Carbon prints and gum bichromate prints are both examples of pigment processes. Rather than the metal salts typically used in the formation of photographic images, pigments and bichromated colloids are used in making these prints. A bichromated colloid is a viscous substance such as gelatin or albumen that is made light-sensitive by the addition of a bichromate. Bichromated colloids harden when exposed to light and become insoluble in water; this is the principle behind many of the non-silver-based photographic processes. Pigment processes were developed in the 1850s and offer superior permanence and control of the appearance of the final print. The resulting prints are characterized by broad tones and soft detail, sometimes resembling paintings or drawings.
Video from the George Eastman Museum