RE: Back-exposing on plastic (was: Re: Gum transfer

Adjustment #2 is not that easy to do. I used to think that when I started
with gum because I thought everything was linear simple math, but I soon
found it wasn't.
 
I started with a simple mix and got a nice gradation though the Dmax wasn't
as black as I liked. It was like a dark gray. So I thought "that's simple,
just add a little pigment." So I did and make another print, and the print
turned out to be lighter! I thought it must be my inexperience in coating,
so I tried a few more and found that they were also lighter. Then I realized
that it was because with heavier pigment, there was a filtration effect, so
the step tablet was underexposed.
 
So I had to double my printing time. It was better, but I still couldn't get
the maximum black. So I exposed more (3x), I still could not. That was
because to achieve the darkest black the UV has to penetrate through the
black pigment, and the deeper it has to go through, unfortunately the more
pigment it has to cut through. The effect is that you have a long long
shoulder on the shadow area.
 
But that time, my exposure time is about 2 hours. I still hadn't got to
maximum black with that coating, and I also hadn't got to maximum black that
I desired by adding more pigment. By step tablets I could estimate that if I
added more black pigment and wanted to achieve the max black, my exposure
time is going to be at least EIGHT HOURS!
 
I did expose one with 4 hours and got a beautiful gradation of about 10
steps! but the gradation is not linear, so one must make a perfect negative
that match the shadow, midtone, and highlight perfectly in order to use the
system. I suppose with a perfect negative and exposure of 8 hours, one might
be able to get a perfect single-coat gum print (perfect in a sense that
every or most tones fall to or close to the desired density).
 
That was when I decided that two coat gum is probably going to achieve the
same thing easier and even faster. You can use a (or two) less-perfect
negatives, but with flashing and different strengths of pigment for each
coat, you can achieve what you want easier. The only thing is that you shift
the problem from making perfect negative, perfect coating, and perfect
exposure to the mechanical problem or registration with multiple coating.
 
 
Dave S

  _____

From: Ender100@aol.com [mailto:Ender100@aol.com]
Sent: Thursday, May 04, 2006 12:48 PM
To: alt-photo-process-l@usask.ca
Subject: Re: Back-exposing on plastic (was: Re: Gum transfer

Hi Katherine,

Thanks for your response, I understand now what you were saying regarding
the tonalities with gum-that's what I thought you meant originally, but my
thick brain intervened.

It would seem then that to get the "best" one print gum, then one would have
to play with some variables, and you gummists have probably already done
this.

Starting with the best paper/sizing combination which prints smoothly with
longest tonal scale, adjust the following variables until you get the
longest scale on a stouffers, the highest Dmax, and smoothest tones.
Obviously there would be tradeoffs.

1. Adjust amount of dichromate higher for longer tonal scale until no gain
is seen.
2. adjust amount of pigment higher for greater dmax until tonal scale
begins to fail.

Other variables you could tweak?

In a message dated 5/4/06 10:31:00 AM, kthayer@pacifier.com writes:

Mark, I understand the theory here, but I also know gum, and I repeat
that I have yet to see a one-coat gum that prints tones from dark to
light, with all the tones in between. Maybe a better way of saying
that is that given the kind of pigment load that you need to get the
deepest DMax, the emulsion is going to be contrasty enough that there
will be gaps between the tones rather than a smooth gradation.
Surely you can see for yourself that Chris's gum print is jumpier in
tone than the platinum print; throughout the portion of the scale
that they both print, there are gradations of tone in the platinum
print that just aren't there in the gum print. So while it seems
theoretically to make sense to say that gum should print all the
little tones between the DMax and the DMin, the fact is that it
doesn't, not in one coat. Like I keep saying, I'm ready to be
pleasantly surprised, and this is what I'm aiming for with the back-
exposure experiments, but I have yet to see a one-coat gum that can
print a delicate gradation of tones throughout a full tonal scale.
Yes, of course, a one-coat gum can print a rather contrasty
approximation of a fully tonal scale, I would have thought that went
without saying, but like I say, there are "fully tonal" scales and
then there are fully tonal scales.

I do think Chris could get a better approximation to the smooth tonal
gradation of the platinum by using less pigment, but then the
question is whether you can get the DMax. Maybe, maybe not. And this
is the struggle always with gum, as Sandy and Terry and many others
have pointed out. More pigment means deeper DMax but more contrast,
less pigment means more subtle tonal gradation but less DMax. This is
the eternal equation of gum; it's got little to do with curves. Yes,
if you've got a really bad curve to start with, as Chris has here,
you can improve it, But you can't improve it beyond the limitations
of the particular emulsion you're using, which I suspect Chris may
have demonstrated in the prints she posted.
Katharine=

Best Wishes,
Mark Nelson
Precision Digital <http://www.precisiondigitalnegatives.com/>
Negatives--The Book
PDNPrint <http://groups.yahoo.com/group/PDNPrint/> Forum at Yahoo Groups
www.MarkINelsonPhoto.com <http://www.markinelsonphoto.com/>
Received on 05/04/06-09:56:54 PM Z