Re: Tanning theory of dichromated colloids (was gelatin

"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."

Speculating about the effect of chrome alum on gelatin, it might be
of the same kind as the dark reaction.

How much is "very little dichromate" ("...is extremely well hardened
with very little dichromate.")? I wonder how that quantity compares with the
fastest
red sensitive DCG system.

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