| Acetate of Silver | + | Nitric Acid | |
| = | Nitrate of Silver | + | Acetic Acid. |
As a rule, it will be better to avoid adding Acetate of Silver to the Bath, since with, pure melted Nitrate of Silver no Nitric Acid can be present, and perfect intensity is easily obtained. When the Bath is saturated with Acetate of Silver, it is in a more reducible state, and hence unless the glass plates are very perfectly cleaned, black lines and markings, the results of irregular action, will be produced on the application of the developer to the film ([p. 104]). Solarization, or reddening by over-exposure, is also promoted by the presence of Acetate of Silver.
Developing solutions for Negatives.—The Protosalts of Iron are not usually employed in developing Negative impressions. They are liable to yield a violet-coloured image, which cannot easily be rendered more intense by continuing the action.
Gallic Acid is too feeble for developing Collodion pictures. Pyrogallic Acid is much superior, and may be used of any strength, according to the effect desired. When the light is bad, the temperature low, and the Negative developes slowly and appears blue and inky by transmitted light, the proportion of the reducing agent should be increased. But with an intense Collodion, on a clear summer's day, the finest gradation is obtained with a weak solution, which does not begin to act until the plate has been evenly covered. A strong developer might in such, a case produce too much opacity in the highest lights, and would probably occasion stains of irregular reduction.
Modes of strengthening a finished impression which is too feeble to be used as a Negative.—The ordinary plan of pushing the development cannot be applied with advantage after the picture has been washed and dried. In that case, if it is found to be too feeble to print well, its intensity may be increased by one of the following methods.—
It must be premised however, that the same degree of excellence is not to be expected in a Negative Photograph which has been improperly developed in the first instance and more especially if the exposure to light was too short. Any "instantaneous Positive" may be rendered sufficiently intense for a Negative, but in that case the shadows are almost invariably imperfect.
1. Treatment of the image with Sulphuretted Hydrogen or Hydrosulphate of Ammonia.—The object is to convert the metallic Silver into Sulphuret of Silver, and if this could be done it would be of service. The mere application of an Alkaline Sulphuret has however but little effect upon the image, excepting to darken its surface and destroy the Positive appearance by reflected light; the structure of the metallic deposit being too dense to admit of the Sulphur reaching its interior.
Professor Donny ('Photographic Journal,' vol. i.) proposes to obviate this by first converting the image into the white Salt of Mercury and Silver by the application of Bichloride of Mercury, and afterwards treating it with solution of Sulphuretted Hydrogen or Hydrosulphate of Ammonia. Negatives produced in this way are of a brown-yellow colour by transmitted light, and opaque to chemical rays to an extent which would not, à priori, have been anticipated.
2. MM. Barreswil and Davanne's process.—The image is converted into Iodide of Silver by treating it with a saturated solution of Iodine in water. It is then washed—to remove the excess of Iodine,—exposed to the light, and a portion of the ordinary developing solution, mixed with Nitrate of Silver, poured over it. The changes which ensue are precisely the same as those already described; the whole object of the process being to bring the metallic surface back again into the condition of Iodide of Silver modified by light, that the developing action may be commenced afresh, and more Silver deposited from the Nitrate in the usual way.