Hi all, I'm looking for somebody who can explain to me the chemistry behind this reaction, and importantly, if the result be deemed "archival." I put acrylic gloss medium over a VDB print a few months ago and got a better dmax and an interesting brush texture. Great. Last night I tried it on a cyanotype ("new" photo formulary kit) and had some really unusual results: the acrylic medium not only enhanced d-max, but it also boosted contrast significantly and shifted the tone toward deep purple! From the acrylic medium MSDS ("Preservative - Non Hazardous At Level Used") I presumed that the active chemical was sodium bisulfite, and a different cyanotype treated in a 10% bath of sodium bisulfite exhibited a similar purple shift. I'm about to start printing a series of large cyanotypes that I intend to show and possibly sell, and I find this effect desirable, but I'm wondering what's going on chemically here, where the purple cast represents a chemical shift in the pigment, or if it's the result of chemical damage. Or more directly, whether this process is archival. Speculation welcomed. Cheers, Conor Peterson FWIW: the attached image was 9 drops of sensitizer + 2 drops citric acid 40% on Arches Hot Press 140lb, bathed post-exposure in a bath of 1% citric acid before final wash.