Thanks for the contribution Nicholas,
I agree with the fact that gallic / tannic acid toned cyanotypes are robust (in terms of light fading), I have numerous tannic acid toned cyanotypes that are still in good shape (color and density) after more than a decade. My concern is the possibility of leaving something alien in the paper, that MAY acidify and damage it in the long term, especially in humid conditions... Papyrus isn't a particularly good example; most of the specimens we have on hands were stored in pretty dry conditions - in Egypt and its structure / mechanical properties is considerably different than what we use now... There's an academic web site about iron-gall ink, it says the specimens should be kept in dry conditions (<70% RH) to inhibit the formation of acid, otherwise you get adverse effects pretty quickly. We often have >=70%RH here in Istanbul in the spring and summer, therefore I don't like the idea anymore... If I want a color which is not possible with cyanotype (or other iron processes I practice), I simply use gum dichromate or casein with extremely lightfast pigments.
The tannic acid I have on my hands (technical grade, pretty uncertain stuff which I bought in an unmarked plastic bag, a rust colored fine powder which takes ages to dissolve) usually stained my papers, and the stain was still present in pure paper base where no coating solution was applied. The severity of the stain was changing from virtually unnoticeable (a very little warming, only noticeable when you observe it next to untreated paper) to clearly noticeable (turning bright white paper base to natural / ecru), depending much on the paper / toning solution strength and toning time... (Edit: Maybe it's mechanical staining, undissolved fine tannic acid particles - of the specimen I have on hands - trapped inside paper fibers???)
Last edited by Loris Medici; 04-13-2012 at 05:12 PM. Click to view previous post history.
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