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.
93. While yet the oldest of all printing processes in present use, the Carbon Process still remains one of the most popular. It has never yet been superseded for permanency and beauty of result, or actual simplicity and cheapness. Yet, with all its simplicity of manipulation, the carbon process requires most careful working and great cleanliness to insure perfect results. Without this care and attention to cleanliness the process is neither easy nor economical, for the least carelessness will lead to spoiled prints and the initial cheapness of material is at once wiped out.
94. Strictly speaking, the carbon process is purely mechanical. Neither the color nor the quality of the finished print is dependent upon the individual will of the worker, as is the case with most other printing processes. A carbon print is either good or bad, there is no half-way measure. But with no other printing process can the worker produce such exquisite range of tone, such registering on paper of all the gradations of the original negative, such depth of transparent shadows, such purity of high-lights.
95. The carbon process is based on the peculiar action of gelatin and certain other organic substances, when treated with an alkaline bichromate solution and dried in the dark. Gelatin alone is soluble in hot water, but gelatin treated as above will become insoluble when exposed to the light. A sheet of paper coated with bichromatized gelatin, dried in the dark and then placed under a negative will faintly register the image on its surface. The high-light portion of the image, being under the dense parts of the negative, will not be altered by the light, consequently will be soluble in hot water, while the half-tones and shadows of the print will be insoluble in varying degrees, according to the density of the deposit on the negative.
96. If pigments be added to the gelatin, they will remain on the paper according to the degree of solubility or insolubility of the gelatin, and in this way an image is formed showing every tone of light and dark as registered inversely on the negative. In the parts of the print unaffected by the light the pigment will be washed away with the dissolved gelatin.
97. For the purpose of giving greater variety, carbon paper is supplied in some thirty different colors, suitable for every kind of subject from portrait to sea views, for ordinary commercial work, or for reproductions of Old Masters. The colors most employed are engraving black, standard brown, red chalk (Bartolozzi), warm black, warm brown, sepia, marine-blue and sea-green.
98. Although the worker can prepare his own tissue, it is not satisfactory to do so, as it is a messy process, so we advise the purchase of the ready-made carbon tissue. As carbon tissue does not keep well in a sensitive state, it is not supplied in that condition. It comes in rolls 2 1/2 feet by 12 feet, or in cut sizes, one dozen sheets to the package. The beginner is advised to buy the cut sheets at first, as they are easier to handle, and can also be bought in assorted colors.
99. Prepared tissue presents a black surface of pigment and gelatin coated on a heavy white paper stock, and has a tendency to roll more or less tightly according to the dampness or dryness of the atmosphere. When too dry or stale it becomes brittle; therefore it is not advisable to lay in a greater stock than can safely be used in a few weeks.