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.
Alum Solution. Prepare the alum solution by dissolving one ounce of potash alum in 30 ounces of hot water. Cold water may be used, but hot water dissolves the alum more rapidly. This alum solution keeps indefinitely.
Drying-Room. The chemicals and paper used for working carbon must be kept in a cool and absolutely dry place. The room in which paper is sensitized must be free from dust. A hard-wood floor is preferable. When the work is done in the home, instead of a studio, and where there are soft-wood floors and no carpets on the floor, we would advise sprinkling the floor an hour before sensitizing the paper. Do this to settle all dust. If the floor is carpeted there is less likelihood of dust being raised during the sensitizing. You cannot be too careful about dust when sensitizing carbon tissue; therefore, a natural wood floor is preferable. A good, clean bathroom is alright. In addition to this room there must be a closet, perfectly dry, free from dust and absolutely light-tight. This closet is used as a drying-room. If such a closet cannot be secured, then an ordinary dry-goods box, made light-tight by being covered with some opaque material, can be substituted. The box can be made into a little cupboard, not smaller than 2x3 feet, and one of the boards should have hinges attached to it to be used as a door, but be sure that the box is light-tight.
134. After preparing the room for sensitizing and drying the tissue, proceed to make up the sensitizing bath. For experimental purposes the small bath previously mentioned will be sufficient; but, if much carbon work is to be done, it is well to make the bath in large quantities, using sufficient of the solution for sensitizing the tissue you expect to use at once. As the chemical is inexpensive, discard the bath after using and thus avoid possible failures.
135. Formula (No. 2) for Large Bath.- Prepare the sensitizing solution in a large jar or stone jug, making up as follows: In the summer months, or in warm climates, dissolve in 100 ounces of water, 21/2 ounces bichromate of potash; in winter increase this strength to 3 or 4 ounces of potash to 100 ounces of water.
Sensitizing The Tissue. A zinc or porcelain tray somewhat larger than the paper to be prepared should be employed for sensitizing. Of this sensitizing solution filter into the tray a sufficient quantity to give a depth of at least one inch. This done, the bath is ready for sensitizing the paper. As the different colors of carbon tissue cannot be judged by the appearance of the surface, always mark the color of the tissue on one corner of the back, with a lead pencil. Do this immediately upon taking the paper from the package. The black side of the tissue is the face side. After dusting the face and back of the sheet of carbon tissue with a camel's-hair brush, immerse it in the sensitizing bath, face side down, and with the camel's-hair brush wet with solution.
137. Carefully sweep the air-bells away from the back and the front of the tissue, watching and moving the tissue constantly to see that none remain. It is equally as essential that no air-bells appear on the back as on the face of this tissue. It is well, therefore, to immerse the print face side down for one minute, keeping the air-bells off the back and occasionally examining the face. If air-bells appear, remove them by touching with the camel's-hair brush, saturated with the sensitizing solution. After one minute elapses turn the sheet over, film side up. In each instance dip the print under the solution, sliding under instead of laying it flat on the solution. In this way the entire print is immersed and the air does not strike the surface at any time. While sensitizing the tissue, the fingers will have to be brought in contact with the bichromate which has a poisonous action on some people. It is, therefore, advisable to wear rubber gloves, or rubber finger stalls during the operation of sensitizing.
138. Allow the print to remain in the solution for about 2 1/2 minutes, one minute with the film side down,
1/2 minute with the film side up-the last minute with the film side down. If desired, immerse several sheets at a time, but for the beginner it is not advised. When the 2 1/2 minutes have expired, lift the tissue slowly and carefully out of the bath, by taking hold of two opposite corners with the thumb and first finger. Place the tissue, face side down, on the sheet of glass, which has been previously cleansed with clear water. Place over the tissue the sheet of rubber cloth; then with the squeegee, which should be long enough to reach entirely across the sheet, carefully, and with even pressure, sweep the surplus solution from the back of the paper, leaving the surface dry. Now, remove the rubber cloth and lift the sheet of tissue from the squeegee plate by catching hold of the corner. Attach photo clips to two corners, and having previously stretched a line in your drying-room, hang the clips on the line. You can continue to sensitize as many sheets as you desire, but we do not advise sensitizing more than a few sheets for the first trial. After all are sensitized place in the drying-room and allow them to remain there for at least twelve hours. It is always well to sensitize the tissue a day before you expect to use it. A good plan would be to sensitize the tissue in the evening and allow it to dry over night. The carbon tissue will work better when a day old.