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Carbon Printing - Theory.
Carbon printing depends on the fact that, if a colloid substance, such as gelatin or gum arabic, is sensitized with any one of several chromium salts and is exposed to light, the salt breaks down, giving off nascent oxygen, this nascent oxygen rendering the colloid more or less insoluble, in proportion to the amount of light action. In practice, the colloid used in carbon work is gelatin, and the tissue consists of moderately heavy backing paper or support, one side of this carrying a layer of gelatin with which some earth pigment such as lampblack, burnt umber, etc., has been incorporated. This tissue is insensitive and is sensitized and dried in the dark just before use. It is then printed under a negative and developed in warm water, when the soluble portions of the gelatin wash off, taking their quota of pigment with them and leaving behind the insoluble portions which with their pigment adhere to the paper, thus giving the print.
The prints are absolutely permanent, as is the case with platinum.
Any one of about 20 different colors may be used.
Practically any support may be used. This includes not only various papers but also glass, Ivory, porcelain, celluloid, etc.
Modifications of total contrast are very easily made.
Carbon has a very long scale of gradation, and gives exceedingly rich blacks, fully equal to those given by platinum.
It is almost impossible, except in very special cases, to avoid the luster of the gelatin emulsion, particulary in the shadows. This may be an advantage, since it adds to the richness of the deeper tones, but it interferes with our enjoyment of the texture of the support.
Like platinum, carbon requires a strong light for printing, therefore it cannot be used for enlarging, except by very special apparatus; it is strictly a contact process.
The technique is slightly more difficult than that of platinum.
It is not easy to handle carbon in either very dry or very damp weather. humidity of 65 to 70 per cent is desirable.
It is extremely difficult to secure absolutely pure high lights, without recourse to brush development. This disadvantage, though, is of minor importance, since almost pure lights may readily be obtained, and it is seldom that absolute purity is desirable.
Carbon being a long-scale medium, if its full possibilities are to be exhausted, the negative must be rather stronger than for bromide paper; about the same quality is desireable as in the case of platinum printing. It is, of course, not necessary to exhaust the scale of the paper; medium or low-keyed prints may be made as well as with any other medium, though when very high key is required, carbon is not so satisfactory as platinum.
"Tissue" is rather a misnomer, since the combination of backing paper and gelatin emulsion is rather heavy. It should be stored in a cool dry place, in which case it will keep indefinitely before sensitizing. If it is allowed to remain moist for any length of time, bacteria may grow in the gelatin, and the prints, on development, will show irregular blank patches.
The word "carbro" is a combination of the first syllables of "carbon" and "bromide", and the process is so named because it is a method whereby a true carbon print can be made from a bromide print. In practice, the sensitized carbon tissue, instead of being dried and printed under a negative, is squeegeed into contact with a bromide print while still wet, the gelatin becoming insoluble not through the action of light on the sensitizer but through the chemical reaction between the sensitizer and the silver of the bromide print. Stripping and development follow in much the same manner as with carbon, the final result being an actual carbon print, exactly as in the previously described process.
Advantages. The advantages of carbro are the same as those of carbon, with the additional ones that no very strong printing light is required, that enlargements can be made without making an enlarged negative, and that multiple prints can be made without the need for registration.
Disadvantages. The disadvantages and possible failures are those of carbon, plus the fact that carbro is decidedly more temperamental than carbon, demanding a closer adjustment of the controlling factors if success is to result.
From an article titled "Special Printing Processes",
by Paul L. Anderson,
from the book
Handbook of Photography
Edited by Keith Henney, Editor, Photo Technique,
and Beverly Dudley, Managing Editor, Photo Technique
Whittlesey House, McGraw-Hill Book Company, 1939