From: Katharine Thayer (kthayer@pacifier.com)
Date: 05/19/01-12:22:22 PM Z
Judy Seigel wrote:
>
has all of that come out in the wash water, or
> does x amount remain in the print, transmogrified ?
>
Hmm. Interesting.
This is exactly the question I was grappling with when Pete and I were
going back and forth about the gum "tanning" process a week or so ago.
I've spent considerable time in the intervene, consulting with experts
and trying to determine what actually happens chemically. I've spent two
hours talking with a physical chemist and some time with a couple of
analytical chemists, and I searched for a used copy of Kosar on the net
and found one only two and a half hours drive away from me, drove and
got it and have taken in the chapter on dichromated colloids. So I know
considerably more about this than I did a week ago, and it's been nice
to find that what I've found out has reinforced what I already
suspected.
The short answer to your question is that it turns out no one actually
knows whether some of the reduced chromium compound is "transmogrified"
into the print or whether it all washes out. The old guys worked on the
theory that the "tanning" of the gum proceeded by exactly the same
mechanism as the chromium tanning of leather, but it's never been
sufficiently demonstrated that that's what actually happens in our
process, and in fact according to Kosar and according to my own
observations, the tanning theory fails to satisfactorily explain much of
what actually happens.
I have an alternative theory which makes more sense to me; I consulted
with the chemists to see if my theory was plausible and was assured that
it is a good theory and chemically sound. My theory says that the
reduced chromium compound is a contaminant which can be washed or
cleared out with appropriate clearing agents, but is not an integral
component of the image. I've already explained some of my reasoning in
the thread on "gum tanning process" and won't repeat myself or expand on
it here, since I get the idea that no one but me cares much about it,
though it's actually rather an important question. At any rate the
whole scheme will be explained in my gum printing manual.
It's an empirical question that could probably be resolved by some
sophisticated analysis. Since I'm not an analytical chemist myself, and
I doubt there's grant money waiting around for such a project, it may
have to remain unresolved.
Katharine Thayer
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