Re: Could someone summarize that gum up or down discussion?

From: "Katharine Thayer" <kthayer@pacifier.com>
> Well, I knew it would be dangerous to try to summarize that discussion;
> anyone who tried it would probably have got something wrong.
>
> But on reflection I think I probably was too succinct in at least one
> sense; I shouldn't have left out Mike Ware. He believes that gum on
> paper hardens from the bottom, and his speculations about that have had
> some influence here over the years. I don't know if he's right or not; I
> don't find his arguments particularly persuasive, as I've said several
> times, but he should be included when answering the question, why do
> some believe or postulate bottom hardening for gum.
> Katharine

To the list:
I received the following post today, and I have Ware's permission to post it
so as to clarify his actual opinion with his actual words. Fascinating
stuff...
Chris

Offlist post from Mike Ware:

It's deeply flattering to be cited as an "authority" on a subject about
which one has published absolutely nothing (I challenge anyone to find a
word of mine in print on the subject of gum dichromate printing). It's even
more amusing to be publicly refuted - and so soundly and authoritatively! -
for presumed opinions one has never held!

Just for the record, a year ago I unwisely shared, privately, some highly
speculative thoughts on the possible mechanisms of gum dichromate printing.
Here is a slightly emended and extended version of part of what I said then:

"According to the Beer-Lambert Law (see any photochemistry text) the
intensity of light, I, penetrating a homogeneous medium falls off
exponentially with depth, d. Recasting the equation in logarithmic, rather
than exponential, form:

log10(Io/I) = ECd = D the Optical Density

where Io is the incident intensity, E is the decadic molar extinction
coefficient and C the molar concentration of the absorber, and d is the
depth (path length).

Hence the notion of 'top-down hardening' - which certainly seems to apply
to carbon printing, as the evidence of the practice of 'carbon transfer'
would show, and the 'bas relief' nature of the images.

However, I would question the assumption that it also applies in just the
same way to the sensitized layer in a gum dichromate print, which is
prepared in a different manner.

In any coating of a normally absorbent paper surface with an aqueous
solution, the dichromate will be partitioned between the liquid and solid
phases (as in the technique of paper chromatography) - to an unknown extent,
which will be strongly dependent on the nature of the paper sizing, among
other factors.

Observations seem to suggest that dichromate ions (actually the photoactive
ionic Cr(VI) species is likely to be [HCrO4]- ) are quite strongly absorbed
onto cellulose (hydrogen-bonding would be a possible mechanism). Depending
on rates of ionic diffusion through the liquid phase, probably controlled by
its viscosity, this absorption will tend to set up a concentration gradient
of the Cr(VI) species, increasing with depth, to replace the initially
uniform distribution.

This implies that the gumbi emulsion layer, when exposed, is no longer
homogeneous, and probably has a higher concentration of the photoactive
Cr(VI) species just near the paper surface - a distribution which will tend
to work contrary to the 'top-down hardening' phenomenon. The Beer-Lambert
Law no longer strictly applies in this system, because of this concentration
gradient. It's probable that relatively more light is absorbed at the paper
surface in consequence, and therefore relatively more hardening goes on down
there than would be predicted by a homogeneous 'top down' model.

This is why I think a comparison with the method described in Maskell &
Demachy's postscript is quite interesting.

All I'm offering is a physico-chemical reason/mechanism for questioning the
assumption that gumbi prints harden 'top-down' just like carbon prints do.
I've no interest in prolonging the dialogue, it's all speculation anyway,
until someone performs some real science, like electron microprobe analysis
on transverse sections."

If anyone can interpret this commentary as advocating "bottom-up hardening",
then I'd be fascinated to hear from them.
Received on Sat Apr 15 20:03:22 2006