Silver Carbro Duotone:
A true duotone process which produces a two layered print. The base layer is Silver and the top layer uses the obscure Carbro process to add a gelatin pigment layer in exact registration.
The Carbro process was discovered in the mid-1800’s after the Scotsman Mogo Ponton found that potassium dichromate could be used to make certain types of gelatin sensitive to light. Later, in 1864 Joseph W. Swann patented a Carbro process which produced an image of carbon or other pigment bound in gelatin on a paper surface. The Carbro process I use employs a warm brown pigment in the gelatin. The beauty of this duotone process is that the photographer does not need to align or register the two images. Rather one begins with a silver enlargement and then applies a carbro "tissue" which has been sensitized with potassium dichromate. The image of the enlargement is chemically transferred to the Carbro tissue, which is then peeled off in a warm water bath. If all goes well, this leaves a Carbro image over the original enlargenent. The silver enlargement, which has been bleached out by the potassium dichromate, is then redeveloped with a selenium sulfide redeveloper. This procedure often fails and ruins both the enlargement and Carbro images. It is one of the most obscure and difficult processes, but when successful creates images with rich glowing shadows and silvery highlights.
