What if you cyanotype (verb) first and then van Dyke (verb). You shouldn't
have any problem with the bleaching of the silver.
What if you protect with gelatin or gum between the two layers.
What if you overexpose the van Dyke print, then try to redevelop it with a
classical paper developer where it has bleached.
Perhaps none of my solution are valuable.
Please let me know. I had this problem once, but I was too lazy to try a
solution.
Philippe,
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Philippe Moroux
http://www.knoware.nl/users/philippe/
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