From: Christina Z. Anderson (zphoto@montana.net)
Date: 07/14/03-07:16:55 AM Z
Good morning Katharine!
Yes, this is why I even tried it in the first place, because as I had said
when I originally posted it, if dichromate is sensitive to sun, why would it
then bleach OUT in the sun?? I only came across that method in one book,
and most of the other tidbits I've picked up I've come across numerous
times.
Anyway, the prints I used were 8x10 one layer exposures (full prints) that I
had developed out, dried, but not cleared. These were done presensitizing
the paper with saturated solutions of the two dichromates, and then painting
the pigment/gum/no sensitizer on top of the dried sensitized paper. Thus I
had a good border of dichromate with no pigment on top. I was observing
just the dichromate stain in the borders of the prints. Every time in
comparison between ammonium and potassium dichromate that border stain (and
it literally is a dichromate stain, no pigment) is darker in ammonium than
in potassium. Thus the ammonium has more ways to go, which is why I covered
up half the print to see the difference in bleaching. The potassium did
bleach out, the ammonium did lighten but was not entirely gone.
It doesn't make sense to me. The sun here in MN is normal sun, not too hot,
filtered through trees. If I had done this in MT that sun is incredibly
powerful, so I wish I had done this test there.
I think it may lead back to what Sil had said previously in the fact that
remaining dichromate in the print after development in water wasn't still
doing its "thing" and endangering the print if left in permanently---Sil,
help me out here, I don't have that post on this computer and can't remember
the scientific reasoning you were sharing.
So you could actually just duplicate this with test strips exposed under
plain dichromate--my exposures were UV BL 5 minutes.
Chris
> Hi Chris,
> This result baffles me. A yellow dichromate stain consists of residual
> hexavalent dichromate, so it stands to reason that exposure to the sun
> would reduce it to a green or brown trivalent chromium compound, not
> make it disappear altogether. And even if it did, there's no chemical
> reason I can think of why one dichromate would do it and not the other.
> So I guess I'd have to see this with my own eyes. (Understand, I'm not
> disputing your observation, I'm just bewildered by it.) Unfortunately
> (or fortunately, depending on how you look at it) I never get dichromate
> stains, so I can't just expose stained prints to the sun; I'd have to
> think of another way to replicate this. Perhaps I should know more about
> your method. Were these standard gum prints that had residual yellow
> dichromate left in the print after being developed and dried?? Or were
> these some of your test prints using dichromate only, no gum or pigment?
> Or were they something else?
> Katharine
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