From: Judy Seigel (jseigel@panix.com)
Date: 07/30/01-12:53:23 PM Z
On Mon, 30 Jul 2001, alnovo@inwind.it wrote:
> However, as for clearing gum someone suggests sodium bisulphite and
> other suggest sulphuric acid, the question is: what is better to clear
> gum? A weak but reducent solution or simpluy an acid one? The first
> works because the yellow staining of dichromate is reduced to
> trivalent chrominum, and perhaps it is carried in solution by the weak
> acidity. Usually are recommended 5% solutions. The second works
> because the trivalent chromium is *surely* dissolved by the strong
> acidity, and perhaps the residual dichromate undegoes to a reduction
> to trivalent chromium, reacting with some reducent yet present in the
> paper, like the gelatine and the gum themselves. Note that the
> reduction of dichromate needs strong acidity. If I remember well,
> there are suggested 1% solutions of sulphuric acid, but it is known
> that acidity is not the best for an archival purpose.
>
I find any kind of dichromate stain that doesn't clear in a plain water
soak very rare: Which could of course be why those times I have cleared,
the bi's & meta's seemed to work more or less interchangeably. I settled
on the one that made me choke the least, but it's so long since I used it
I forget which. (I've also heard of hypo clear, essentially sodium
sulfite, used for clearing gum.)
Judy
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