Gum: Supersize it

FWIW I've been testing some curves to make digital separation negatives
for gum printing and have made an interesting observation concerning
sizing and exposure. (Well, at least it is interesting to me.) I've
created a square 101-step file (0-100% K value) in Photoshop and have
been printing the file along with several adjacent Stouffer 21-step
wedges at each edge.

I'm trying to equalize printed pigment strengths and exposures for
C,M,Y, and K negatives so I've been masking part of the paper to guage
the exposure and strength of each separate color. When I print the
yellow layer I've masked off the areas I'll later place the step wedges
upon for the cyan and magenta layers. Then, I'll mask the printed
yellow step wedge area as well as the cyan area while I print the
magenta layer, and so on. As a result I have a step wedge printed on
the top that gets the full CMYK printings, one at left that gets cyan
only, the bottom is magenta only and the remaining side is yellow only.

What I've noticed is that the step wedge that is getting the full CMYK
set of exposures is building density slower than the areas which receive
only one color. In other words, if I start with yellow and get 6 steps
printed with it, when I do the next layer of magenta, the magenta may
print a distinct 6 steps in the area that was masked but only 5 steps
over the previous yellow area at the top of the file. Same goes with
the subsequent cyan and black layers: the effect where all colors are
accumulating is less than the individual exposure/development effect.

I'm guessing the previously exposed image is acting as a very efficient
size and affecting the subsequent density. This makes me think that an
exposed unpigmented gum/dichromate layer (ala Sarah van Kuernan [sp?]
and others) might provide a more predictable sizing effect. Looks like
I'll have to test that against my standard sizing procedure now.

On a related note, I overprinted a yellow layer on an actual pictorial
image today on a paper from the same batch. I gave the paper an
extended soak in hot water in order to remove some additional yellow
color. The border of the image was a half-inch dense black frame on the
digital negative. This frame prevents exposure of the underlying gum
layer and under normal cirmcumstances, the area beneath usually clears
to paper white without a hint of stain if I'm doing things right. (This
frame also aids in visually registering negatives on subsequent emulsion
layers.) However, when I printed the subsequent magenta layer today I
found the masked edge of the image speckled but the almost clear area of
the sky (minimal yellow and magenta components) within the image did not
speckle or stain. My guess is that the sky area received the equivalent
of a tanning exposure like the unpigmented sizing layer might produce
and that the hot soak removed the original gelatin size from the border
area previously masked during the yellow exposure. As a result, part of
the print speckled and stained while another essentially clear area did
not. This again leads me to think a minimally exposed yet unpigmented
gum layer might work better than hardened gelatin as a size.

The original size I added to these papers was 260 bloom gelatin hardened
in an aluminum sulfate bath. This size prevents staining with my normal
exposure and processing procedures.

YMMV.

Joe
Received on Tue Dec 21 19:20:42 2004