PROBABLY everyone knows the fundamental basis of the carbon process, a name that has clung since its first inception, although carbon is no longer the pigment used. Briefly, some water-insoluble pigment is suspended in gelatine and coated on paper somewhat thickly; this is then sensitized with a bichromate and, after drying, exposed to light under a negative. Where the light acts, the gelatine is rendered insoluble in warm water, so that on treatment of such a print with hot water the insolubilized gelatine, being undissolved, remains to form the image in varying thicknesses of gelatine imprisoning the coloring matter. There is, however, one unfortunate fact, that, the exposure being from the front of the tissue, as the pigmented gelatine is called, the insolubilizing action proceeds from the surface downwards, so that the underlying strata, those nearest the paper, are still soluble, and will wash away in the hot water, leaving the surface image or picture more or less without support. This action is made clear from Fig. 12, which represents the tissue exposed under a strip negative; P is the paper and G the exposed gelatine, the amount of light action being shown in black. It is obvious that only the patch I on the extreme left of the diagram is anchored to the paper, where the light action has penetrated right through to the support; in the remaining patches there is still an underlying stratum of soluble gelatine varying in thickness from II to VI. If the tissues be now immersed in hot water, this soluble gelatine will be liquefied, dissolve in the water, and leave the last five patches unsupported in any way, except where they are attached to one another. Possibly two or more might be strong enough to hold thus, but the lighter patches, say V and VI, would break off, and these represent the details in the highlights of the picture.
To overcome this trouble, it is necessary to use a temporary support, and squeegee the exposed tissue to this for development. The condition in this stage is shown in Fig. 13, in which P as before is the paper, G the gelatine and T the temporary support. It is obvious that the patches II to VI are now supported and will not be lost in development.
Three-color tissues can be obtained commercially and it is not wise for the amateur to attempt to make his own, as, while the actual making of the pigmented gelatine is quite easy, to obtain an even thickness of coating is not so. The tissue has to be sensitized with bichromate, and the following is an excellent formula:
Potassium bichromate 25 g.
Citric acid 7 g.
Ammonia 30 ccm.
Water to 1000 ccm.
This should make a lemon-yellow solution, smelling distinctly of ammonia. If it does not smell, a little more ammonia should be added. The tissue should be immersed in this for three minutes, and the temperature of the solution should preferably be 150 C. (6o° F.); then the tissue should be placed between two sheets of filter or fluffless blotting paper, lightly rubbed with the hand to remove excess solution, and hung up in the dark to dry. It does not become light-sensitive till dry.
While the time required for this drying is not a serious matter, as tissue sensitized in the evening will be ready for use the next morning, yet by the use of an alcoholic or acetone bath it can be dried in half an hour. For this bath, however, it is advisable to use ammonium bichromate, and, as the alcohol or acetone mixture will not keep, it is as well to make a stock of the ammonium solution as follows:
Ammonium bichromate 25 g.
Distilled water to 400 ccm.
When required for use, mix ten volumes of this with fifteen volumes of methyl or denatured alcohol, or acetone. Then pin the tissue on a board by the four corners and paint the surface with a broad flat brush saturated with the spirituous liquid. Use the brush first lengthwise and then across; leave for five minutes, and then paint again and allow to dry. The brush should not be bound with metal.
No matter how the tissue has been sensitized, it will not keep longer than a week or ten days, and it is advisable to use it within three days.
The exposure has to be judged by an actinometer, which can be obtained from any large dealer.
Possibly it may be helpful to describe the making of an actinometer, or print meter as it is sometimes called. Procure a pill box from a drugstore - a one ounce box is quite large enough - place the lid, top side down, on a piece of wood, and with a sharp penknife cut two parallel slits half an inch long and about a quarter of an inch apart. Pass the knife blade through these slits with a little sideways pressure so as to widen them sufficiently to allow a strip of paper to pass freely. Procure some gelatino-chloride printing-out paper, Solio or the like, about 4x5 or larger, and cut into strips just wide enough to pass through the slits. Expose the paper between the slits to diffused daylight until of rather a deep color, then paint the top of the lid with water colors which when dry will match the color of the paper. A slit must also be cut in the side of the lid, parallel to and in line with the top slits, and through this the end of the paper is brought, forming a little tag, which enables one to shift the paper after exposure.
To use this actinometer, the box is placed by the side of the printing frame containing the tissue and the negative, and as soon as the small exposed strip of paper darkens to the standard tint, a fresh piece is pulled into place and the action of light watched until the standard tint is again reached. This operation is repeated as many times as necessary. It is usual to classify negatives according to the number of tints they require; thus one speaks of a 2-, 4-, or 7-tint negative. Trial and error will alone give an idea as to the number of tints required; but the average negative will usually need about five tints. By using the long strips of paper, the unexposed part can be coiled up in the box.