The carbon tissue prepared for this process consists of paper coated with gelatine containing carbon, lamp-black, or other pigments.
The Autotype Company, of London, manufacture a special "transparency" tissue.
Cutting up the Tissue is performed by unrolling it gently upon a zinc cutting plate, cut square and true, with the inches marked at the bottom and right-hand side. By using a T square and observing the numbered inches marked on the plate, it will not be difficult to cut the tissue to any dimension. If the tissue is very curly and unmanageable it should be kept down with convenient weights. After cutting it up to the required sizes, which should be conveniently smaller than the dish to be used for sensitizing, it should be kept flat under a metal plate.
Sensitizing the Tissue is the next operation. This is performed in a solution of potassium dichromate rendered alkaline with ammonia. Tie over the mouth of a two-gallon jug a piece of muslin, to form a kind of bag, into which place fifteen ounces of potassium dichromate, then fill up the jug with water and allow it to stand until the dichromate is dissolved and the solution becomes cold. It is sometimes advisable to regulate the quantity of dichromate. In hot weather, or for very thin negatives, the proportion of water should be doubled, while for very hard negatives only half the quantity should be used. In very hot weather it is often advantageous to replace about 30 per cent. of the water with the same quantity of alcohol.
The operation of sensitizing the tissue must be carried on in a room lighted by a window covered with a yellow blind. A flat dish of porcelain, glass, or papier maché, a squeegee, and a sheet of glass or zinc larger than the tissue, will be required.
The solution is poured into the dish, and should be at least two inches deep. The tissue is then immersed in it, and the air-bells that form immediately brush away from both sides with a broad camel's-hair brush. The temperature of the bath should not be higher than 60 deg. Fahr.; and the time of immersion should be from three to five minutes. After the tissue has remained in the solution for the allotted time it is gently removed and laid face downward upon the glass or zinc plate, and the back squeegeed, removing all superfluous solution. The tissue is removed from the glass and laid over a sheet of cardboard, bent into the form of an arch, to dry.
Another method (H. J. Burton's) of sensitizing carbon tissue is to lay it flat on a sheet of clean blotting paper, and sponge on the back a very strong sensitizing solution composed as follows:
| Potassium dichromate | 4 ounces |
| Liquid ammonia fort | 1 ounce |
| Water | 20 ounces |
First mix the ammonia with the water, then grind up and add the dichromate.
Drying the Tissue should be accomplished in a room perfectly free from the noxious fumes of other chemicals, and lighted only by non-actinic light. Tissues sensitized during the evening should be dry on the following morning. It should then be cut to the sizes required and kept flat in a pressure frame, or other similar contrivance.