Re: Tanning theory of dichromated colloids (was gelatin

From: Katharine Thayer <kthayer@pacifier.com>
Subject: Re: Tanning theory of dichromated colloids (was gelatin
Date: Mon, 17 May 2004 18:44:47 +0000

> But by sending me a paper that was supposed to prove that the chromium
> complexes with PVA, and then the authors said clearly that they couldn't
> assess whether the chromium complexed to the PVA, you lost considerable
> credibility with me. I realize that mistakes can be made, (Lord knows I
> make them all the time) but you haven't said anything since that to
> restore my faith. The invisible chromium was especially detrimental to
> the faith-restoring effort.

I just sent you whatever related at hand in PDF form. I have file
cabinet full of papers related to chemistry of photographic process
but I'm not your librarian or consultant to find out exactly what you
ask for. I just do what I can do with limited time. Remember my main
business is something unrelated to photography and my main
photographic interest is silver gelatin process and making prints with
this process. I don't have a good organization of papers for
dichromate processes and iron based processes. I just have a mixed
drawer for those and it takes more effort to recall which paper
answers your question most directly.

At least, the paper I gave you cited important works which you could
get on your own. If you are in scientific field you know that is where
people spend most time, next to their experimental work.

At that time you didn't ask the same question. You wanted to know if
chromium is contained in the washed image-forming material. Complexed
is a different question, because it requires that chromium be
covalently bonded at its coordinates. Anyway, Ducalf they cited was
enough for the former question and Grimm et al papers show enough for
both of them. What Grimm et al's conclusion is coming to is that in
PVA model molecule, hardened products are crosslinked with ordinary
hexacoordinate chromium complex at carboxyl groups, very analogous to
the case with gelatin.

> For anyone else who might be reading this, the observations that make me
> wonder about whether chromium is present in the image are these: (1) gum
> can be completely hardened without any visible chromium being present,
> and (2) when visible chromium is present, it can be completely removed
> without altering the image in any way. There is one argument that would
> allow these two things to be true even if chromium does form a part of
> the complex, and that is that so little chromium is needed to link the
> molecules together that there's just not enough there to see. This is
> certainly possible, but must be demonstrated empirically if it is so.

Gelatin can be hardened by chromium without visible color. I think
this is a good enough counterexample to abandon the "visible color"
criteria. Anyway, gelatin case is well studied, and major papers are
cited in the book chapter I sent you. (More recent papers on gelatin
hardening shifted their focus to glutaraldehyde and other aldehyde
hardeners, as well as epoxides, peptide couplers,
1,3-bis-(vinylsulphonylmethyl) ether, etc. and not much more work on
chromium hardening. I frankly think there's no much need to study
chromium hardening with gelatin anyway.) Grimm et al did the PVA case
using a model molecule which is basically short version of PVA. If you
want to study the gum arabic case, you know what kinds of experiments
are necessary. Whether you do it or not is up to you.

> One reason among many to question whether the tanning process is an apt
> model for our process is the fact that when collagen is chrome-tanned,
> it turns blue, due to a chromium compound that is either complexed with
> the collagen or deposited in it.

Can you tell me what kind of concentration of chromium they use and
how it compares with the amount needed to harden simpler molecules
like gelatin, oxidized PVA or oxidized gum?

The "tanning reaction" mentioned in hardening reactions of binders
means not much more than crosslinking. It seems that you are expanding
the term as whole "leather tanning process" and negating this doesn't
happen in photographic binders. It's not surprising to me.

> If the chromium is in the image, why
> doesn't it color the image with characteristic chromium colors?

The amount used is very different, that is one obvious factor. The
"color" of chromium compounds mentiond in literature as well as what
your friends said, are either in aquaous solutions, dry form, or in
rather insoluble compounds that are precipitated out (like dyes and
pigments that contain chromium). In dried organic polymeric matrix,
this is a different story. Leather tanning as well as embalming and
other tissue fixation has many different requirements than hardening
of photographic binders and I wouldn't assume much more than the basic
chemical reactions to apply to your problems.

> Sorry we missed on that. I can't get papers from my university library,
> if the library doesn't carry the journal (as an alum I'm not entitled to
> services like interlibrary loan, or even to the use of the photocopy
> machines if the library does carry the journal, which I consider cruel
> and unusual punishment) but I recently discovered that my tiny sleepy
> little library down the road can order photocopies for me, which is
> wonderful though it takes forever.

That's surprising. In Eastern Massachusetts and Providence, RI area,
non-academics can register for interlibrary patron status at a public
library, and you can go to many university libraries (including
private schools, but except Harvard University libraries, for which
you must have current affiliation). Harvard libraries photocopiers
need money on the University ID, but you can just buy prepaid card
from the vending machine for photocopiers in MIT libraries... (Or one
can load a camera with copy film and bring a light copy stand...)

--
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)