Re: Tanning theory of dichromated colloids (was gelatin

From: Katharine Thayer ^lt;>
Date: 05/17/04-12:44:47 PM Z
Message-id: <>

MARTINM wrote:
> "Grimm et al also showed one more step from there. If the dichromated
> PVA does not contain certain amount of moisture, this PVA-Cr(III)
> complex undergoes further photosensitive reaction, which loses water
> and chromium drops out of the PVA complex and renders the PVA soluble
> again. This, they say agrees with the experience from phosphor coating of
> color CRT's <snip>
> That's interesting. This seems to be confirmed by dichromated gelatin
> systems. Actually, relatively recently a Russian group introduced even kind
> of a wet recording system: a mixture of dichromated gelatin/glycerol is
> exposed to yield a real-time image (no further processing required).

This is actually quite interesting, if I remove the illogical attempt to
relate it to gum printing.

I haven't heard anything in my years of participating in gum discussions
that would suggest that this has any relevance to gum printing in
practice; people certainly print gum in the arid desert and I've never
noticed anyone mentioning that their gum prints dissolve because of the
driness. But just the same I'll want to see how Grimm et al know that
the chromium is in the complex and how they know that it has dropped

I'm not sure I understand how Martin's example relates to the Grimm
research reported above.

I've just realized that the entire original message from Ryuji is
attached below, so I've read it and as long as I've read it I will
comment on it; comments embedded in text below.


> ----- Original Message -----
> From: "Ryuji Suzuki" <>
> To: <>
> Sent: Friday, May 14, 2004 8:57 PM
> Subject: Re: Tanning theory of dichromated colloids (was gelatin
> > From: Katharine Thayer <>
> > Subject: Re: Tanning theory of dichromated colloids (was gelatin
> > Date: Fri, 14 May 2004 07:05:50 +0000
> >

> >
> > Anyway, you have been negating complexing of chromium with gum (and
> > PVA) for a while. I read over old emails just now and it's very clear
> > that your argument was towards that direction.

This is not the case, whatever you wish to believe. My question has
always been, "How do you KNOW that the chromium complexes with the
gum?" and I am completely open to the answer, whatever it is, as long
as the answer is consistent with rules of evidence. I wouldn't have
asked you the question if I hadn't wanted to know, for crying out loud.
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.

The reason I've ever asked the question is that the complexing of
chromium doesn't seem consistent with what I know about gum printing,
and that's why for five years, I've been asking the question here and
elsewhere: How do you KNOW that's what happens? So far the answers have
been underwhelming, to say the least. People have answered the question
by sending me the famous paragraph from Eder, as if that proved
anything, or as in Ryuji's case, sending me a paper that doesn't prove
what it is claimed to prove. Since I will only ever be convinced by
data, I need to see some data. If that strikes you as being biased in
any particular direction, then you and I have very different ideas about
what constitutes a scientific mode of thought.

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.

Another argument I was offered to account for these observations was
that the form of chromium that participates in the crosslinking of gum,
or of gelatin for that matter, must be colorless and that's why you
don't see it. I've spent quite a lot of time studying about chromium and
talking to analytical chemists who use chromium a lot in their work, and
nobody seems to know anything about any colorless forms of chromium
("That's why it's called CHROME-ium," said one chemist) so I think that
explanation is just not credible.

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. Most of what I've read on leather
tanning seems to support the consensus that the chromium is only
deposited in, rather than coordinated with, the collagen. The latest
thing I'm working through seems to provide better support for the idea
that it the chromium is not just deposited, but I'll need to understand
how that is consistent with electron microscopy that shows the chromium
distributed in random clumps rather than uniformly, even though the
leather is uniformly tanned. But whether it's deposited or complexed,
the question remains, if this is the right model, why don't our prints
turn blue when they are "tanned"?

One analytical chemist I know was really interested in my prints from
the beginning. All he knew about the process was that chromium was
involved, and he assumed (I tend to like to print in blues, greens and
browns, all of which he recognized as chromium colors) that the magic of
the gum process was achieved by controlling the chromium to produce the
different colors. He was terribly disappointed when I explained that the
colors were from added pigment, and that the chromium in the process
doesn't color the print. "How could it not?" was his response, and
that's still what I want to know. If the chromium is in the image, why
doesn't it color the image with characteristic chromium colors?


And recently you said
> > that "tanning" reaction cannot happen because there is not enough
> > carboxyl groups in gum.

I don't think I said that exactly, or even remotely. I've never said
that the "tanning" reaction cannot happen in gum, I've only asked what
the evidence is for it. My remark about the carboxyl groups followed a
stronger remark about amine groups; the carboxyl comment was much more
moderate, but neither was a categorical statement. I questioned strongly
whether amine groups are involved in gum crosslinking, because there is
almost no such material in gum, and most of that is buried deep in the
molecule. And then I asked a milder question about the carboxyl groups.
It's interesting that you chose to challenge me on the carboxyl groups
rather than the amine groups, because obviously no amount of dichromate
is going to be able to create amine groups in gum.


> >
> > Can you explain the reason why you think Cr(III), which is soluble,
> > cannot be washed out in the way Duncalf and Dunn did, even if it is
> > not complexed with insolubilized gum?

Every time chromium goes down a step in valence, my physical chemist
consults tell me, it becomes less soluble, and that certainly is borne
out in gum practice, where if visible Cr(III) is left in the print after
washing it must be removed with a clearing agent such as sodium
bisulfite or potassium metabisulfite. In other words, not all the
Cr(III) is easily soluble, even if it's not part of the matrix. As to
Duncalf and Dunn, I wouldn't expect there to be a lot of Cr(III) in a
washed print if chromium doesn't coordinate to the matrix; I was
thinking trace amounts. But in a qualitative analysis, you wouldn't know
whether it was a little or a lot, was my point.

> >
> > At least one more paper cited by Mannivannan et al (besides Duncalf)
> > studied this mechanism and they said that up to 3 terminal carboxy
> > groups (they showd the cases of alpha-ketonic acid and beta-hydroxy
> > acid) are complexed around hexacoordination of Cr(III). It's Grimm,
> > L., Hilke, K.-J. and Scharrer, E. 1983. The mechanism of the cross
> > linking of poly(vinyl alchohol) by ammonium dichromate with
> > U.V.-light. J. Electrochem. Soc., 130, 1767--71. Also get the other
> > papers from the same group for more details, because this paper only
> > gives a roundup of their findings. I don't know why Mannivannan et al
> > did not discuss their results, especially because the results
> > presented there are compelling.

I will send for this paper, thanks, and of course if they actually
demonstrate the complexing of the chromium I will be very happy. All
I've ever wanted is to KNOW; I don't care what way the evidence comes
down, as long as it finally comes down. But as I said before, I won't be
convinced by someone telling me that it's so; you have to show me that
it is so.
 (which I was going to send you by fax but you said
> > your fax didn't work or something. Now you figured out how to get
> > papers from your library so you can get them.)

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.

Received on Mon May 17 19:40:43 2004

This archive was generated by hypermail 2.1.8 : 06/04/04-01:20:53 PM Z CST