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

Perhaps we could test some of the hypotheis put forward. For example: the
light penetration of the gum layer. With some assumption one could calculate
if the light penetrates to the paper interface or not. Anybody still
remember their freshman chemistry or physiscs. If I get bored enough with
gum transfer experiments I might do the calculations as well. For the very
thick, heavily pigmented gum layers that I am using for back printing there
is no visible ligt passing through and since carbon absorbs everything
indiscriminately no UV passes through as well. I will test that to make
sure.
Marek

>From: "Christina Z. Anderson" <zphoto@montana.net>
>Reply-To: alt-photo-process-l@usask.ca
>To: alt-photo-process-l@usask.ca
>Subject: Re: Could someone summarize that gum up or down discussion?
>Date: Fri, 14 Apr 2006 15:45:12 -0600
>
>Au contraire, no rebuttal at all...just stating Ware's actual words on the
>record re: gum hardening, which may or may not have to do with you or
>anyone else for that matter on this list.
>I just find the way he phrases it infinitely interesting and applicable to
>the topic at hand.
>Chris
>
>>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 20:09:05 2006