CHAPTER XVI 



SPECIAL PRINTING PROCESSES 



By Paul L. Anderson 



Carbon Printing. Theory. — ^Carbon printing depends on the fact that, if a colloid 

 substance, such as gelatin or gum arable, 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" as purchased consists of a 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 sold 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. 



Advantages. — 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. Very beautiful miniature portraits 

 are sometimes produced by carbon printing on ivory. 



Modifications of total contrast are very easily made. 



Local modifications of values may be made to a slight extent. 



Multiple printing is very easy, not only in one color, but in various combinations of 

 colors. Some very beautiful special effects may be attained in this manner. 



Carbon has a very long scale of gradation, and gives exceedingly rich blacks, fully 

 equal to those given by platinum. 



Disadvantages. — It is almost impossible, except in very special cases, to avoid the 

 luster of the gelatin emulsion, particularly in the shadows. This may be an advan- 

 tage, since it adds to the richness of the deeper tones, but it interferes with our enjoy- 

 ment of the texture of the support. 



Like platinum, carbon requires a strong light for printing, therefore it cannot be 

 be used for enlarging, except with very special apparatus; it is strictly a contact 

 process. 



The technique is slightly more difficult than that of platinum, though no more so 

 than that of gaslight or bromide printing. 



It is not easy to handle carbon in either very dry or very damp weather; a humidity 

 of 65 to 70 per cent is desirable. Still, by proper precautions, this difficultj^ may be 

 overcome. 



It is extremely difficult to secure absolutelj^ 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. 



The Negative. — Carbon being a long-scale medium, if its full possibilities are to be 

 exhausted, the negative must be rather stronger than for bromide or gaslight paper; 



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