From: Katharine Thayer (kthayer@pacifier.com)
Date: 07/14/03-04:53:42 AM Z
Christina Z. Anderson wrote:
>
>
> 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---
I think Sil was referring not to a visible stain (correct me if I'm
wrong, Sil) but to what little residual chromium remains in a print
that has no visible stain (the question, as I recall, was whether all
prints should be cleared as a rule, as many experts recommend, even if
there is no visible stain; for whatever it's worth, I agree with Sil
that prints need only be cleared if visible stain is present) and was
also referring, as I recall, to reduced chromium, not to hexavalent
chromium. So I'm not so sure it relates. Since your post last night said
that the old source specified that two hours in the sun would remove a
YELLOW dichromate stain, I assumed you were talking about hexavalent
chromium, which is yellow or yellow-orange, not reduced chromium, which
is green or brown. It wasn't entirely clear from your description, but
it sounds like maybe your tests were done not on hexavalent dichromate,
but on reduced chromium. If we're going to do scientific testing, we
need to be able to agree on basic terms and on what it is we're testing.
>
> So you could actually just duplicate this with test strips exposed under
> plain dichromate
Actually, I couldn't, as long as it stays cloudy and showery; I may not
get two hours of sun this week. But at any rate I'm still a bit puzzled
about what it is exactly we're testing and how it relates to gum
practice. The fact that your prints were exposed the same although the
speeds of the dichromates are different at saturation concentration
(which as I recall is the concentration you used) suggests that the
residual chromium may be at a different stage in its reduction in the
two prints to start with, which makes me wonder if your experiment is a
valid test of whether the two dichromates really behave differently in
this circumstance, as you concluded. If you started with each print
exposed properly, meaning different exposures to allow for the different
speeds of the dichromates, or used concentrations at which the speeds
are similar, would the two dichromates behave more similarly as far as
this question?
But even more crucial to how this relates to normal gum printing: in
order to get something to see using just dichromate, you'd have to WAY
overexpose compared to how you'd expose for a normal gum print (To
overstate the obvious for non-gum printers trying to follow this
discussion: in normal gum printing, the image consists of hardened gum
and pigment, not chromium; in a properly exposed print most of the
dichromate doesn't change color and just washes away in the development;
the only way to make a visible dichromate stain IME is to overexpose.)
Maybe the overexposure overrides the speed difference; perhaps all the
dichromate is completely reduced in both cases in your experiment,
though it's hard to know. One thing that experience does suggest is that
if the potassium dichromate was overexposed enough to darken the
dichromate, then the ammonium dichromate at the same exposure would be
done to a crisp.
I am still curious and will run a couple tests when I can, but I may end
up insisting on a different protocol for the tests that I run, because
I'm not persuaded that your test provides adequate support for the
conclusion you drew from it, quoting your post, "that if someone is used
to pot bi (di) and makes a switch, am di "seems" to present problems."
Katharine
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