Re: Sodium Bisulfite

Katharine Thayer wrote:
>

> > What I see is that if I give a very lonk soak (say 8-12 hours) to "my
> > prints", the colour changes from brownish-green to pale gray-green, and "my
> > water" turns pale yellow. What is in the water?
>

Maybe what we need is some kind of taxonomy of dichromate stains:

1. The kind of yellow "stain" Alberto talks about here, I wouldn't
call a stain because my definition of stain requires not being washable
in water. But I have to admit I've never seen anything like what Alberto
describes. In my practice, the soluble dichromate gushes out of the
crosslinked gum in the first five or ten minutes, and after that I can
leave the print in clear water for days without any further leaching of
yellow dichromate. But yes, if you had a situation like this, where
there is still soluble dichromate that hasn't been completely washed
out, then the basic chemistry Alberto has described here with regards
to Cr(VI) and sulfite/bisulfite should apply, and combining this stain
with bisulfite should form a green product, with my only reservation
being that the proof comes from adding sulfite or bisulfite to
dichromate in solution, and it's clear from research that dichromated
colloids in film (thin layer) don't behave the same as dichromated
colloids in solution.

2. A yellow dichromate stain (an actual stain that can't be washed out
no matter how long the soak) which I have seen only a few times: once
while testing papers I found that certain papers and sizes in
combination with each other would hang onto the dichromate and not let
go of it, and several times when I have dripped dichromate onto the
glass of the coating surface and not noticed it, and the dichromate
makes a spot on the back of the paper. This kind of stain I daresay
would behave chemically the same as the "stain" in (1), since it is

3. The stain that I mean when I say "dichromate stain," this is the
stain that is tan or brown and contains no soluble dichromate. This is
the stain that turned green when I scraped the gum while it was still
wet. As far as I know, the chemistry of this stain is not well
understood. One chemist suggested to me that perhaps the brown stain is
an intermediate chromium compound (somewhere between Cr(VI) and Cr(III)
that got stopped mid-transition, but it was just a speculation and it
was just thrown out as an aside in a discussion of issues more central
to the chemistry of the gum process, and perhaps he wouldn't want to be
held to that speculation. At any rate, this stain, in my experience,
combines with either sulfite or bisulfite to form a blue product the
color of chrome alum, but only if the overexposure hasn't gone too long.
Past a certain point, this stain does not respond at all to bisulfite.

4. There's another type of "dichromate stain" that may or may not be
mythical, and that's a green dichromate stain. As I said here sometime
back, I used to think I'd seen green stain in severely overexposed gum,
but then when I tried deliberately to make green stain, I couldn't do
it. I think I asked here a few months ago whether anyone had actually
seen green stain, and no one answered.

katharine Thayer
Received on Fri Oct 15 12:00:00 2004