Carbon printing is an old photographic procedure, in which carbon black (lamp black) was used for making black & white prints. This technique is (probably) developed by Alphonse Poitevin in 1855. This technique was adapted in 1868 by Louis Arthur Ducos du Hauron into color printing with pigments. These techniques were commonly used well into the middle of the 20th century.
(carbon print by Mark Sink, "The Great Sand Dunes #7")
Carbon printing is - besides gum printing - one of the most durable and lasting printing techniques. The carbon prints will stand out as if they were made yesterday among all kinds of prints from the 19th century available in foto archives.
How does carbon printing work? A sheet of paper is coated with a layer of pigmented gelatin. When the paper has dried it is made sensitive to light with dichromate (applied with a brush). After the paper has dried again, it can be exposed by contact printing with a negative. The exposed paper is then put under water and it makes contact with an other sheet of paper: the final carrier of the image. Some light pressure is applied to this sandwich for some time. When removing the sandwich from the water, keep the sandwich together and wash it gently in warm water. The original sheet of paper will come lose from the pigmented layer: it is now transferred to the new sheet of paper. This sheet with the image is now washed with cold water and will harden.