Re: Sodium Bisulfite

From: Katharine Thayer ^lt;[email protected]>
Date: 08/21/04-10:13:57 AM Z
Message-id: <>

Martin Angerman wrote:
> The sulfite acts as a reducing agent. It clears the dichromate by reducing
> it to an inactive form.

Hmm. I've always wondered just what a clearing agent does, chemically,
but this explanation doesn't work very well for me, since what is called
"dichromate stain" in my experience almost always consists of Cr+3 not
Cr+6. I tried a few years ago to introduce a new name here for the stain
to reflect this reality, but had little luck with this campaign and gave
it up after a while.

Except for unusual cases where a peculiar combination of sizing and/or
paper hangs onto the bright yellow dichromate and won't let it go, the
hexavalent dichromate, being very soluble, almost invariably washes out
in the developing water without additional clearing needed. It's the
less soluble reduced chromium species that comprise a tan, brown, or
green stain that must be removed to clear the print. I don't know what
the clearing agent does to the reduced chromium to get rid of it, but
the point is that the chromium in the stain is already reduced before
the clearing agent is introduced, and that we've just confused ourselves
by calling this reduced-chromium stain a "dichromate stain."

It's true that the same clearing agent works for both the less typical
dichromate stain and the usual reduced-chromium stain, (except for very
heavily overexposed prints that are so stained that sulfuric acid may be
needed to clear them) but I'm not sure what that means in terms of what
happens chemically.

At any rate, my own experience is that sodium bisulfite can be re-used
many times without diminishing its effectiveness. I had a gallon of it
mixed up years ago that I used for 3 or 4 years before it stopped
working. I only need to clear very occasionally, so the number of
prints that were cleared with that gallon over the 3 or 4 years was
rather small, perhaps 12 or 15 at the most, but it would have been a
waste of materials and money to mix up new stuff each time, when the old
stuff was fine for re-use.

You know you need a new batch when the solution loses its pungency of
odor; in my experience that happens comcomitantly with loss of ability
to clear effectively.

I no longer keep a big jug of clearing agent around, because I've
learned that immersing the paper in the clearing bath is only necessary
in odd cases such as the sizing/paper problem mentioned above or when
I've accidentally spilled some dichromate on the coating table and not
noticed it and it's soaked into the back of a piece of paper. But for
simply clearing an image, I've found that simply brushing the clearing
agent on the image works well, and then a short dip in water is all
that's needed to rinse it out of the print, whereas when the paper is
immersed, you need a more extended soak to make sure you've got the
stuff out of the paper. So now I just keep a pint mixed instead of a
gallon, but that pint is as fresh and effective today as it was when I
mixed it months ago; the extra air in the liter bottle that I keep it in
doesn't seem to affect it at all.

Katharine Thayer

> Air can also oxidize sulfite to sulfate, ruining it as a clearing agent.
> Therefore, I would not recommend storing it for more than a day. Going from
> Saturday to Sunday would probably be OK, however.
> My thoughts would be similar to reusing glyoxal and over-using fixer. The
> sulfite (and others) are relatively cheap, particularly compared to your
> time and other materials. How many people would dilute platinum salts or
> short a mixture of good pigment in gum printing, just to save money? The
> same applies to the rest of the process.
> ----- Original Message -----
> From: "Darryl Gage" <>
> To: "AltPhoto" <>
> Sent: Saturday, August 21, 2004 7:06 AM
> Subject: Sodium Bisulfite
> > After clearing a 2 or 3 gum prints, is the sodium bisulfite solution
> > storable for later use or should I just dump it and make a new solution
> > when needed? Thanks.
> >
> >
> > Darryl M. Gage
> > Forestville, NY
> >
> > "Strange and beautiful are the stars tonight..." Blue Rodeo
> >
> >
Received on Sat Aug 21 17:09:50 2004

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