From: Sandy King (sanking@clemson.edu)
Date: 07/14/03-02:53:40 PM Z
Chris,
>
> But get this, speaking of weird thoughts: I came across several times
>in different authors, both current and past, that at a certain point, the
>concentration of the dichromates does not increase speed!! In other words,
>once past 5% it doesn't really matter much!!! Now, I have NO CLUE why some
>people would say that or think that but it is yet another thing to test!
>Because, if it is true, I'd save a lot of dichromate. I know Suzanne Izzo
>uses her concentration less than saturation. Maybe at a certain point it
>does its hardening thing and past that it doesn't matter. Please note I am
>NOT saying this is true--but this, along with the bleaching of the sun on
>dichromates, seems so bizarre to me and contrary to logical thinking and
>what others say on this list if I remember, that I can't leave it alone as
>a fallacy--aside from the fact that if it IS true, it would be of benefit.
>Hence the testing. I think Sandy has probably done this type of test of
>dichromates at different concentrations, or maybe you have, and can dispel
>this heretofore myth (or confirm its truth!) and chime in here.
In carbon printing there is an increase in speed, and decrease in
contrast, in dichromate concentrations beyond 5%, but the increase is
only useful up to a maximum of perhaps 10% with extremely contrasty
negatives. By extremely contrasty I mean those negatives that have a
density range of more than the typical log 3.15 range of most step
tablets.
What happens with carbon is that as you increase the dichromate
percentage over about 6% the resulting image becomes very flat, and
there is a greatly increased probability of staining. I suspect that
the use of saturated solutions in gum printing, together with very
efficient lights such as UV tubes, is one of the reasons for image
fogging. In carbon for sure we us strong dichromate solutions with
very weak light, and weak solutions with very strong light.
> And, furthermore, Nadeau, as does Gassan and a few more people, points
>out the fallacy that dichromates are not sensitive when wet, a fallacy as
>Nadeau says has been continuing for 80 years. So there are lots of these
>things floating around, as we all on this list know.
It is true that dichromates are sensitive when wet, but with carbon a
wet sensitized tissue is much less sensitive than a dry one, and this
on the order of several stops. In other words, a tissue that might
need 10 minutes of exposure when dry might require up to 40 or 80
minutes if exposed when wet.
Sandy
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