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

Goodness, things get more, and more, and more muddled.

I was referring below to things that have been attributed here to
Mike Ware by others, when I said that I believe his influence has
been partly responsible for the opinion expressed by some here that
gum hardening is at the paper surface. I was not referring to my own
correspondence with him, from which the below appears to be taken
verbatim, and which I have always referred to as speculations, just
as he refers to them. So while I think this was intended to be a
rebuttal to what I said, it in fact corroborates exactly everything
I've said on the subject.
Katharine

On Apr 14, 2006, at 8:42 AM, Christina Z. Anderson wrote:

> 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 20:08:15 2006