The Products of Combustion of Coal-gas a cause of Fading.—Coal-gas contains Sulphur compounds, which in combustion are oxidized into Sulphurous and Sulphuric Acids; other substances of a deleterious nature may also be present. A plate of polished silver suspended in a glass tube, through which was directed the current of heated air rising from a small gas jet, became tarnished with a white film in the course of twenty-four hours. Positive prints exposed to the same, absorbed moisture and faded; the action resembling that of oxidation, in being preceded by a general darkening in colour. Of four prints exposed, an Iodide-developed print was the least injured, and next, a print upon Albuminized paper.
ON THE ACTION OF DAMP AIR UPON POSITIVE PRINTS.
In order to ascertain this point, more than six dozen Positives, printed on every variety of paper, were mounted in new and perfectly clean stoppered glass bottles, at the bottom of each of which was placed a little distilled water, to keep the contained air always moist. They were removed at the expiration of three months, having been kept during that time, some in the dark, and others exposed to the light. As the prints were prepared by various methods, toned in different ways, and mounted with or without substances likely to exercise a deleterious action, this series of experiments will possess considerable value in determining some of the intrinsic causes of fading of Positives.[27]
[27] For a more detailed account of the experiments, see the original paper in the 'Photographic Journal,' vol. iii.
The general results obtained were as follows:—Positives which had been simply fixed in Hyposulphite of Soda remained quite uninjured. Whether developed by Gallic Acid on either of the three Salts of Silver usually employed, or printed by direct action of light, the result was the same. Hence we may infer that the darkened material which forms the image of Photographic Prints does not readily oxidize in a damp atmosphere.
Toned Positives were found in many cases to be less permanent than Positives simply fixed. This was especially the case when the toning had been effected by Sulphur; all the sulphuretted prints, fixed in solution of Hyposulphite which had been long used, became yellow in the half-tones when exposed to moisture. Positives fixed and toned in Hyposulphite containing Gold were variously affected; some prepared when the solution was in an active state being unchanged, others losing a little half-tone, and others, again, fading badly. These latter were prepared in a Bath which had lost Gold and acquired sulphuretting properties; and it was noticed that they were more injured by the action of boiling water than those Positives which proved to be permanent under the influence of the moisture.
Toning by means of Chloride of Gold appeared to be highly satisfactory, but the number of prints operated upon was small. The Sel d'or process also did not injure the integrity of the image, no commencing yellowness or bleaching of half-tones being visible after exposure to the moist air.
This series of experiments confirmed the statement made in a former paper, that some tints obtained in Positive printing are more permanent than others. Violet tones produced by Sulphur invariably passed into a dull brown by the action of the moist air; and even when Gold was employed in toning, these same purple colours were usually reddened. This was especially the case when English papers were used, or foreign papers re-sized with Serum of Milk containing Caseine. The chocolate-brown tints which best stand the action of boiling water, and in particular those upon Ammonio-Nitrate paper, were least affected by the damp air; and indeed it was evident that the two agents, viz. moist air and hot water, acted alike in tending to redden the print, although the latter did so in the most marked manner.
It seemed also, from the results of these experiments, to be a point of great importance that the size should be removed from the print in order to render it indestructible by damp air. This was evidently seen in two cases where Positives, toned in an old Hyposulphite and Gold Bath, were divided into halves, one of which was treated with a strong solution of Ammonia. The result was that the halves in which the size was allowed to remain, faded, whilst the others were comparatively uninjured. The Albumen proofs especially suffered when the size was left in the paper, a destructive mouldiness forming, and fading the picture. The use of boiling water obviated this, and the prints so treated remained clean and bright. A partial decomposition of Albumen however occurred in some cases even when hot water was used, the gloss disappearing from the paper in isolated patches. With Caseine substituted for Albumen there was also a loss of half-tone; thus seeming to indicate that both these animal principles, although stable under ordinary conditions, will, even when coagulated by Nitrate of Silver, decompose if kept long in a moist state.
The use of improper substances for mounting proved to be another determining cause of fading by oxidation. Those bodies which combine with Oxide of Silver, are likely upon theoretical grounds to destroy the half-tones of the image; and it was found, that if the picture were left in contact with Alum, Acetic Acid, etc., or with the substances which generate an acid by fermentation, such as paste or starch, it invariably faded.