It only now remains to describe the method of using the modification in development, which will extend the scale of gradation.
So far nothing is known that will decrease gradation in the development of bromide papers, except the addition of bromide of potassium in large quantities, which at the same time very materially alters the colour of the deposit and is occasionally employed for that purpose ; consequently, if a negative is so thin that it will not give a good black print on gaslight paper, the only remedy is to deal with the negative itself and intensify by means of one of the well-known processes. Hitherto the greatest difficulty in bromide printing has been to obtain good prints from negatives which, having been developed to give considerable contrasts as required for P.O.P., platinum, etc., are too hard for representation by means of the most rapid papers, notwithstanding that they have a long scale of gradation. This difficulty is more especially found in enlarging, because the negative is needed to be so much thinner then than for contact printing.
Experiments upon the action of bichromate of potassium (and other salts) upon the latent image or its development (Photo. Journal, Feb., 1904) showed that by means of weak solutions applied for a short time between exposure and development the scale could be greatly lengthened out, thus extending the detail in the shadows without materially altering the result already obtained in the highlights or changing the colour. The result is, therefore, that whilst the speed of the paper remains practically unaltered, so far as it affects the rendering of the high-lights, and the exposure required is therefore the same, a print on gaslight paper can be made to give a similar result to that obtained upon slow or rapid bromide paper, or even if desired, a longer gradation than either.
Consequently good prints can be obtained upon gaslight papers from almost any negative, except, perhaps, some that have been developed excessively. These may be dealt with by extending the scale of the rapid bromide paper in the same manner.
The method is exceedingly simple, for nothing else is required but the addition of a very small quantity of bichromate of potassium to the washing water before development. The alteration made is dependent upon the strength and time of immersion. The action is so rapid that it is well to keep to one length of time, say, 1 minute, and vary the strength only. Exposure will be the same, but development should be continued until the desired strength of the picture is obtained. The high-lights soon cease to increase much in depth, but the shadows gradually put on strength like the development of a negative plate. This process may be described as the flattening of too great contrasts, and, of course, if overdone will result in too flat a print just as in the case when in the ordinary way the negative is too thin for the paper (Table 4,3). A full description, with examples of the effect obtained were given in Photography, Jan. 30th, 1904, p. 94.
So far, only one make of paper has been found practically unaffected by bichromate of potassium, but, speaking generally, the slower the paper the less strength required to bring about a change.
For gaslight papers 1 part of bichromate of potassium in 500 water will be found to have a very marked effect, and 1 part in 100 will be sufficient for trial upon the bromide papers.
The best way to commence is to take a negative which gives a print, say, upon gaslight paper, in which, when the high-lights are correct, the shadows are black and without detail (Table 4, 1).
Having exposed a piece of the paper under this negative for the proper time for the high-lights, cut it into four strips. Immerse one strip in a bichromate of potassium solution, 1 to 500, for one minute, rinse in changes of water for 15 or 20 seconds altogether, and develop with one of the other pieces. If the difference is not sufficient take another of the strips and place in the same way in the 1 to 100 bichromate of potassium and develop. If the action is now too great and the print too flat altogether, another trial can be made with the fourth strip. When a very great alteration is desired, it may be found necessary somewhat to increase the exposure, because there is a tendency after a time when strong bichromate of potassium is used to practically destroy the weak portions of the image.
An acid fixing bath should be used, as any stain remaining from the bichromate of potassium is removed during the fixing.
The same course is to be carried out when the gradation of the slow and rapid bromide papers is to be extended. Should an excessive alteration be desired, it is most readily accomplished by substituting permanganate of potash, about 1 part in 1000 water, for the bichromate. A deep brown stain may be left on the paper, but this is cleared away in the acid fixing bath.