>Philip Jackson (pjackson@nla.gov.au) wrote
>Judy's comments on pigment stain in gum remind me of an earlier discussion
>regarding different chemicals for clearing chromium stains. I vaguely
>remember sodium bisulphite and potassium metabisulphite being suggested at
>the time, but recently came across a new candidate that might be worth
>considering. It comes from a dichromated gelatin hologram article by Jeff
>Blyth (Applied Optics, 30 (1991) 1602) and consists of a 1% solution of
>sodium dithionite.
>
>Sodium dithionite is also known as sodium hydrosulphite Na2S2O4 * 2H2O which
>to a non-chemist looks a bit like sodium hyposulphite Na2S2O3 * 5H2O. I have
>no idea whether hypo (which is also used as a reducer in chrome dyeing)
>could be substituted, or indeed whether it would be as effective as sodium
>hydrosulphite, which is used as a chromium removal reagent in water
>treatment. It's also supposed to be a photographic developer ingredient, but
>I don't recall ever coming across it before. I should also make it clear I
>have no idea of how toxic sodium dithionite might be or what effect it might
>have on paper or pigments (it was being recommended for clearing glass plate
>dichromated gelatin holograms).
Chapter V, (on the restoration of platinum prints) in the most recent (3rd)
edition of my platinum book has a discussion on the use of a combination of
sodium dithionite/EDTA for the removal of foxing and yellowing. My sources
include references to thesises Mike Ware supervised, among others.
Unfortunately, the method also removed all paper fillers such as calcium
and zinc. This is not to say that the same thing does not happen with other
types of treatment.
Unfortunately, further studies (e.g., accelerated aging tests) would be
required to assess the long term effect of such treatments.
Luis Nadeau
awef6t@mi.net
Fredericton, New Brunswick, Canada