Re: tonal inversion and pigment loads

Katherine,

You are perhaps right about the fact that not everybody is using the
same terminology in this. This is only to be expected since this
'inversion' effect has seemingly received little attention until now.
Several of us have noticed it, in my case years back when I just
attributed it to bad practice and ignorance of the procedure on my part.
Very recently I have read a reference to inversion in a note by Judy
Seigel in the P-F Journal. And, latest of all, the post by Joe Smigiel
some two months back. As far as I know, that is all. So, a terminology
has not yet evolved.

In every example of inversion that I have seen, there are three regions:
1. A first region corresponding to what we could call the shadows in the
negative. Speaking in terms of step tablets, the lower numbered zones.
In this region everything happens as expected.

2. A second region where the density of the negative (or step tablet) is
such that it doesn't produce any density on the gum positive. This
region is actually an extension of the first. It should go until the
darkest zones of the negative or tablet, and in general it does, except
that but sometimes it doesn't, when there's inversion.

3. A third region where the inversion happens, when it happens. In this
region the density is in general lower than in region 1, and in some
cases a few steps are differentiated. In other cases, as my examples on
glass, no steps are visible.

This three regions move up or down with exposure, so they are exposure
dependent, in contrast with pigment stain which as far as I know, is
exposure independent.

This definition doesn't attempt to explain the inverted numbers in the
inversion area, which in first instance seem related and exposure
dependent too. You say that 'for some reason they didn't flake off', and
I agree. Precisely, I'm attempting to find what the reason is or at
least to pinpoint under what circumstances it happens.

But to answer your questions: I used a Spanish paper called Guarro
Acuarela. It is a rough grain, acid free, 60% cotton, 240 g/sq.m
cost-effective paper . It has been twice brush-coated with 3% gelatin
and hardened with formaline. The pigment level is low and the same
gum/pigment mixture does not produce inversion or stain in a more sized
paper, as I told in the same post.

If it were paper related stain, as you say, there would not be
discernible steps in the inverted zone, in my opinion. I take paper
stain to be an exposure-independent, global effect.
If you are interested, I have scans of tests where inversion occurs in
addition to heavy staining.

And thanks for taking the time to analize my experiments. Perhaps
someday, between all, we will have a better understanding of these things.

BTW I respect very much Demachy as an artist, and he certainly knew the
gum technique admirably well. But one could expect that in almost a
hundred years our understanding of the mechanics of gum dichromate has
advanced to the point that his explanations should have been superseded
by something more up to date. I'm not sure that this is so.

Tom Sobota
Madrid, Spain

Katharine Thayer wrote:
>
> 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 Fri Jan 27 15:09:43 2006