CARBON PROCESS (Synonym, process based upon the fact that gelatine and other organic substances, if heated with a bichromate, become sensitive to light which renders them insoluble in the parts exposed. This fact was discovered by Mungo Ponton in 1838, but it is to the subsequent discoveries and improvements of Becquerel, Poitevin, Pouncey, Swan and Sawyer that we are indebted for the simple and efficient carbon or autotype process of the present day.
The carbon tissue prepared for this process consists of paper coated with gelatine contain ing carbon, lampblack or other pigments (see Carbon Tissue).
It can be purchased readily sensitized, but in this condition will not keep for any length of time.
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 dimensions. 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 use for sensitising, it should be kept flat under a metal plate.
Sensitising 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 till the dichromate is dissolved and the solution be come 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 a good plan to replace about 3o per cent. of the water with the same quantity of alcohol.
The operation of sensitising 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 machi, a squeegee, and a sheet of glass or zinc a size 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-hair brush. The temperature of the bath should not be higher than 6o deg F., 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 downwards 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 sensitising carbon tissue is to lay it flat on a sheet of clean blotting paper, and sponge on the back a very strong sensitising solution composed as follows : Potassium dichromate 4 ounces Liq. ammonia fort Water 20 " 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. Tissue 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.
Exposing the tissue can be exposed behind the negative in an ordinary printing frame, or in special frames having no joint in the back, as no image is visible. The negative must be furnished with a safe edge, made by painting an edge about one-eighth of an inch round the negative with black varnish, or by pasting on strips of red or black paper. Exposure must be judged by an actinometer. Several forms of this instrument are described under the heading Actinometer. A very suitable instrument for toning the exposure of carbon tissue is Sawyer's actinometer. It consists of a rectangular tin box with a glass lid, bearing twelve tints gradated from slight discoloration to a degree of opacity, representing the extreme amount of deposit upon the lights of the densest negatives, each division of this screen of tints bearing a number in opaque pigments ; a roll of sensitive paper is placed in the box, and the end pulled forward so as to pass under the tints. When this arrangement is placed in the light, the silver paper commences to discolor underneath the gradated screen, beginning of course at the lightest division, but the number on the tint being in an opaque pigment is preserved white, and serves to register the progress of printing ; for if, when the lid is opened, the number one, for instance, shows clearly on a tinted ground, the instrument is said to have registered one tint ; by that time the number two will have begun to make its appearance, and if sufficient exposure be given the light will print through the whole scale by successive steps, and show up the numbers one to twelve. With an instrument of this kind it is evident that, by exposing at the same time as the carbon tissue and determining the number of tints requiredior the proper exposure of that negative, the same number of tints with the same negative will always prove correct. A little practice will enable one to judge the number of tints required for every class of negative.