The desensitizing of color plates has been dealt with elsewhere and the same process may be utilized for all screen-plate work. With the separate method, the saf-ranin dyes may be used; but for the autochrome plate the makers advise the use of the ammonium salt of au-rantia, in 1: 2000 solution, for one minute, as this does not delay the appearance of the image nor prolong development as does safranin.
The original developer recommended for the auto-chrome plate and applicable to all other combined plates was pyro-ammonia, and it is still considered the best by many workers. The following solutions and method are advised:
A. Sodium bisulphite solution 3 ccm.
Pyrogallol 26 g.
Potassium bromide 26 g.
Water to 1000 ccm.
B. Sodium sulphite, dry 100 g.
Ammonia, sp. gr. 0.880 100 ccm.
Water to 1000 ccm.
As the particular ammonia solution advised above is not always available, in America at least, one-fourth more of the weaker solution, of specific gravity 0.9, should be used. For use, dilute one part of this B solution with three parts of water. The above solutions are mixed in the following proportions:
Solution A 1 part.
Solution B diluted 1 part,
Water 8 parts.
The duration of development is dependent on the time of appearance of the first sign of the image, exclusive of the sky in landscape work; and the temperature of the solution should be 160 C. (6o° F.). The temperature is important, as any material increase will make the gelatine soft, and trouble may be caused. The duration of development for other temperatures may be calculated from the following table, which gives the factors by which the total time of development should be multiplied to obtain the same result:
10° C. (500 F.) multiply by 1.6
160 C. (61° F.) multiply by 1.0
200 C. (68° F.) multiply by 0.8
25° C (770 F.) multiply by 0.6
The developer is modified according to the appearance of the image, more of the dilute B solution being added in proportion to the delay in its appearance. The following table shows the additional quantities of dilute B solution that should be added to every 100 ccm:
Time of appearance in seconds
Add diluted B solution
Total lime of development in minutes and seconds
Almost any developing agent may be used, the following, however, in the author's opinion, being one of the most satisfactory, as it gives brilliant results without fog:
Metol 4 g.
Hydrochinon 12 g.
Sodium sulphite, dry 50 g.
Potassium bromide 6 g.
Ammonia, sp. gr. 0.92 33 ccm.
Water to 1000 ccm.
Rodinal, amidol, adurol, metoquinon and chloranol have all been suggested, but as they present no particular advantages, adherence to one of the above is advised. In the separate method, in which a panchromatic plate is used, one would naturally adopt the normal developer, such as metol-hydrochinon. With these plates, ammonia should be avoided as far as possible, and it is assumed that the worker knows how to develop such plates. There is only one caution that need be given, that development should not be prolonged until the results are hard and contrasty; they should tend to the side of softness, and for those who follow the directions given on the cards enclosed with the plates, the times for "portraits" should be adopted. Freedom from fog, as far as possible, is desirable, but if the desensitizing method be adopted this need not be feared.
The preliminary development of the combined plate, as has already been explained, gives a negative image in the complementary colors, and to convert this into a positive this primary image is dissolved. This is effected by treatment with acid potassium permanganate or acid bichromate solution, and the latter is on the whole preferable, as the stock solution keeps better and there is no chance of black spots being caused, which may happen with old permanganate solution. The latter should be prepared in two solutions, and the permanganate should be kept in the dark:
A. Potassium permanganate 4 g Distilled water 1000 ccm.
B. Sulphuric acid, pure 20 ccm Distilled water to 1000 ccm.
Mix in equal volumes immediately before use. The bichromate bath is:
Potassium or ammonium bichromate 5 g Sulphuric acid, pure 10 ccm.
Water to 1000 ccm.
As soon as development is complete, the developer should be poured off and the dish filled up with water and gently rocked for a few seconds. The water is then poured off and the dish again filled with water; this can be repeated three or four times until the plate has been washed for from thirty to forty-five seconds, and then the reversing solution can be applied. Plenty should be used, and the action should be allowed to continue for two or three minutes; then one can turn on white light or go out into daylight. The reversing bath should be allowed to act not less than five minutes, and it is here that so many beginners go wrong, in not giving the solution time enough to dissolve the primary image. The author prefers to throw the first lot of solution away and pour on fresh and allow to act another three minutes. It is not wise to be economical, either in time or solution, at this point.
As soon as the primary negative image is dissolved, the picture in colors will be seen on looking through the plate, as well as the still unreduced silver bromide film. It has been suggested that the plate, after treatment with the reversing solution, should be washed and dried, but while this method saves a little time and trouble it should not be adopted, as the color rendering is never as good.
This has the purpose of reducing to the metallic state the emulsion which has not been affected by the primary exposure and development. This reduced silver prevents any light from penetrating through the color elements where it should not, and increases the brilliancy of the picture.