General Non-Silver processes

Carbon print. Edizioni Brogi. Temple of Neptune, Paestum. c. 1910. 15 3/16 x 19 11/16" (38.5 x 50 cm). The Museum of Modern Art, New York. Gift of Richard Benson. Carbon prints render photographic tone with seamless perfection. The difficulty of making them largely confined the process to professional shops, which produced large prints in significant editions. Carbon printing found its greatest use in reproducing works of art.

There is a large difference in the light energy needed to make a negative and to make a print. The negative is usually made in a camera, whose lens focuses the light that goes through it so that any point on the negative only receives light from a single point on the subject.

The lens might be “fast,” which means that it has a large aperture in relation to its focal length, but even so the illumination landing on the negative is minute compared to the light running in all directions out in the world. Only silver compounds are sensitive enough to record the faint levels of light that come through the lens. When we make a print by contact—that is, by holding the negative directly in contact with the material to be printed—the light we use has no need to carry information. It can be as bright as we wish, and can come from a large source, so that many points on the surface of the light send their energy to every point on the print. The result is that compounds with low sensitivity can be used in contact printing, and there are many materials sensitive enough for this use. Silver salts are of course one such material, and were used as printing-out media as well as for making the latent image in the camera. Iron too exists in a whole range of light-sensitive compounds that, when coated and handled properly, can leave a deposit of their own or reduce other compounds to a visible metallic state. A third group of materials is the colloids, a rather loosely defined family of glues and proteins that can be sensitized to light in such a way that they will harden upon exposure and lose their natural solubility in water. The most commonly used colloid for photographic printing purposes is gelatin, the same material found in photographic films and plates.