Re: cyanotype question
My interest in the bleeding was wondering if the FAC was the culprit, if its molecules are the bigger ones that maybe don't sink in as well into the paper fibers, because it is the increased FAC that bled.There is an interesting patent on blueprints, cited also by M. Ware but not deeply enough IMHO (Crawley et al., US 2,350,991), where the author specualates about bleeding / non bleeding formulations regarding their ratio of (light-sensitive ferric compound)/(iron cyanide compound). For ratios above 2 (that is, an excess of light-sensitive iron) bleeding occours due to a lack of reacting iron cyanide (this patent deals with mixtures of ammonium iron oxalate and potassium ferri- and ferro- cyanide). Ammonium iron citrate (green) is given with an iron content ranging from 14.5 to 16% as Fe. My calculations are that, for a 10% potassium ferricianide, the non-bleeding equivalence would be a solution of ammoniium iron citrate (green) between 20 and 23%. If so, 25% is in the bleeding range. My experience is that, despite the enhanced exposure times due to the filtering effect of ferricyanide, a lower concentration of am fe citrate (or an higher concentrtion of k ferricyanide up to 1:1 to am fe citrate) gives no bleeding, deeper blue and contrasty images than the usual 10:25 (K ferri : am fe citrate) formula. As for the effect of the acid washing, I have noted an increasing staining of highlights leaving the prints in acidified water for more than 10-20". Ware himself recommends a "half a minute in very diluted hydrochloric or nitric acid" (10 cm3 to 1 litre water)" ("Cyanotype", pag. 107) and that "a lesser dilution of a more diluted stock solution of acid may be preferred". Also, "These [acid] developments baths tend to accumulate Prussian blue, which will eventually begin to stain the paper" (p. 108). BTW: I have noticed a very different behaviour between 300 and 600 g/m2 Fabriano Artistico, the latter giving deep blues and contrasty prints than the former. I don't know if this is due to a different absorption or a different sizing. Alberto
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