Due to the massive toxicity of hexavalent chromium and its prohibition in the EU, photographers have searched for a number of years for alternatives to potassium and ammonium dichromates in the gum dichromate and carbon transfer processes. Additionally, in gelatine-based processes like carbon transfer, there is a "dark reaction" where dichromate salts can fog the emulsion.
There are basically three major lines of inquiry: one is the Chiba system and variants, and the latter two are photoreactive azide dyes which act as more or less direct substitutes for dichromate.
The Chiba System uses a ferrous photochemistry to initiate polymerization, exploiting photosensitivity of iron salts used in cyanotype, vandyke, and Pt/Pd processes. It is thorougly documented in a paper available online for carbon and gum printing: