I agree that it is possible that some Calcium Citrate remains in the paper even after prolonged wash post buffer neutralisation with Citric Acid. Would this affect the light fastness of Cyanotype prints made on buffer neutralised paper is something I'm not sure. Does insoluble Calcium Citrate in the paper act as a reducing agent for Prussian Blue over time? I hope the chemists in the forum can throw more light.
Calcium citrate is acidic in nature as far as I can discern, moving up the pH scale as more of the acid group H+ are swapped for Ca++ One might expect though thay the solubility is higher for partially reacted cirtic acid (monocalcium > dicalcium > tricalcium) so perhaps in excess of citric acid, it is more likley to get washed out. In other words, calcium citrate is more soluble in citric acid than in plain water. So as far as "bleaching" is concerned, I would expect, less so (or none) with citric treated paper compared to untreated.
Having said that, it is important to remember that the fading or bleaching of cyanotypes has 2 sources. One is the presence of an alkaline environment which results in conversion of the ferric ferrocyanide into ferric hydroxide which is yellow or light orange giving the perception of fading. However, this is reversible if one were to treat the print with K ferrOcyanide, regenerating the Prussian blue.
The second source is having to do photo-sensitivity of the Prussian blue itself. When exposed to UV, ferric ferrocyanide (blue) is photo-reduced to ferrous ferrocyanide (Prussian white.) This too is reversible, either by simply putting it back in darkness or treating with hydrogen peroxide.
Frankly, with acid-treated paper, I would be more worried by disintegration of the paper itself than the fading of the print over a long period of time. That's reason why buffer is added to paper in the first place.
:Niranjan.
