Ed Buffaloe's cite, The Unblinking Eye, has a concise and well written article on archival processing. In that article, I came across the following:
"Doug Nishimura indicates (in a letter to Jennifer Scott) that complete protection with Kodak Selenium Toner requires a dilution of not more than 1:9, and a toning period of 3 to 5 minutes at 68Ί. Some fine art photographers find this depth of toning not to their taste."
What is interesting to me personally about this is that the paper I use, Ilford MGIV Fiber, requires almost exactly this dilution and toning period to get a discernible increase in DMax. While there is, to my eye, almost no change in image color, is it possible that, after all, I have been toning sufficiently for archival permanence? To put this another way, for an area of the picture to be protected by the toning, must there have been a visible change in density and/or image color? Do any of this forum's scientist contributors have an opinion on this?
I also found, in the same article, some information on Agfa Sistan:
"Other Treatments: Agfa makes a product called Sistan, and Fuji makes a similar product called AG Guard, which is used to treat prints after washing. Thomas Wollstein has corresponded with Agfa regarding Sistan, and tells us that Sistan contains potassium thiocyanate and a wetting agent--it works by converting oxidized silver ions in the emulsion to a stable, insoluble salt. Robert Chapman states that Sistan ...precipitates any silver ion formed by oxidation in the form of silver thiocyanate (AgSCN). Silver Thiocyanate is colorless and virtually light-insensitive. But Sistan only works as long as the thiocyanate stays in the emulsion, so Agfa recommends that Sistan be used as a final treatment, after washing and before drying--if it is washed out, archival benefits are probably lost.
According to Doug Nishimura, before any silver deterioration can occur, silver must be oxidized into silver ion. Even air and moisture can act as a strong enough oxidizing combination to cause damage. He notes that ...there is always a small amount of ionic silver in equilibrium with silver metal in a photographic image. But, whether the ionic silver already exists in the emulsion or is caused by pollutants, thiocyanate combines with it, thereby stabilizing it as an inert salt which will not cause image degradation. Sistan is said to be fully compatible with toning treatments. I should note that Dupont 6-T Gold Toner contains potassium thiocyanate, and Kodak GP-1 contains sodium thiocyanate, but I do not know if either is in sufficient quantity to be as effective as Sistan is alleged to be--also, prolonged washing would negate any benefit derived therefrom. Robert Chapman states that, though he has inquired several times, Agfa has not provided him with substantive documentation to prove the effectiveness of Sistan. He allows, however, that ..it makes sense on theoretical grounds. Other sources on the world-wide-web hint that Sistan may not be effective over the long term, but there is no hard data to back this up either."
Obviously, then, for those who want the effectiveness of the treatment demonstrated before adoptiong it Sistan would not fill the bill.
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