Thanks Loris! I'll definitely have to give that a try.
Additional information
I've noted that the instructions I've found in books and information on websites for making Van Dyke Brown prints frequently contradict each other when it comes to processing the paper. I've been looking into finding a method of processing the paper to make a Van Dyke Brown print as permanent and stable as is practical (short of toning it and losing that nice brown image).
One key idea that seems to be common with the other iron based printing methods like Kallitypes and Platinum / Palladium is that the development or clearing steps are acidic. The acidity helps the unused iron sensitizer to wash out. I haven't seen this mentioned for Van Dyke Brown.
I contacted Mike Ware with a couple of questions about the processing of paper for Van Dyke Brown. I asked him about the acidity of processing solutions (the water development bath) and the possibility of using EDTA as a chelating agent to remove iron salts from the paper fibers--leftover iron sensitizer trapped in the paper is said to be a major cause of staining and lack of permanence in VDB images. With his permission I'm posting his comments here:
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[from Mike Ware]
There are several intertwined issues here:
1) EDTA. If you look at the chemistry of colour photography processing, (in which I'm far from expert) you'll see that in one version, ferric EDTA is used as a silver bleach, because it dissolves metallic silver. So EDTA is altogether NOT the thing to add to any of the iron-sensitized silver processes. Fortunately, this is not the case with palladium, platinum and gold, where disodium EDTA is a good first bath.
2) Clearing of ferric ion. This cannot be done effectively in an alkaline environment, and even neutral washing may leave problems. It is also very important to avoid chlorinated water, and 'hard' water. It would be best to use pH 4 or less (which can be obtained from a spoonful of citric acid) to suppress hydrolysis of the ferric ion. However, if the silver salt used was silver nitrate, then the 'nitric acid' formed will tend to redissolve some silver image. Which explains why Kallitype uses alkaline developers, like Borax, and Rochelle salt, and Van Dyke uses a neutral wash, at best, otherwise image may be lost.
3) To be able to use acidic conditions with silver, one must have a non-oxidising anion. This was my logic in replacing silver nitrate with silver sulphamate in my Argyrotype process, which work in an acid environment. See:
http://www.mikeware.co.uk/mikeware/Argyrotype_Process.html
4) Nanoparticle silver images are still very vulnerable, especially on plain paper, and most workers with Kallitype and Van Dyke recommend gold toning for permanence. I believe my Argyrotype silver images already become sulphide-toned in the thiosulphate, so have a better chance of stability without gold toning. But I would not issue guarantees of permanence.
So, the short answers are : 1) forget EDTA. 2) Try a spoonful of citric acid in your wash water. 3) Gold tone your Van Dykes. or 4) Consider using Argyrotype.
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