Re: Tanning theory of dichromated colloids (was gelatin

From: Katharine Thayer <kthayer@pacifier.com>
Subject: Re: Tanning theory of dichromated colloids (was gelatin
Date: Fri, 07 May 2004 15:32:34 +0000

> Yoiks! Are you now arguing that the tanning model is useful for
> explaining the gum process, just to be argumentative? According to
> everything you've just said about how useless leather is even for
> explaining anything about gelatin, I'd have thought you would agree
> that it's even less likely that it would be useful to explain
> anything about gum, which was my point.

See, you're predetermined as to what your conclusion is and just
trying to collect whatever fits with your theory and not making fair
effort to see all possible reactions gum and dichromate can engage.

> And you've argued Duncalf and Dunn, which I am waiting to read
> myself before I draw any conclusions about its utility, but which
> according to Manivannan et al doesn't establish whether the chromium
> is bonded to the gum. It's hard to see how this study could address
> that question anyway, since it's pretty obvious even from the brief
> summaries that it's a qualitative study. There's always going to be
> some trivalent chromium there; the question is whether it's bonded
> to the colloid or not.

See, you're predetermined that their study is useless and irrelevant
before reading it. Their study is quantitative. Their data show that
the amounts of chromium washed out from exposed and unexposed
materials differ. At the time you wrote me before, you were arguing
that the final washed image does not contain chromium, so I said that
data is enough to show that chromium remains.

Even if you want it be shown whether chromium is chemically bound in
finished gum, you can't criticize other works not showing it. And you
can't take lack of this data as a proof of chromium not bound to gum.

> And in fact I've come across a citation (I've sent for the paper but
> haven't seen it yet) to a qualitative study that found NO chromium
> in gelatin hardened by the dichromated gelatin process.

What's the exact citation for this?

> You've argued that the process of dichromated gum must necessarily be
> the same as the process of hardening gelatin with chrome alum, without
> giving any evidence or compelling logic that this must be the case.

I said the chemical reaction closely analogous to chromium (III)
hardening of gelatin is very viable in gum as well and this
possibility should not be ignored until shown otherwise.

> You say, so dismissively, "consider the building blocks." Well, I've
> considered them, and I don't get it. In the one case carboxyl
> groups, in the other, what?

Are you saying there is no carboxyl group available for reaction in gum?

Anyway, if you believe that gum hardening in dichromate process is in
no part related or analogous to chromium (III) hardening of gelatin,
feel free to believe it. Just don't tell me and others that is already
the truth. Also, I think it's unfair that you criticize people who
deal with gelatin for them not solving your problem. At least a lot of
gelatin literatures give you several ways you could approach this
problem. One can selectively disable some functional groups and see
how that affects the gum dichromate process. One can use radioactive
tracer. One can control pH and see its effect in relation to the pK's
of functional groups. Tracer study is difficult, but the other two
aren't impossible to do in a small lab. Indeed, I use those techniques
combined in a small darkroom as a part of my silver gelatin emulsion
making because it makes the work flow much easier and shorter than
other methods I used to use... nothing now, photographic manufacturers
started using that technique in 1960's. I'm just 40 some years behind.

--
Ryuji Suzuki
"You have to realize that junk is not the problem in and of itself.
Junk is the symptom, not the problem."
(Bob Dylan 1971; source: No Direction Home by Robert Shelton)