Bichromated

Bichromated is an adjective used to describe a material, most commonly gelatin, that has been treated with a bichromate salt such as potassium dichromate or ammonium dichromate. The term stems from the use of dichromate (a chromium(VI) compound) as a photoactive hardening agent. When exposed to light, the chromium(VI) in the coated layer is reduced to chromium(III), triggering crosslinking of the gelatin molecules. This crosslinking renders the exposed areas insoluble in water, while unexposed areas remain soluble and can be washed away during development.

Historically, bichromated gelatin has been central to several photochemical and printing processes. In photography, it underpinned early wet-plate and lantern slide technologies, where image formation relied on differential solubility after exposure. In printmaking and ceramics, dichromated gelatin coatings served as relief layers or stencils. In modern practice, similar chemistry is used in the creation of photopolymer plates and certain screen printing stencils, where a light-exposed, hardened film forms the printable or etchable image.

Safety and handling considerations are important, as bichromate salts are oxidizing, toxic, and potentially carcinogenic. Proper ventilation, protective equipment, and disposal procedures are required. The technique is largely historical in some fields but remains relevant in specialty printing, photography, and photopolymer plate production.

Etymologically, bichromated reflects the presence of chromium in two redox states during processing, transitioning from chromium(VI) to chromium(III) upon light exposure to achieve the crosslinked, water-insoluble state.