Johnson's process is adapted to supply the place of albumenised paper and silver, gold, and hypo, solutions; the manipulations are more simple than silver printing, and less skill is required for producing prints by this method than by the usual silver one. The most troublesome portion of any carbon process is the preparation of the tissue, that is, the sheet consisting of the layer of gelatine and carbon or pigment. This carbon tissue consists of a layer of gelatine containing the carbon or other permanent pigment spread on paper. As sold it is not sensitive to light, but requires the action of a solution of potash bichromate to render it sensitive. So far the process resembles the silver printing one - the tissue corresponding with albumenised paper, the bichromate sensitising solu-lution with the silver one. When the paper is dry, the coloured surface is placed in contact with the negative, and exposed to light; the exposure should be about one-third the time required for silver printing. The pigmented paper is prepared in long rolls, so that much larger sheets can be obtained than of albumenised paper. It should be cut into convenient sized sheets for sensitising. A solution, 20 gr. to the oz. of potash bichromate is provided in a flat dish.
The sheets may be placed in the solution one at a time until all are immersed. Each should be turned over to see that no air-bubbles form. They must remain in for one minute, but may stay longer without injury. They should then be taken out, and hung to dry. This sensitising and drying must be done in chemical darkness, like sensitising silvered paper; more caution must, however, be taken, as the carbon paper is so much more sensitive. When the paper is dry, it must be placed in contact with the negative to be printed. It is advisable for carbon printers to classify their negatives. Let those negatives that print the quickest be called No. 1; those that require longer printing, No. 2; and those still denser, No. 3. By the use of an actinometer, the amount of printing given in a certain time can be measured. This simple little apparatus consists of a round japanned tin box, with a slot in the lid about £ in. wide and 1 in. long, like a money-box. Inside the box is a strip of Carrier's sensitised albumenised paper, about 1/2 in. wide, coiled up in a roll. The lid of the box is painted a chocolate colour, like the tint that sensitised albumenised paper quickly takes when exposed to the light.
By simple means, a portion of this paper is pulled out of the box, and in doing so a portion is exposed to light through the slot in the lid, the rest of the strip being screened from light. The paper when exposed begins to darken, and presently arrives at the same tint as that surrounding it on the lid of the box. Let us suppose a negative to have sensitised pigmented paper placed under it, and the actinometer to have a piece of the white sensitive silver exposed through the slot, then let the actinometer and the negative be both exposed simultaneously to the same light; by the time the light has darkened the silver paper to the standard tint, the actinometer and the negative will both be said to have received one tint, that is, they will both have re-reived that amount of action from the light necessary to produce on the silvered paper that particular tint. In the first instance each negative, or each class of negative, will have to be tested by the actinometer, how many tints have to be darkened before the carbon print is made, and the negatives may then be marked accordingly. When a negative has been once tried and marked the number of tints it requires, no mistakes will be made afterwards as to the exposure that will be required.
The next operation is to attach the print to a temporary support during the development, or removal of the unacted-on pigmented gelatine. Plain gelatine is not sensitive to light, but is easily soluble in hot water. The potash bichromate makes it sensitive to light, and the change effected in the gelatine by light renders it insoluble in hot water, but the rest of the gelatine still remains soluble. The insoluble portion constitutes the picture, and it is necessary to dissolve everything but that which light has rendered insoluble. The print has to be attached for this purpose to a temporary support. Almost any substance impermeable by water will answer, but some substances are more convenient than others, such as the surface of ground opal glass, or zinc plates that have a finely-ground surface. To facilitate the removal of the print from this slightly-roughened surface, rub the support over with a dilute solution of rosin and wax in turpentine, using a soft rag, and leaving only a very thin film of the solution on the surface.
The pigment print is first immersed in cold water, gelatine side downwards; the print at first curls inwards as the paper on the back expands with the water, but in a few seconds it flattens and shows signs of curling outwards; at this juncture take it out, and previously wetting the glass or zinc that you are going to develop it on, lay it on gelatine side downwards, and with a rubber scraper, or squeegee, press the print in close contact to the support to expel the water. Sweep the squeegee backwards and forwards once or twice to get rid of all moisture that can be driven out. Allow the print to remain thus for a few minutes, and if you have other prints ready to goon with,you may serve them all the same until you have several ready. This pressure ensures the perfect adhesion of the print to the surface of the support through all the subsequent hot and cold water washings. The glass or zinc with the print thus firmly attached by atmospheric pressure may now be immersed in hot water at say 100° F. Let it remain for a few minutes.
When the coloured gelatine begins to show itself oozing from the edge of the paper, try one of the corners of the paper if it will lift easily; if so, lift it slowly and steadily from the support, and it will come off, bringing with it a great deal of the unaltered gelatine. If it does not lift off easily, allow it to remain until it will do so. On no account force it up. The time it takes for the paper to come freely away depends on the temperature of the water it is immersed in; the water need not be hotter than the hands can bear. When the paper is removed the rest of the unaltered gelatine will speedily flow away, and the pisture will gradually emerge from the dirty mass that envelops it. Allow it to remain in the hot water till all the soluble gelatine is removed; this is known by the ceasing of the dirty or coloured streams that previously have come from the picture. There is no fear of the print itself being dissolved away, for the altered gelatine. that forms it is insoluble. When all that will come away has come away, remove the glass from the warm water, and well wash in cold water; the picture may then be set aside to dry, still adhering to the glass or zinc.
When the print is in this state it can easily be seen if the exposure to light under the negative has been too little or too much. If it has been too little, the print will be too light, that is, there will not be enough pigmented gelatine left on the glass to properly represent the negative, showing that sufficient time was not given for the light to render enough of the gelatine insoluble. The print will betray the deficiency of exposure by the absence of the half tones. If the print is too dark, then the exposure has been too great, and too much of the gelatine has been rendered insoluble. If either error has been committed a mark should be made on the margin of the negative showing the greater or lesser number of tints that the negative should receive in future printings. Gelatine prints never look sharp when they are wet; they will be sharp enough when the gelatine is hard and dry. After the print is dry, proceed to transfer it to the permanent paper base on which it is to remain, Ordinary plain paper, or even paper slightly gelatinised, is not sufficient for finally attaching to the image on the glass or zinc.
If such papers be attached to the gelatine image, the finer parts of the high lights and half-tones are so attenuated that this kind of paper will be sure to leave them behind. There is, however, a paper provided with a coating of insoluble gelatine that readily attaches itself to the image, and brings it all off the glass.