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

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 20:08:47 2006