Fixing the image.—Hyposulphite of Soda (one ounce to four of water) will be found preferable to the Cyanide of Potassium, as the latter has a solvent effect upon the Albumen. An unusually long time will be required, as the fixing agent must penetrate the Albumen, to reach the Collodion beneath.
Careful washing in water for five or ten minutes removes the excess of Hyposulphite, and the plate may then be varnished in the usual way.
THE DRY COLLODION PROCESS.
The earlier attempts to employ sensitive Collodion plates in a desiccated condition were unsuccessful. The film of Pyroxyline shrinks on drying, and becomes almost impervious to moisture: hence, the developing solution not penetrating properly, density cannot easily be obtained. We are indebted to Dr. Hill Norris, of Birmingham, for establishing the theory of the subject upon a more correct basis. He has pointed out the importance of distinguishing two different conditions of the Collodion surface,[53] viz. the contractile, common in newly-mixed Collodion,—and the short or powdery, in Collodion which has been iodized with the alkaline Iodides, and kept until much Iodine has been set free. The latter is the most suitable condition for the dry process; and the practical mode of distinguishing between them is by sensitizing a plate and passing the finger across it; if it can be easily pushed away in a firm and connected skin, it will be unfit for the purpose required. In order still further to preserve the film in a condition permeable by the developer, it is recommended to coat it whilst moist with a solution of Gelatine.
[53] See these states of the film more fully described at [page 83].
The dry Collodion process, although less sensitive, is more simple than that on Collodio-Albumen, and possesses many of its advantages; but it is less universally applicable, since it depends entirely for success upon the peculiar state of the Collodion, resembling in this respect the Oxymel process already described.
Mode of preparing the plates.—The glasses are coated with the Collodion in the usual way. Blistering during development being liable to happen in this process as in the last, every care must be taken to make the films adhere with the greatest possible tenacity, both by cleaning the glasses with extra care (see [p. 294]), and also by allowing the Collodion to set firmly before dipping in the Bath. The plate may be held from twenty to thirty seconds previous to immersion, or even longer, provided the film, when lifted out of the Bath, appears of uniform thickness throughout (see [page 218]).
The sensitizing having been completed, wash the plates with plain water, exactly in the same way as for Oxymel ([p. 292]). If Nitrate of Silver be left, clouding will take place in the process of development. After washing, drain for a few seconds, and immerse in the solution of Gelatine.
To prepare this Bath, take of
| Nelson's patent Gelatine | 128 | grains. |
| Distilled water | 14 | ounces. |
| Alcohol | 2½ | ounces. |