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CARBON AND CARBRO 433
the obvious suggestion that the exposure should be made through the support. This increased tremendously the exposure time, since the paper backing was only slightly translucent. It also superimposed the pattern of the paper grain upon the image. W. Blair got around this by proposing to treat the paper with waxes and oils to make it transparent (Phot. Notes, Vol. 4 (1859), p. 331. Risler (Bull. Soc. franq. Phot., Vol. 10 (1864), p. 271; Eng. P. 2954/63) suggested the transparent substance mica as a base. This was evidently the first use of a completely transparent material to serve as a carrier for the gelatin. Soon afterward Placet and Despaquis (Bull. Soc. franq. Phot., Vol. 13 (1867), pp. 170, 227) proposed collodion, a material also suggested by du Hauron in 1869 (cf. above).
The final step that made carbon completely acceptable as a print medium came in 1864 when Sir Joseph Swan published his procedure not only for the preparation of the tissue, but also for processing the material (Phot. News, 1864, p. 85). In order to give the tissue pliancy, he advised the addition of sugar to the gelatin, a practice that is used to this day, although it could be replaced by other polyalcohols such as glycerin, etc. The gelatin is prepared free of sensitizer, which was to be added just prior to coating. A typical formula as suggested by Swan, was the following:
Gelatin 2 parts
Water 8 parts
Sugar i part Coloring matter to suit
This solution could be coated upon paper, in which case the tissue was to be sensitized by bathing and drying just prior to use. If it was desired to coat a sensitized paper, then to ten parts of the gelatin solution was added one part of saturated dichromate. This was done just before coating. The unsensitized paper would keep indefinitely under proper conditions, whereas the sensitized tissue would keep only a few days or a week.
After drying, the tissue was exposed by contact. Swan's real contribution to the art lay in the next step. He discovered that the tanned gelatin image would adhere to a polished surface. Therefore by mounting the exposed tissue upon a polished support so that the polished surface was in contact with the exposed surface of the tissue, he was able to wash away all the untanned gelatin, and retain the tanned image with all highlight detail intact, upon the new support. This technique is utilized to this day. The Swan disclosures were absorbed by the Autotype Company.
While carbon was being developed, the same procedure and chemistry were utilized by other people. In 1858 John Pouncy sensitized a piece of paper by brushing it with a solution of dichromated gelatin. In this technique, the gelatin was later replaced by gum arabic, and the procedure became known as gum-bichromate. In 1873 A. Marion (Brit. J. Phot., Vol. 20 (1873), p. 242; Vol. 47 (1900), p. 709) introduced a new wrinkle. He exposed a