Re: tonal inversion and pigment loads

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.

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.

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.

To give an example: I prepared some sheets of Fabriano 5 for the oil
process. I took one of the heavily gelatined papers, fixed in formalin,
and applied to it the same mixture as previously. I found no trace of
inversion nor of stain.

The pigment used is Ivory Black from Maimeri in a rather low
concentration. This pigment has low covering power, perhaps because the
grains are relatively big. They are so big that some percentage of them
can be seen and manipulated under a not-too-large magnification.

Looking at the 'inverted' areas through a binocular loupe, what is
inmediately striking is that the dark areas consist of mostly loose
pigment grains. These grains, far from being imbedded in the paper
fibres, just lie on the top of it. They are too large to get between the
fibres anyway. When the gum is still wet, the grains can be seen movin
in the small water 'puddles'. They should just wash away ... but they don't.

Fascinating, but far from having been explained, in my opinion. Some
more experimentation is necessary. And even so, it is very difficult to
abstract one's working methods and materials so that results can be
extrapolated to other workers. But we already know that...

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.

Tom Sobota
Madrid, Spain

Katharine Thayer wrote:
>
> I haven't yet seen any credible evidence that one can eliminate
> either pigment stain or inversion by increasing exposure, but I have
> eliminated both of them simply by changing the gum/pigment ratio. I
> have seen some evidence, in Tom's (I think it was Tom) posted test
> strips, that one can move the inversion up or down the step tablet by
> changing the exposure, but again that seems rather obvious and trivial
> to me; of course changing the exposure alters where the tones appear
> on the step tablet; more exposure blocks the shadows and sends the
> meaningful tones farther up the tablet, but so what? The meaningful
> tones are still the same tones, and the pigment stain is still there;
> the only difference is your shadows are all blocked. The crucial
> variable in the inversion, as far as I have seen, is overpigmentation,
> and if the gum is overpigmented, you'll either get stain or inversion
> or flaking or something, because the extra pigment has to go
> somewhere, and the way to fix it is not to expose more, but to reduce
> the pigment, in my experience. (It should be clear from the test
> prints that I posted that reducing the pigment enough to eliminate the
> stain does not mean giving up printing with a fully saturated pigment).
>
> One caveat: I did see an oddball thing where someone sent me an
> inversion he got with a very small amount of pigment; he then got a
> positive image with the same pigment mix by exposing more, but at the
> same time he increased the exposure, he changed other variables as
> well, so it's impossible to say what caused the improvement. Which is
> why you should never change more than one thing at a time.... at any
> rate, he was using blue photofloods for the exposure and my experience
> with them suggests they don't behave like normal lights do. So that
> one seems like an anomaly to me, unexplained and unexplainable without
> further investigation, one variable at a time. When someone can show
> me an inversion with a light pigment load that goes away with
> increased exposure, all other variables held constant, indicating that
> inversion is a function of exposure rather than pigment load, then
> I'll eat my words, or a few thousand of them anyway, but I can't eat
> my pictures. If anyone's interested in a full treatment of my
> position on pigment stain, I just uploaded that revised page last
> night. There isn't anything particularly new there; I've said and
> showed all this here before, I think. One of my goals on revising the
> site was to cut down on the text, as no one seems to read it anyway,
> but I don't seem to have accomplished that.
>
> http://www.pacifier.com/~kthayer/html/stain.html
> Katharine
>
>
>
>
Received on Thu Jan 26 18:09:06 2006