c. Moisture as a cause of fading.—Although. Photographs properly printed are not readily injured by damp air ([p. 153]), yet as there are impurities of various kinds constantly floating in the atmosphere, a state of comparative dryness may be said to be essential to the preservation of all Photographs. In collecting evidence upon the subject, "wet" and "damp" are frequently alleged as having been causes of fading—the prints were hung against a damp wall during frosty weather, in a room without a fire: or the rain had been allowed to penetrate the frame! No pictures will long survive such treatment, and Photographs, like engravings and water-colour paintings, require common care to be exercised in their preservation.
d. The modes of Mounting the Proof.—This subject has been alluded to in the abstract of the Author's papers at [p. 155]. All cements which are of an acid nature, or which are liable to become sour by acetous fermentation, should be avoided. Flour paste is especially injurious, and many cases of fading have been traced to this cause. The addition of Bichloride of Mercury, which is often made to prevent the paste from becoming mouldy, would still more unfit it for Photographic use ([p. 151]). Starch is not much preferable. No substance appears better than Gelatine, which does not readily decompose, and shows no tendency to absorb atmospheric moisture. The deliquescent nature of many bodies is a point which should be borne in mind in mounting Photographs, and hence the use of a salt like Carbonate of Potash, which the writer has known to be added to paste to prevent the formation of acid, would be unadvisable.
e. The effect of Imperfect Fixation.—The earlier Photographers did not always succeed in properly fixing their prints, since old Photographs are often found thickly studded with spots and blotches in the tissue of the paper. These prints however are not invariably faded upon the surface, and hence it cannot be said that imperfect fixation will certainly end in the total destruction of the picture. Still a notice of the subject may properly be introduced in this place, and the attention of the reader be once more drawn to the importance of washing the print in water on removing it from the printing frame; a decomposition invariably occurring when paper Positives saturated with free Nitrate of Silver are plunged in a dilute solution of Hyposulphite of Soda, containing an insufficient quantity of the salt to dissolve away the Hyposulphite of Silver before it begins to undergo spontaneous change.
f. Exposure to an impure Atmosphere as a cause of Fading.—The five causes of fading which precede, have mostly reference to an intrinsically faulty condition of the print. This, the sixth, explains the mode in which a Photograph carefully prepared may yet suffer injury from deleterious matters often present in the atmosphere. The air of large cities, and particularly that emanating from sewers and drains, contains Sulphuretted Hydrogen, and hence articles of silver-plate become tarnished unless placed beneath glass. The injury which a print sustains by exposure to air contaminated with Sulphuretted Hydrogen, is less than the tarnish produced upon the bright surface of a silver plate (see [p. 148]); but it is recommended as a precautionary measure, that Photographic Pictures be protected by glass or kept in a portfolio, and that they be not exposed too freely to the air.
The products of the combustion of coal-gas are probably more likely than the cause last named, to be a source of injury to Photographs suspended without any covering. The sulphur compounds in gas burn into Sulphurous and Sulphuric Acids, the latter of which, in combination with Ammonia, produces the sparkling crystals often observed upon the shop windows.
The question as to the manner in which the Photographic Image may best be protected from these extraneous causes of fading has been mooted, and many plans of coating prints with some impervious material have been devised. If the pictures are to be glazed or kept in a portfolio, this of itself will be sufficient, but in other cases it may perhaps be useful to apply a layer of spirit or gutta-percha varnish. The use of wax, resin, and such bodies is likely, by introducing impurities, to act injuriously rather than otherwise.
g. Decomposition of Pyroxyline a source of Injury to Collodion Photographs.—Collodion Positives and Negatives are usually esteemed permanent; but some have been exhibited which, having been put away in a damp place, gradually became pale and indistinct. The change commences at rough edges and isolated points, leaving the centre, as a rule, the last affected. On examination, numerous cracks are often visible, thus seeming to indicate that the Collodion film has undergone decomposition. The result of this would be the liberation of corrosive Oxides of Nitrogen, which destroy the image. Substitution compounds containing Peroxide of Nitrogen are known to be liable to spontaneous change. The bitter resin produced by acting upon white sugar with Nitro-Sulphuric Acid, if not kept perfectly dry, will sometimes evolve enough gas to destroy the cork of the bottle in which it is kept; the solution of the resin has then a strong acid reaction, and rapidly fades an ordinary Positive Print.
These facts are interesting, and indicate that Collodion Pictures, containing in themselves the elements of their destruction, should be protected from moisture by a coating of varnish.
Comparative Permanence of Photographic Prints.—There is every reason to think that the Photographic Image, however formed, is permanent, if certain injurious conditions are avoided;—in other words, that prints do not necessarily fade, in the same manner as fugitive colours, by a simple exposure to light and air. But supposing a case, which is the common one, of injurious influences which cannot altogether be removed, it may be useful to inquire what mode of printing gives the greatest amount of stability.
Positives produced by a short exposure to light and subsequent development with Gallic Acid, may be expected to be more permanent than ordinary sun-prints; not that there is any reason to suppose the chemical composition of a developed image to be peculiar, but that the use of the Gallic Acid enables us to increase the intensity of the red picture first formed, and to add to its stability by precipitating fresh Silver upon it. This point has not always been attended to. It has been recommended to remove the print from the developing solution whilst in the red and early stage of development, and to produce the dark tones subsequently by means of gold; but this plan, although giving very good results as regards colour and gradation of tone, appears to lessen the advantage which would otherwise accrue from the adoption of a Negative process, and to leave the picture, as regards permanency, much in the condition of an ordinary print obtained by direct action of light.