Re: Tanning theory of dichromated colloids (was gelatin

"...which according to Manivannan et al doesn't establish whether the
chromium is bonded to the gum."

I assume that's the Canadian group (Lessard, Couture, Changkakoti, Bolte,
Solano, Capollo etc.)...

Martin

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Friday, May 07, 2004 5:32 PM
Subject: Re: Tanning theory of dichromated colloids (was gelatin

> Ryuji Suzuki wrote:
> >
> >
> > > My goal is to show that the "tanning" model is not at all useful to
> > > explain the chemistry of dichromated gum.
> >
> > What an anti-scientific mind it is. You already have your conclusion
> > and you are simply looking for whatever pieces you can use.
> >
>
> 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.
>
> I don't of course deserve that insult above, not even a bit of it. I've
> spent most of my life learning and practicing a scientific mode of
> thought, and have hardly abandoned it suddenly now in spite of my
> unfortunate turn of phrase above. Reading it over, I can't even parse
> what I thought I was trying to say. Sometimes my fingers type different
> words than I'm thinking; perhaps that's what happened there. As a matter
> of fact, I am still quite open to the idea that the tanning theory works
> for gum; I'm certainly not invested in thinking otherwise. But only data
> will convince me.
>
> You and I have already discussed the question so there probably isn't
> much more for us to say to each other about it. You've already sent me
> an article about dichromated PVA which you claimed would prove to me
> once and for all that chromium complexes with gum in the gum process,
> but there was this slight problem, that the article stated quite clearly
> that the researchers were not able to ascertain whether the chromium
> complexed with the hardened PVA or not. So no, I wasn't convinced,
> sorry.
>
> 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. 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.
>
> I had hoped that there would be a way to do a quantitative analysis on a
> sample of a finished and washed gum print, based on the expected value
> of chromium to be found if the theory works or if the theory doesn't
> work, to finally answer the question one way or the other. But
> unfortunately we don't know enough to know what the expected value would
> be, and the usual methods of stoichiometry fail us when dealing with
> entities whose molecular weight is partly 50,000 and partly 250,000 and
> partly somewhere between a million and two million, give or take a
> couple hundred thousand.
>
> 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. 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? for example. You have said elsewhere that chrome
> alum is not a very good hardener for gelatin. I've demonstrated myself
> that it's even less good for gum, requiring at least 10 times more
> chrome alum as for gelatin to produce even minimal hardening. And yet,
> gum hardened with dichromate and light is extremely well hardened with
> very little dichromate. So I'm not getting why it's so obvious that gum
> and dichromate must work exactly like chrome alum and gelatin.
>
> I've been corresponding with an expert in gum arabic who says there's
> no compelling reason why the chromium has to be bonded to the gum, and
> I've also been told now by two physical chemists that the crosslinking
> could occur without chromium being directly involved in the matrix. If
> someone could give me some compelling reason to believe it does, I'd be
> very happy to be persuaded. But someone who will invent a fictitious
> transparent form of chromium to make the theory work rather than to
> consider the possibility that the theory might be wrong, is operating
> more on faith than on science, and is going to have to accept that I'm
> not convinced. I'm not from Missouri, but you've still got to SHOW me,
> not just tell me, why it is so. This is what science is about.
>
> Katharine Thayer
Received on Sun May 9 12:53:40 2004