Re: An exchange with Mike Ware (on Argyrotype)

From: Loris Medici <mail@loris.medici.name>
Subject: RE: An exchange with Mike Ware (on Argyrotype)
Date: Thu, 23 Mar 2006 11:12:28 +0200

> I will recall the point I make according to EDTA (but not citric acid)
> since I've just read that it will dissolve elemental silver - ruining
> the image!!! Definitely no good for our Vandykes, Kallitypes,
> Argyrotypes :)
>
> Anyway, AFAIK (and if I'm not wrong) citric acid can dissolve iron
> compounds making no harm to elemental silver).

The complexing effect of citric and other acids depending on the pH.
Relative effects between ligand-Fe(III) and ligand-Ag depend mostly on
the type of the ligand. If you want to make it most specific to
iron(III), I'd go for oxygen ligands, such as oxalic acid,
acetylacetone, gluconic acid, etc. and buffer at their optimal pH to
complex iron.

Another approach is to use compounds that make very insoluble complex
with silver to protect the image while using very powerful iron
chelating agent(s). For example, there are several substituted
tetrazoles, imidazole, etc. potentially usable for this
purpose. Benzotriazole may or may not work, but
1-phenyl-5-mercaptotetrazole is pretty strong in its adsorption as
well as having very small ksp with silver.

One problem with residual iron is that it can generate hydroxyl
radical in presence of air and moisture, and this WILL attack silver
image. To solve this problem, it's most effective to incorporate iron
ligand chelator that inactivates iron's redox catalysis, a radical
scavenger in the paper base, or more preferrably both. Most effective
ligands have some affinity to silver.

Regarding relative longevity of those nongelatin silver processes,
it's probably the best to run a peroxide fuming test with very low
peroxide concentration. (If you use a test condition used for toned
silver-gelatin prints, I expect the image to fade very fast and won't
be able to see any relative difference.)
Received on Fri Mar 24 00:42:13 2006