Smieglitz@aol.com
Date: 04/25/00-05:32:11 PM Z
Hi Garet,
It was a continuous tone, flat negative specifically developed to print well
in gum. (Halftone negatives print much easier in my experience, but I don't
like the resultant look.) I tried to hold everything but the light source
constant in the testing because I want to standardize, not clear one print
for 10 minutes, another for 30 minutes or another overnight. And yes, I
varied the exposure times until I was satisfied I had achieved maximum
density along with adequate clearing of the highlights. The distance of the
quartz light was varied in previous tests until max D and clearing matched
that of the UV exposure source given a standard 30 minute autodevelopment. I
choose yellow as the pigment specifically because yellows are more opaque and
harder for me to print as a result of their wavelength absorption. (I even
ran a spectrographic analysis of the K dichromate absorption spectrum and
adjusted the UV lamp exposure to print unpigmented gum optimally which
assumes that the K dichromate sensitivity correlates with the actual printing
speed of a pigmented gum emulsion. I don't think I overlooked anything in
this test.) The results are shown in the scan. The picture is worth a
thousand words....
The quartz light simply printed better for me. The *midtone contrast is
flat* with the UV tubes. The max D is the same and the highlights clear with
both light sources at a 5 minute exposure and 3 10-minute water baths for
clearing.
For years I've used the UV tubes and have achieved some good results. But,
my success rate was nowhere near what I wanted it to be. I've changed
pigment brands, papers, coating techniques, sizing, you name it. The only
thing I never thought of changing was the light source. I took it on faith
that that UV tubes were the way to go since *every* alt text I've ever seen
recommended them. They work fine for cyanotype, VDB, and probably Pt/Pd,
but, I now believe based on years of experience and testing a seemingly
infinite number of permutations in materials, that the quartz halogen, metal
halide lamps, and the sun are much, much, much, much better for gum printing
than the UV tubes. Several tests after the one I scanned also indicate this
to be the case. I don't want to keep testing and speculating. My brain
hurts. I want to move on and use my eyes now. I want to print some images.
I won't be printing gum with UV tubes again.
Joe
In a message dated 04/25/2000 17:47:59, garet@rmi.net wrote:
<<Joe,
I remember from your tests of several months ago that perhaps there
was a clearing issue. The BL vs. Halogen light sources in your tests
are certainly printing differently, but I still wonder if it is more
than just a speed issue. Did you try different lengths of exposure?
Did you try clearing the BL prints longer? Perhaps the different
light source affects the clearing rate (pure speculation)? I seem to
remember you mentioned a 30 minute clearing bath. My prints sometimes
don't even start to clear within 30 minutes. Not that I would wish an
8 hour clearing time on anyone, but it works for me and my prints do
clear given enough time (yes, Judy, even with Joe's rabbit skin glue
size which I have been using because it clears better than others).
>Just a thought. Assume a layer of particles. Light striking straight
down
>would expose less just beneath the particle than a diffused source
which
>could angle around and expose under it.
>
>--Dick
>
This may have something to do with it also, particularly since you
used a yellow pigment, which should tend to absorb more of the
blue-ultraviolet waves than other pigments. The diffused source would
tend to cut in under the particles more, as Judy said. I'll also
second what Judy said about the time lag. I've coated two sheets at
once and exposed one up to 48 hours later with only a barely
perceptible decrease in clearing of the highlights (granted it was
cool weather and typical low Rocky Mountain humidity).
>Halogen lamps appear to print cleaner, with better separation and
>contrast with respect to the gum emulsion.
Odd, it seems that most of us are going for less contrast (longer
range) in a gum print. Or was this from a half-tone negative?
>
>>
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