Gum Bichromate is one of the most versatile alternative photographic processes, invented in 1855 and popularised at the turn of the 20th century. The technique relies on the property of colloids, such as gum, gelatine, or glue, which harden and become insoluble when mixed with a bichromate and exposed to light.
By mixing a coloured pigment with the colloid, photographers can create an image that remains after washing away the unexposed areas with water. Typical daylight exposure times range from 30 seconds to 2 minutes. Advanced practitioners often build up multiple layers with varied pigments, re-sensitising, re-exposing, and re-developing the print to achieve rich tonal depth and artistic effects.