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:
download/original: http://polychrome.nl/file_download/4/TheChibaSystem-HR.pdf
alternate/view online: https://archive.org/details/the-chiba-system-a-non-toxic-alternative-to-the-dichromate-processes
Further, the photographer Heather D'Augustine has developed the D'Augustine/Chiba method for casein prints.
Photographers have reported success in replacing dichromates in alternative processes with a couple types of photosensitive azides. A survey of this may suggest further research. A sulfite clearing bath after development is indicated, since azides are dyes.
The first is known as Diazidostilbene Sodium Sulfonate, or simply Diazidostilbene or DAS. This is, in essence, the UltraStable process, which was a branded carbon transfer. This gives reportedly excellent results with protein bases such as gelatine, casein, and albumen. It's also known to work well with various synthetics, reported widely in the literature.
DAS is the sodium salt/tetrahydrate of diazidostilbene disulfonic acid, which is the form found in a variety of patents if one wishes to research the chemistry via pubchem. Pubchem also lists hydrate forms and a dipotassium salt, but no suppliers.
The second uses relatives of Diazo sensitizer (DSR), also known as Diazo 22S, which is seen in Gum Diazo Printing v1.4. DSR works adequately with gum. Research in a photrio thread (see below in thread.md) indicates that it works adequately with gelatine processes, but gives excessive contrast. Speedball Diazo sensitizer, which is DSR but with a phosphate ion instead of sulfate, works extremely poorly with gelatine processes. It is reportedly adequate with gum. Additionally, decent carbon printing results have been reported using Diazo 22Z sensitizer, a complex of DSR with zinc chloride.
These chemicals are salts of (often with a formaldehyde byproduct locked in polymer) 4-Anilinobezenediazonium. Systemic naming is "Benzenediazonium, 4-(phenylamino)-, [ion]" and "4-Diazodiphenylamine [ion]" is also commonly used by suppliers.
DSR/22S is commonly sold as a sensitizer for screen printing emulsions, except by Speedball which sells the phosphate salt. Most other salts are harder to get, or available from the same suppliers.
Diazo sensitizers is colloquially used as a family name for these chemicals.
Major bulk supplier of these in raw form appears to be Secant Chemicals which is referenced below, but according to reports they do not sell in small quantities.
CAS: 2718-90-3 pubchem: https://pubchem.ncbi.nlm.nih.gov/compound/2724999
noted as a photochemical in the literature
parent compound Diazidostilbene disulfonic acid (CAS 15874-22-3) fairly common in the literature, may be a good ground for further research
also known historically as Hardener No. 3. A 1987 datasheet from a now-defunct supplier was found and shared by photrio/APUG user CMB
Secant Chemicals calls this "SCL 3".
pubchem: https://pubchem.ncbi.nlm.nih.gov/compound/6419866 CAS: 41432-19-3
no references in the literature
parent compound CAS 16072-57-4 along with sibling compounds (see below) appear extensively in patents
Secant Chemicals "Diazo 22S"
CAS: 71550-45-3
two pubchem entries, I have no idea why
same parent compound as DSR, but phosphate instead of sulfate
not offered by Secant Chemicals
Readily available in solution, distributed by Speedball for screen printing.
CAS: 68988-17-0
Diazo 22Z is Secant Chemicals name for this compound.
parent compound is DSR, essentially complex with zinc chloride