Re: Gum : dichromate concentration

> > Yes, after the first 10 minutes all the dichromate seems to have leached
> > out, tho the pigment has generally not begun to go.
>
> Funny, I started to say in my post yesterday that the dichromate usually
> goes completely before the pigment starts going, and then I thought it
> might generate another unnecessary argument, so I deleted it. But this
> is my observation as well.
> k
>
Judy, Katharine, Keith, Joe, Sandy, Dave, Clay, whew!!,

As per above, so does mine; the dichromate seeps out from under the gum
immediately upon hitting the water, but is not immediately cleared from the
overexposed borders when that happens.

Keith, I use am di, not pot di, which is speedier and less contrasty.

Dave, I have never disagreed with the assertion that the less am di the
greater the contrast and the slower the speed. I am ONLY talking about
diginegs here, OK?? And throwing out the related question of "at what
point of am di concentration is speed and contrast sufficient?"

Let me describe my negs: they are VERY thin negs, I will bring one in and
measure on a densitometer, but they are thin and low contrast enough to look
like they wouldn't print well. This is perhaps another variable. There is
no way they would ever print on silver paper. If I remember correctly,
Sam's method is molded around using this very thin, low contrast neg. I
have also read in the past somewhere that the Edwards BL units (blue light?)
print very low contrast and "are not suitable for gum", so it could be that
is another factor in the mix, although seems that Sandy thinks not, and that
doesn't explain why Sam's results are perfect with the NuArc, nor why
Suzanne Izzo is able to print outside in sunlight so successfully with an
even lesser dilution of pot di.

><Katharine said>Thanks for the clarification. This is what you said:

>"The first attempt was the same thing--a flat image. THEN I realized my
>error. I was ASSUMING that such a low dilution would necessitate a
>longer
>exposure--NOT the case."

>It doesn't seem unreasonable to read that statement as an assertion that
>a low dilution does not necessitate a longer exposure, but ..... we'll
>just say, my mistake, and leave it at that. Sorry,

No, certainly not unreasonable. Yes--I should have said "way longer
exposure" and maybe that would've been clear...but I thought I made it clear
by the rest of the post. Whatever, I hope that we are all clear on this
point. When I get to take a breath (probably it'll take the same amount of
time as writing this post is) I will expose the analog neg to the solution
and give you a report on the exact time it exposed in, for comparison's
sake.

I'm gonna guess that most of this has to do with the digineg. Sam and I are
both using Epson 2200s.

How about we all agree on this: **if** a gum printer is printing diginegs
with a saturated ammonium dichromate solution, ***unsuccessfully***, because
it is printing ***so flat as to be unusable, and taking way too long to
develop***, then he/she can cut the am di down to a 6% solution and use
that. Is that possible to agree upon? But, of course, everyone here
already knows this--just I didn't...dare go...so low....

However, I, too, as Sandy is, am puzzled by the incredible variations we are
reporting, which is probably exactly why every time the subject of gum comes
up on this list we all get tizzied up. Maybe it leads back to the
acidity/alkalinity thing that we talked about a while back...and, as Judy
was reporting a 3-6 stop variation of exposure in different gums, that might
account for some of it. Who knows.

Back to my diginegs, this time for screenprinting :)

Chris
Received on Mon Dec 1 15:22:42 2003