Gum bichromate printing is one of the most interesting alternative techniques. It is based on a different principle than cyanotype, platinum printing, and similar methods, specifically on the sensitivity of chrome-tanned gelatin. The gelatin layer is sensitized with dichromate and then exposed to UV light. The illuminated areas harden and become water-repellent, while the unexposed areas remain soft and absorb water. After moistening, the gelatin swells in the unexposed areas, allowing oil paint to adhere specifically to those regions.

It is the only technique that allows manual interventions in the image, making it the most creative.