From: Judy Seigel (jseigel@panix.com)
Date: 05/20/01-04:21:19 PM Z
On Sat, 19 May 2001, Katharine Thayer wrote:
> > But I have read (either on this list or from authorities "met" on this
> > list), assurances that our process leaves the dichromate in its TRIVALENT
> > form already. Which is (some of) what I mean about lack of confidence in
> > "advice" given.
>
> Where reduction is completed, dichromate is trivalent, but since only a
> small amount of the dichromate is used in order to oxidize the gum
> sufficiently to harden it, (Kosar, 1965) most of the dichromate is still
> in the hexavalent stage. All that yellow-orange in the water is
> hexavalent. Only what's green is trivalent.
My only formal study of chemistry was in the 10th grade, so long ago that
the periodic table was still a shelf, so I realize I may be missing
something here. But if my serial surmises that,
a. the trivalent stage is, if not entirely benign, not pernicious, and
b. the *reduction* which "oxidizes" the gum is caused by UV light, and,
c. the reason the still-yellow-orange dichromate remaining was not reduced
was because it was protected from the UV light by the dark part of the
negative,
are correct, then,
d. exposing the still hexavalent dichromate to UV light should reduce it
without resort to further chemical treatment.
In my experience, the wash water turns orange quite promptly. In fact, by
the time the pigment starts running (8-10 minutes) the "orange" is almost
out. Making that the "save" tray, then exposing it to sunlight (or
electric UV light) should, as in (d) above accomplish the necessary, not
only simply, but *visibly.*
If not, why not?
You could of course save the tray with the dichromate AND the pigment in
it, reduce as usual, and then stir the goup in sunlight, but in my
experience, the gum & the pigment exceed the dichromate in volume, and
obscure the color. They might also serve to *mask* the dichromate.
I've said (eg., P-F #1): "Institutions that have the capability
collect first wash water for gum prints for purification. Or first wash
water can be evaporated outdoors, with residue disposed of properly. The
straight solution should not go down the drain..." Etc.
But in a way that's flying blind -- you've got this dark mix which doesn't
visibly change... whatever you do. Seeing the color change might simplify
the whole business.
Judy
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