Re: tonal inversion and pigment loads

On Jan 12, 2006, at 1:31 PM, Christina Z. Anderson wrote:

> A couple of points:
>
> Why I think Mike Ware and Judy are right to link exposure in with
> the variables of why tonal inversion--the steps on the step wedge
> that invert, do so STEPPED--they go from light to dark just as they
> do from dark to light. If it were totally a factor of pigment
> stain, there would not be steps.

Sure there would. If for example the the pigment overload is
sufficient to cause the emulsion to flake off, and also sufficient to
cause staining, what you end up with is a thin layer of hardened gum
(left behind when the emulsion flaked off) that's differentially
hardened; this hardened gum acts as a resist to the pigment stain, in
an incremental way. Where there's no exposure (no gum) there's
maximum stain. Where there's a little gum, the stain is a little
lighter than where there's no gum. Where there's a little more gum,
there's a little less stain. Where there's maximum gum, there's no
stain. Hence: steps; hence: inversion. As I said before, of course
exposure is necessary to create an inversion, because exposure is
necessary to create an image. What's cool about the inversion is
that it's a tonal (though reversed) image, but the image itself isn't
made of hardened gum; the hardened gum only acts as a resist. The
visible image is made entirely of pigment, deposited as stain. It's
very much like a gumoil print, where the gum is a resist and the
positive image is made of oil paint, except that rather than applying
paint in a separate step, the pigment stain is deposited in the same
printing that gives you the gum resist. For gumoil you would use a
positive rather than a negative to create the gum resist; since in
this case you're using a negative to create the gum resist, you end
up with a negative image instead of a positive. But if you wanted to
play with this effect (if you could figure a way to reproduce it
reliably, as I can't seem to) you might want to use a positive
rather than a negative.

>
> As far as pigment load, I thought I would share my method that has
> taken me through 1000 (no s--t) prints: I mix 1 tube of 14/15ml
> watercolor paint (not pigment powder) into 45 ml gum--this makes
> 60ml solution. At time of use, I use this stock solution in this
> ratio:
>
> 1 tsp stock pigmented gum:1 tsp plain gum:1 tsp water: 1 tsp
> saturated am di.

It's interesting all the different ways to get to the same place.
Instead of mixing the same stock solution for all pigments, I mix
different stock solutions for every pigment. I don't measure, I just
dump in paint til the color looks "right" for that particular pigment
(usually at maximum color saturation, but less saturated for tricolor
reds and blues for example) and then I use that mix 1:1 with
saturated ammonium dichromate. I don't add more gum, unless for one
reason or another I want to print the color paler than the stock
solution, and I never add water.

Chris's basic mix looks to be about 6% paint, which is what I figured
both of our pthalo mixes to be when we had that earlier conversation
about pigment loads. But it sounds from this like she mixes strong
pigments (which I would consider pthalo to be) at less than that, in
which case I may be using more pthalo, which makes my caution about
not paying attention to labels ("a lot," "not very much") even more
appropriate. And I use considerably more of other pigments than I do
of pthalo. I've put a whole tube of pigment into 20 ml of gum, but
that was when I was trying to make a light and weak pigment (Bohemian
Green Earth in that case) stronger and darker by using more of it,
which is kind of like trying to make up for a unit loss on items by
selling more of them.

The point about pigment loads is that gum can easily handle any
pigment at full color saturation, with no problem. Once full color
saturation is reached, there's no point in adding more pigment. If
you keep adding pigment beyond color saturation, you will reach the
point where the gum simply can't hold all of the pigment in
suspension, and then you run into those weirdnesses we've been
talking about, like stain and inversion. People who have never run
into these things, as I said the other day, have been smart enough or
lucky enough to keep their pigment loads under the limit of gum's
capacity for holding pigment. I wasn't, in the beginning; I used too
much pigment and experienced a lot of staining, and that was on sized
paper. But once I understood that it takes less pigment than I
thought to achieve full color saturation, then I never had pigment
staining any more, and could print on unsized paper without a problem.

Tonight's Pineapple Express is roaring in right on schedule, and
especially ferocious tonight.

Katharine
Received on Thu Jan 12 23:55:18 2006