The time of immersion has also a certain influence on the results. The less the tissue is allowed to absorb the solution the less sensitive it is, but also the more the tendency of the half tints to be washed off during the development. Generally the tissue should remain immersed until it lies flat and the edges just commence to curl up, unless white and black impressions are desired, but even then it is preferable to operate as said above, using a bath at 2 per cent.
For use the bichromate bath should be cooled down to 15 deg. C. (59 deg. Fahr.), and much lower in summer, say 10 deg. C. (50 deg. Fahr.), and kept at about this temperature by placing pieces of ice around the tray. At 20 deg. C. (68 deg. Fahr.) the prints are more or less granulated; above this the gelatine is softened and the reticulation greater; at 25 deg. C. (75 deg. Fahr.) it may dissolve.
The addition of alcohol to the bichromate bath—sometimes recommended to harden the film and allow it to stand a higher temperature, and to hasten the desiccation of the tissue—is objectionable, for the spirits tend to reduce the bichromate, which is transformed into the green salt, and, therefore, a partial or complete insolubilization of the gelatine is the result.
Aqueous ammonia added to the sensitizing solution has for its object to permit one to keep the sensitive tissue for a somewhat longer period, but it renders it less sensitive. If enough be added to turn the solution yellow weak prints are obtained.
The bichromate bath should be renewed often. It does [pg 90] not keep owing to the presence of gelatine and other organic matters which it dissolves and which cause the reduction of the chromic salt even in the dark. The tissue prepared in such a bath is not very sensitive and the image develops with difficulty, and even cannot be developed at all.
As said above, the tissue is well sensitized when its edges commence to curl up. It is then removed from the bath by drawing it on a glass rod fixed at the end of the tray, and placed, prepared side down, on a slightly waxed glass plate, rubbing it with an India rubber squeegee to remove the superflous liquid, when it is hung up to dry.
While wet the bichromated tissue is insensitive; the sensitizing can therefore be made by daylight, but the drying should of course be done in the dark room, that is in a room lighted by a candle or the sunlight filtered through a deep orange window glass.
Caution.—The soluble bichromates are very poisonous. By absorption they produce skin diseases not without danger and very difficult to cure. Hence when handling the wet tissue the fingers should be protected by India rubber tips, and any yellow, stains on the hands should be rubbed with a dilute solution of aqueous ammonia, and the hands well rinsed in water.
Drying.—When the tissue dries rapidly it adheres well on the support upon which it is applied for developing and yields brilliant images which are easily cleared. On the other hand, were it allowed to dry slowly the adherence would not be so complete, the image dull and developing with difficulty. They may even refuse to develop at all from the insolubilization of the gelatine.
In winter and in the cool days of spring and autumn, the gelatine dries quick enough in the air, but when the weather is warm and damp, the gelatine, drying very slowly, may be so softened as to run off, or to produce an entirely objectionable reticulation, or the defects above mentioned. This may be avoided by drying it pinned up in a box, or a closet, over quick-lime.