So far as the mere application of the gelatine solution is concerned, the process is simple enough, and varies not at all, whether the negative be one of collodion or gelatine; but, actually, the treatment required by the negatives themselves depends upon the nature of the image bearing film. In the case of collodion negatives, absolutely no previous preparation is required, as the collodion itself assists the dried gelatine film to "strip." Where, however, the plates are prepared by the operator himself, and are intended for stripping, adopt the practice of polishing the glass with talc previous to coating, even when it is not adopted for other reasons, as absolute immunity from accident is thus secured. The negative having been levelled, and covered with the bi-chromatised gelatine, is allowed to set thoroughly, and is then placed in an airy situation to become thoroughly dry. It is advisable not to resort to artificial heat, as it causes a tendency, when even a temperature far below that required to liquefy the film has been employed, to unevenness of the film after drying and insolation.
It is desirable, also, that the plates be screened from strong light during drying, for the same reason, as otherwise the portions first dry become partly insoluble oefore the proper exposure is made, with the result that after stripping the film is apt to "cockle" on the slightest provocation.
After thorough desiccation, the negative is exposed to light for a sufficient time to thoroughly harden the whole thickness of gelatine. If sunlight be available, a few minutes will suffice, but as no harm arises from extending the exposure, it is as well to give enough. In diffused light, and during the winter season especially, two or three hours at least should be given, or even a whole day, the negative being placed in a printing frame and backed with white paper, which will reflect back the light that passes through the film. It should be turned . once or twice during exposure, in order that both sides may be "attacked."
After exposure, the negative is placed to soak in clean water, preferably cold, or at least at the normal temperature of, say, not more than 60° F., though the gelatine is now quite insoluble even in boiling water. But there is a chance, with increase of temperature, of" reticulation," or a species of "frilling," from the gelatine partly leaving the glass during washing. This is more especially the case with gelatine negatives than with collodion. At first the washing water will be strongly tinged with bichromate, and mast be changed at intervals until it remains colourless, when the whole of the soluble chromic salt may be considered to be eliminated. The transparent portions of the film will now be of a bright yellowish-brown colour," somewhat resembling a badly "pyro-stained" gelatine negative. To remove this objectionable tint, the plate is next soaked in a weak acid bath - either hydrochloric or sulphuric - of the strength of 1 oz. of the acid to 1 pint of water, in which it is allowed to remain until as free from colour as possible.
A better decoloriser is aqueous sulphurous acid, but it is an unpleasant solution to use indoors. Those who care to venture may secure the benefit of the sulphurous acid by adding the hydrochloric acid to a 10- or 20-gr. solution of sodium sulphite or bisulphite instead of to plain water; but this part of the proceeding should be performed out of doors, certainly not in' the dark room.
The acid bath, if properly and sufficiently applied, will render the transparent portions of the film practically colourless, though a faint tinge may remain of yellow in the case of hydrochloric or sulphuric acid, or smoky green if sulphurous acid be employed. When this stage is reached, nothing remains but to wash the negative in three or four changes of water, and then to drain and dry it. When thoroughly dry, the point of a sharp knife is passed round the edges of the film, one corner is raised, and the whole film strips easily from the glass, forming, if the proper quantity of gelatine has been used, a stout, tough, and flexible film, perfectly flat, and in every way equal to glass for the purpose.
The only difference in the treatment required by gelatine negatives consists in the preliminary preparation by immersion in a 5-gr. solution of chrome alum acidified with sulphuric acid. This serves the double purpose of hardening and loosening the film from the glass, though it is not absolutely necessary - at least, so far as the loosening is concerned, as the subsequent acid bath suffices to effect this. But it is a sine qua non that the film be rendered thoroughly insoluble. After draining, a little of the bichromatised gelatine is poured into the sink; the negative is placed on the level, coated with the stripping solution, and, in fact, treated in precisely the same manner as already described. When it comes to the operation of stripping, the only difference will be that the film clings somewhat more tenaciously to the glass than is the case with collodion. It is also proportionately thicker than a stripped collodion film. If any difficulty should be experienced in stripping, a bath of very dilute hydrofluoric acid, as recommended by Plener, would no doubt settle matters.
It is, of course, a matter of necessity that the paper film should have been prepared for stripping, otherwise the adherence of the gelatine to the paper will be of such a tenacious character as to resist all treatment. But, given a properly prepared film, the action of the bichromatised gelatine, applied in the ordinary manner, would not have time or opportunity to penetrate the negative film, and reach the underlying layer of soluble gelatine. The only risk of failure lies in the chance of the soluble layer being hardened, and so cementing the image to the temporary support.