From: Sarah Van Keuren (svk@steuber.com)
Date: 03/31/01-09:43:27 AM Z
>
> On Thu, 29 Mar 2001, Sarah Van Keuren wrote:
>
>> In gum printing the top of the gum bichromate emulsion can harden like a
>> layer of ice over a puddle but slough off entirely in the wash because it is
>> not firmly attached to the support. I expect that you know what I'm talking
>> about even if it seems cryptic to those who don't do gum printing.
>
and Judy replied:
> Sarah, I wonder about that. I've always ASSUMED that the coat was simply
> too thick, or too dark, that is that the light didn't reach down to the
> part that was connected to the paper -- or is that what you're saying?
> However, I THINK (tho haven't tested) that if you expose enough it will
> hold... tho maybe no *steps* just 2 tones...??? (And would certainly
> require contrasty neg so it didn't fog.)
>
> Judy
>
>
Judy, I think we're saying the same thing. Gum bichromate would have a
longer tonal scale like carbon printing if it weren't for the fact that gum
prints are exposed from the top instead of from behind. Even when the
emulsion is not too thick or too dark, there is usually a point at which it
sloughs off because the partially hardened emulsion is not attached to the
paper. With a halftone negative this may not happen, though I think it can
in dense areas with high resolution such as 200lpi. With a high contrast
negative it would not happen, as you wrote, because there would be just two
tones ‹ attached emulsion and unprinted paper base. I actually like the
short tonal scale of gum because of the way individual layers of emulsion
can be moved up and down the full scale and thus placed where desired. Now,
with digital negatives, masks can be made (if desired) to protect open areas
that would be overprinted as one exposed layers of emulsion into the denser
parts of a continuous tone negative.
I look forward to the upcoming issue of P-F where you show how length of
scale can be varied by increasing or decreasing pigment. It makes sense to
me from my own experience.
Sarah
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