It is plain from data thus collected, that Photographs do not necessarily fade by time; and the fact that in one and the same portfolio are constantly seen prints which appear permanent, and others in an advanced state of change, cannot but lead to the inference that the main causes of deterioration are intrinsic, depending upon some injurious matters left in the paper; which is confirmed by experiment.
Causes of fading.—The Author believes that the fading of Photographic Prints may almost invariably be referred to one or other of the following conditions:—
a. Imperfect washing.—This is perhaps the most important of all, and the most frequent. When Hyposulphite of Soda is allowed to remain in the paper, even in minute quantity, it gradually decomposes, with liberation of Sulphur, and destroys the print in the same way and quite as effectually as a solution of Sulphuretted Hydrogen or an alkaline sulphuret.
Imperfect washing may be suspected, if the Photograph, within a few months from the date of its preparation, begins to get darker in colour: the half-tints, which are the first to show the action, afterwards passing into the yellow stage, whilst the dark shadows remain black or brown for a longer time.
The proper mode of washing Photographs is sometimes misunderstood. The length of time during which the print lies in the water is a point of less importance, than that the water should be continually changed. When a number of Positives are placed together in a pan, and a tap turned upon them, the circulation of fluid does not necessarily extend to the bottom. This is proved by the addition of a little colouring matter, which shows that the stream flows actively above, but at the lower part of the vessel, and between the prints, there is a stationary layer of water which is of little use in washing out the Hyposulphite. Care should therefore be taken that the pictures are kept as far as possible separate from each other, and when running water cannot be had, that they are frequently moved and turned over, fresh water being constantly added. When this is done, and especially if the size be removed from the paper in the manner presently to be advised, four or five hours washing will be sufficient. It is a mistake to allow the pictures to remain in the water for several days; which produces no good effect, and may tend to encourage a putrefactive fermentation, or the formation of a white deposit upon the image when the water contains Carbonate of Lime.
b. Acid matters left in the Paper.—Upon examining collections of old Photographs, it is not uncommon to find prints which are stated to have remained unaltered for a long time after their first production, but in the course of time to have lost their brilliancy, and become pale and indistinct. This kind of fading often commences at the corners and edges of the paper, and works inwards towards the centre. The Author's experiments have shown that it is principally caused by a slow process of oxidation.
The Photographic Image does not appear readily susceptible of oxidation unless it be previously darkened by the action of Sulphur, or placed in contact with acids or bodies which act as solvents of Oxide of Silver ([p. 146]). The materials often used in sizing papers, such as Alum and Resin, being of an acid nature, are directly injurious to the image; and the removal of the size, which may easily be effected by means of a dilute alkali or an alkaline carbonate, without injury to the tint, has the additional advantage of carrying out the last traces of Hyposulphite of Soda, and also the germs of fungi, which if allowed to remain would vegetate and produce a destructive mouldiness on exposure to damp (Chap. III. Part II.).
The fact that acids facilitate oxidation of the image is likewise a hint that Photographic Prints should not be handled too frequently, or touched with the finger more than is necessary; the warm hand may leave behind a trace of acid[29] which would tend in time to produce a yellow mark.
[29] The writer has seen blue litmus-paper immediately reddened by being laid upon the arm of a person suffering from acute Rheumatism. This acid is probably Lactic Acid!