This section is from the book "Modern Photography In Theory And Practice", by By Henry G. Abbott. Also available from Amazon: Modern photography in theory and practice: A hand book for the amateur.
Carbon prints are not made by the average amateur, on account of the somewhat tedious processes involved but those amateurs who have once mastered the process and noted the beautiful results are well satisfied to put up with the extra trouble. Prints made by this process possess the property, which cannot be attributed to all other varieties, that of being absolutely permanent and the black or carbon prints proper, possess a purity of tone found only in platinotypes. If pure blacks and whites are admired, we may say that in this respect they even surpass platinotypes. Carbon prints are not made on prepared paper, as are other prints and we cannot therefore speak of "carbon paper" in a strict sense. In order that the amateur may thoroughly understand the carbon process, let us explain the underlying principles. The carbon process is founded on a peculiar faculty which gelatine has. If gelatine be impregnated with bichromate of potash and then exposed to light under a negative, those portions which have received the light's rays become insoluble in hot water. The gelatine is not applied to the regular paper, for reasons which we will explain later but is applied to thin tough paper and is technically known as "carbon tissue." Carbon tissue may be purchased from all the large photographic supply stores, though the smaller dealers do not keep it as a rule. It is generally to be had in black, which is the true carbon and in browns, greens and reds. It is usually carried in
England in two forms, known as sensitized and plain but we know of no dealer in the United States who carries the tissue senitized but the amateur can easily sensitize it before use, by immersing in the following bath for thirty or forty seconds:
Bichromate of Potash...............................................1/2 oz.
Distilled Water.....................................................12 oz.
Aqua Ammonia..................................................... 2 drops.
It must be hung up to dry in the dark room for several hours. If the sensitized variety was on the market this trouble might be avoided but the plain paper will keep indefinitely and the sensitized is limited in its keeping qualities. The great drawback which carbon prints have had in the eyes of the amateur photographers is that the progress of the printing cannot be observed from time to time, as with printing-out papers and the fact that carbon prints have to be transferred. Let us try and explain why the transfer becomes necessary. As pointed out above the gelatine when treated with bichromate of potash and exposed to the light, becomes insoluble in hot water. It then follows that if we expose a piece of the tissue under a negative in the printing frame, that the light will act upon it as it does on other printing paper and that those portions which lie under clear glass in the negative receive the greatest amount of light, those under the half-tones next and those under the black portions of the negative no light. In the case of the portions of the tissue directly under the clear glass the light strikes clear through the gelatine on the surface of the tissue and if we were to place the exposed tissue in hot water those portions would be unaffected, while those parts which were under the black portions of the negative and received no light, would be dissolved and run away. How about the half-tone, or those parts which received but a portion of the light? The light not having free access to those portions, the surface of the sensitive film only has been affected, and if we were to place the tissue in hot water the unaffected portions underneath would be dissolved and the entire half-tone portions be washed away with it. Now it is obvious that in order to avoid this we must transfer the film before developing, or, in other words, turn it upside down and this is done by what is known as transferring to a temporary support and removing the tissue which previously held it in place. We can now see why, in the carbon process, it is impossible to print and develop the picture without removing it from the paper on which the sensitive ground was originally laid, as in printing out papers.
Now if the film on the paper is black, how are we to know when it has been printed sufficiently? There are two ways of determining: one as we did in the case of bromide, or velox paper, by experiment, i. e., by printing a small portion and then developing it and two by means of an actinometer, or by comparison.
There are several actinometers on the market as Burton's, Sawyer's and Johnston's, or the amateur can make a simple actinometer, with very little trouble, as follows: Procure a piece of ordinary glass, say four inches long and a half inch wide and a piece of thin wood, (cigar box will do) of the same dimensions. Cut up some white tissue paper into strips a half inch wide and four inches long. Make some starch paste and paste the face of the glass lightly and press on one of the tissue strips. Paste again and put on another strip, letting it recede about three-eighths of an inch and proceed in this manner until you have say eight or nine tissues pasted one over the other, with three-eighths inch steps between them thus