'S AMERICAN JOURNAL OF SCIENCE.]
EVERYTHING connected with the permanence of photographic products has an interest so vital to photography that nothing connected with it can be considered as trivial. We are yet groping in the dark as to the causes of failure. "Sulphuration" is a convenient word; but it would be more satisfactory if we had some idea as to the nature of the obnoxious insoluble sulphur compound. The hypothesis which has for some time past become current is that the fading depends upon the presence in the print of some sulphur compound, which, with time, acts upon the silver, converting it, as is said, into sulphide. Sulphide of silver we know as a jet black substance, and we habitually convert the reduced silver of negatives into sulphide when we wish to intensify them. Why, then, should the production of sulphide of silver strengthen a negative and destroy a print? MM. Davanne and Girard answer as follows:--Sulphide of silver prepared by itself is, they say, black; but, formed in presence of organic matter, the organic matter enters into the composition, and the compound is yellow. To prove this they precipitate a solution of silver with sulphuretted hydrogen, and find it violet black. They again precipitate the same solution in presence of starch, and obtain a yellow precipitate. I have paid much attention to the subject of the fading of prints, and had been forcibly struck by the anomaly above referred to. The explanation of the gentlemen just mentioned seemed very satisfactory; but the question appeared to have too much importance to pass it by without verification. The following results were Obtained:--
In a test-tube. was placed a little boiled starch, in a second tube some water. Two or three drops of a five-per-cent. solution of nitrate of silver were added to each, and then hydrosulphate of ammonia. Both solutions gave a yellowish-brown precipitate without the slightest difference in shade or colour. The precipitate afforded by hydrosulphate, of ammonia could not here differ from that produced by sulphuretted hydrogen; but to remove any doubt on this score, the experiment was carefully repeated with sulphuretted hydrogen. The same result precisely was obtained. As a further confirmation, the experiment was varied by the substitution of another organic substance, viz., collodion, instead of the starch. The result was precisely similar.
The different result obtained by MM. Davanne and Girard can perhaps be explained as follows:--The sulphide of silver is a heavy substance, and when thrown down from a simple watery solution it quickly subsides. But solutions thickened with organic matter, such as gum, starch, gelatine, &c., retain a precipitate for a long time in a state of suspension, thus preserving the original yellowish-brown appearance. If, therefore, we prepare solutions as above, and pass H S through them, allowing them to subside, and examine them only after a time, we shall find in the one case a black precipitate below a colourless solution; in the other a brownish viscid liquid. But if we watch the process from the outset, we shall see that the reactions are chemically identical, and differ only in respect to the mechanical suspension which takes place. in the one case. The experiment which I here cite may throw some light on the origin of the yellow colour. It appears that when the sulphide of silver is in a state of very fine division its colour is yellowish brown, as may be easily ascertained by treating a very dilute solution (e.g., 1-5,000th) of nitrate of silver with hydrosulphate of ammonia. Many substances are only black in consequence of excessive intensity of colour. Lampblack, for example in a state of excessively fine division is yellowish brown. Ink diluted is purple. Claus has shown that the intensely black hydrated sesquioxide of ruthenium is, when very finely divided, green.
I therefore conclude:--1st. That it is as yet not absolutely demonstrated (although probable) that the current opinion, ascribing the fading of pictures to the production of sulphide of silver, is correct. 2nd. That if this opinion be correct, there exists no evidence that organic compounds have anything to do with the production of a yellow colour; the tint of sulphide of silver found in their presence and in their absence being quite the same shade of yellow-brown.
M. CAREY LEA.