This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Sensitizing Chemicals. Bichromate of potassium, bichromate of ammonium and bichromate of sodium are all used for sensitizing carbon tissue, the first named being most generally employed. It should be purchased as chemically pure as possible, the cheaper or commercial brands being often too acid to react properly. It is usual to make the bath of 2% strength for summer use and from 3 to 5% strength for winter use. Various other chemicals are frequently employed to give greater sensitiveness or increased contrasts, and for summer use or in hot climates the addition of alum, salicylic acid or other astringents is recommended to harden or toughen the gelatin film.
Drying Of Tissue. Sensitized paper must be dried in a darkened room or closet which is entirely free from noxious fumes, dust, etc. The time it takes to dry has considerable influence on its sensitiveness; the quicker it dries the more sensitive it is. Sensitized in the regular way, it should dry in not less than five hours, or more than eight. A recently introduced "spirit sensitizer" is applied by brushing it over the surface of the tissue, and then the paper dries in ten to twenty minutes. This is a great convenience to the worker who has to get out his prints rapidly. (See Chapter XII (Elementary Tank Development), page 111.)
Theory Of Process. When sensitized carbon tissue is placed under a negative in the printing-frame and exposed to the action of light, those portions of the tissue which receive light are altered and rendered insoluble in water, in inverse ratio to the amount of light they receive. The highest points of light-those under the densest parts of the negative-barely receive any light; consequently the gelatin film is almost wholly soluble. On the other hand, the portions of the print under the clear glass parts of the negative-the shadows-receiving the full strength of the light, will become almost totally insoluble. Between these two extremes the insolubility of the tissue will depend entirely upon the range of gradation and the density of the various parts of the negative.
103. The light strikes through the tissue to varying depths, according to the varying densities of the negative. If a cross section of the print could be shown enlarged, it would be seen that the layers of gelatin which have become insoluble are of varying depths, looking like the cross section of a mountainous country, showing hills and depressions.
104. As all parts of the tissue are slightly exposed to the action of the light when under a negative, it follows that there will be a layer of insoluble gelatin on top of the print immediately under the negative film. Were this print now placed in water, the image could not be developed but the soluble parts of the tissue underneath the top layer would dissolve out, leaving the top layer without any support, with the result that there would be no print. The picture, therefore, is really underneath the top layer of tissue, not on the surface as with other papers.
105. To reach the picture so that it may be developed out, some means must be employed of supporting the top of the tissue-the insoluble part-while the lower or original paper support is withdrawn, thus exposing the pigment, the superfluous parts of which may then be washed away leaving the print itself intact.
106. This is effected by what is termed "single transfer." A piece of gelatinized paper is squeegeed to the surface of the exposed tissue while under water. The paper backing of the tissue is then removed, leaving the pigmented gelatin adhering to the surface of the second sheet of paper. The picture is not yet visible, but on subjecting the tissue to the action of hot water, the soluble parts of the gelatin and pigment are carried away, leaving the print, which, of course, is now reversed; i. e., presents a picture such as you see of yourself in a looking glass.
107. A reversed picture is of little value for architectural and record work, but would be immaterial in pictorial or landscape work. Therefore, this single transfer picture is frequently used by amateurs and others who desire pictures for exhibition purposes or where a reversal of the image is of no importance.