Understanding Gum chemistry (Was: Re: Sausage and dichromate

The mention of Mike Ware reminds me of another post I came across a
couple of weeks ago while skipping briefly through the archives from
the time I was away. I clicked on what I thought was a "next in
thread" link but it took me not to the next in thread but to the next
in time, which was on a different topic, so I left it and went back
to the thread I was following. But after a while I thought, "Wait a
minute. *What* did that say?" and went back and found it and read it
more carefully. Here's the reference:

/lists/alt-photo-process/2006/feb06/0002.htm

It says that Mike Ware had done "some extended study of gum" and "in
both theory and findings made sense of the hitherto unexplained."
This was a surprise to me on both counts, since I had some
correspondence with Mike Ware a year or two ago, when I was spending
a lot of time trying to find out what is actually known about the
chemistry of the gum process. He shared with me some hypotheses, a
couple of which have been discussed here before and which I haven't
found compelling because my own tests, observations and experience
don't support them; he also shared some findings from a student's
thesis research on dichromated PVA, which didn't particularly relate
to my questions.

I'm not at liberty to share the contents of the correspondence, since
it was a private correspondence, but I wouldn't say that anything he
shared with me suggested that he has done any extended study of gum
or has provided an explanation of the process. I would have been
glad if he, or anyone, had provided such an explanation, because I
like being able to understand how things work, and it's been
frustrating to me that so little information is available. I'd be
glad to be able to lay these questions down and be able to finish and
upload my web page on the chemistry of the gum process. So if there's
information out there that would clear this all up, I'd be very very
happy to know about it.

I do agree with another statement in the above-referenced post, to
the effect that knowing how the gum process works won't make anyone a
better gum printer, but I strongly disagree with extending that
argument to say that since understanding the gum process won't make
anyone a better gum printer, we shouldn't be interested in, and
shouldn't discuss here, issues that might lead us to a better
understanding of the mechanism of the gum process. This forum is
hosted by the University of Saskatchewan "as a service to the
academic community" and it seems to me that trying to understand how
the gum process works is a legitimate concern of the academic community.

In the meantime, misconceptions and outdated ideas about the
mechanism of the process continue to be repeated here, which I don't
think serves our community well. For one example, it is often stated
here that the crosslinked matrix involves Cr(III), but there is
little actual research evidence to support this widely-held belief,
and a recent body of research suggests strongly that for dichromated
PVA, (the research literature on dichromated PVA is the closest
approach we have to understanding the mechanism of dichromated gum),
the chromium involved is Cr(V). (See for example Bolte et al, 2005.
"Hologram formation reconsidered in dichromated PVA: polymer
crosslinking around chromium (V). In Practical Holographyy XIX:
Materials and Applications, Proc of SPIE vol 5742, p. 195-204.)

I hardly think it can credibly be said that we already know
everything that needs to be known about the mechanism of the gum
process, and that anyone who is still trying to understand it is
doing the equivalent of re-inventing the wheel. The truth is that
while we know some general outlines, we know very little about the
specifics of the process, and IMO it doesn't serve us well to pretend
to know more than we do.

Katharine
Received on Sun Mar 19 14:52:50 2006