About the same time Mr. Talbot, Sir John Herschel, Dr. Fife, and myself, discovered the very remarkable property of the iodides in bleaching the darkened salts of silver. Many very beautiful results may be thus obtained. The manipulatory details published by Dr. Fife were simple in their character, but arrived at by a long series of inquiries. It is now quite easy to prepare photographic papers, on which the iodine solutions shall act with perfect uniformity :—
Soak the paper for a few minutes in phosphate or muriate of soda, removing with a soft brush any air bubbles which may form on it. The superfluous moisture must be wiped off with very clean cotton cloths, and the papers dried at common temperatures. When dry, the paper must be pinned out on a board, and the silver solution spread over it, boldly but lightly, with a very soft sponge brush. It is to be instantly exposed to sunshine, and, if practicable, carried into the open air, as the more speedily evaporation proceeds the less does the silver penetrate the paper, and the more delicate it is. The first surface is very irregular, being as before described, and represented in fig. 2. As soon as the surface appears dry, the silver solution must be again applied as before, and the exposure repeated. It must now be exposed until a fine chocolate-brown colour is produced equally on all parts of the surface, and then, until required for use, be carefully preserved from the further influence of light. If the paper is to be kept long, the darkening must not be allowed to proceed so far as when it is to be speedily made use of.
In darkening these papers, the greatest possible attention must be paid to the quantity of light to which they are submitted, everthing depending on the rapidity of the blackening process. The morning sun should be chosen, for the reasons before stated. A perfectly cloudless sky is of great advantage. The injurious consequence of a cloud obscuring the sun during the last darkening process, is the formation of a surface which has the appearance of being washed with a dirty brush. This is with difficulty removed by the iodides, and the resulting pictures want that clearness which constitutes their beauty. Papers darkened by the diffused light of a cloudy day are scarcely, if at all, acted on by these salts. Great care must be taken to prevent the silver solution from flowing over the edges of the paper, as thereby an extra quantity of darkened silver is formed on both sides, which requires a long-continued action of the iodides and sunshine to bleach.
The kind of paper on which the silver is spread is an object of much importance. A paper known to stationers as satin post, double-glazed, bearing the mark of J. Whatman, Turkey Mill, is decidedly superior to every other kind I have tried. The dark specks which abound in some sorts of paper must be avoided, and the spots made by flies very carefully guarded against. These are of small consequence during the darkening process, but when the bleaching wash is applied, they form centres of chemical action, and the whitening process goes on around them, independently of light, deforming the drawing with small rings, which are continually extending their diameters.
The saline washes may be considerably varied, and combined to an indefinite extent, with a continued change of effect, which is singularly interesting. In their application we should be guided, as in the negative process, by their combining proportions. The following list of the salts which will give the best effects, selected from upwards of seven hundred combinations, will show the variety of colours produced. They are placed in the order of the sensitiveness they appear to maintain, when used as nearly as possible under the same circumstances :—
Muriate of Ammonia. . Red, changing to black in the sunshine.
Chloride of Sodium... Ditto, ditto.
Chloride of Strontium A fine brown.
Chloride of Barium ...A rich brown, inclining to purple.
Sol. Chloride of Lime Very red.
Sol. Chloride of Soda A brick red.
Iodide of Potassium. ... Yellowish brown.
Chloride of Potas- Variable, sometimes yellowish, often a sium.................. steel blue.
Phosphate of Soda.....Mouse colour.
Tartrate of Soda........Dark brown.
Urate of Soda...........Yellowish brown.
Chloride of Iron.......Deep brown, which blackens.
Bromide of Sodium.....Red brown of a peculiarly rich tint.
The change mentioned in the colour of the finished picture is that which arises from a fresh exposure to the solar rays ; where no change is mentioned, it is too slight to be worth notice.
When papers prepared with any of the above, except the phosphates, are soaked for a little time in water, and dried in the sunshine, the picture produced,—it matters not what iodide is used,—is rendered peculiarly red, and does not change by re-exposure. By washing some of the papers with weak solution of ammonia, this peculiarity is produced in a very striking manner.
In the other divisions will be found some further remarks on the very peculiar physical phenomena presented by the action of the compounds of iodine on these darkened salts of silver, and details of yet more perfect forms of manipulation.