Re: tonal inversion and pigment loads

On Jan 26, 2006, at 4:08 PM, Tom Sobota wrote:

> Probably the reasons for tonal inversion are simple and
> straightforward, once we understand them, but for now the situation
> looks pretty complex and not easily interpretable.
> Please look at this test image: http://usuarios.arsystel.com/
> tksobota/Inversion-1.jpg where in the leftmost strip we see not one
> but two separate inversions:
>
> 1. From step 1 to step four we see the expected gum response to
> increasing negative density.
> 2. From step 8 to step 16 we see an inversion with several well
> marked inverted steps. The letters are black.
> 3. From step 17 to 21 the black letters turn to white, but the
> inverted background does not change.

Hi Tom,
Again we see different things here. I see the normal tones as you
describe (1) with the numbers in steps through 15 being part of the
normal tones, the stepped inversion (2) except that it's not a
complete inversion except in steps 17 through 21, where the gum
emulsion has washed or flaked off the numbers.
>
> In the middle strip this 'secondary' inversion of the letters has
> disappeared, except possibly in step 21. In the leftmost strip no
> trace remains. The only difference between the three strips being
> exposure I can say positively that yes, at least _some_ inversion
> is affected by exposure.

And I'd just say that in the middle strip, for whatever reason, the
gum numbers stayed put and didn't flake off, although it looks like
21 is starting to go. I guess we'll have to agree to disagree about
what to call this, because to me this is just what I was saying
before, that with more exposure you just move the inversion up the
scale by blocking the lower steps, but nothing is really changing,
you've still got stain expressed as inversion expressed in the same
tones, the stain tone is just moved farther up the tablet. You
haven't changed anything about the stain or the inversion by exposing
it longer, it will still look the same; all you've done is block up
the tones of your gum emulsion, makiing a longer development
necessary, but the stain will remain as is when you're done
developing the gum.

When people have said that stain is a function of exposure, I guess I
would expect they must mean that exposure makes stain happen, or
affects the level of stain in some way, and I sure don't see it in
this example. That's my whole point, when I say that stain doesn't
seem in my observations to be a function of exposure. If more
exposure doesn't change anything about the stain, how could it be
said that stain is a function of exposure? A more reasonable
conclusion to draw from this, if conclusions could be drawn, is that
the stain doesn't change with exposure, but the flaking improves
(less flaking with more exposure) although in the tests I ran last
week, I didn't find that neat a correlation.

>
> However, before someone feels the urge to eat her words, I must add
> that in other tests, with different sizing and/or different paper
> (but identical gum/pigment mix and exposure) the inversion does not
> happen at all. I think that the substrate is far more determinative
> of inversion that pigment concentration or exposure. But this is a
> tentative opinion, for now.

What's the substrate you used for these examples?

If it's an unsized paper that doesn't print well unsized, if the
pigment level is low, and if the same pigment mix doesn't stain on a
more sized paper, as I think you're saying, then this is what I
would call paper-related staining, not pigment-related staining,
(distinction made clear on my page on pigment stain) and by
extension, paper-related inversion. And if so, then I must eat a few
words at least.

>
>
> By the way, I am not of the opinion that gum has a limit in the
> quantity of pigment that it can hold, and that staining is the
> result of the pigment 'having to go somewhere'. Solutions have a
> saturation point but gels do not. Any tube of watercolour is the
> proof that gum can hold more pigment than we ever use in gum
> dichromate.

I've thought about that, and I know what you're saying, but I think
we're talking about different things, and maybe I should use a
different vocabulary for what I mean. The point is that if you took
that pigment/gum mixture straight from the tube and tried to print
with it, it would stain like crazy, and you need a certain amount of
more gum to keep the pigment from depositing on the substrate.
Demachy's quote on my page says what I mean, and my experience and
observations coincide with Demachy's, whatever you want to call it.
Katharine
Received on Thu Jan 26 20:40:36 2006