Re: Image formation in gum (2)

Tom, most of this just looks like pigment stain to me. One thing you
may have misunderstood: while I have been proposing to define tonal
inversion as a special case of pigment stain, that certainly isn't to
say that I think tonal inversion is the only way that pigment stain can
present itself. Pigment stain can manifest as speckles, as a light
color cast on an otherwise well-toned gum image, as brushmarks across a
gum image, as a heavy pigment layer that completely obscures a gum
image, and everything in between, as well as this tonal inversion
thing. I think you've shown several different ways that pigment stain
can look, but that's all that I see here.

The things you showed the other day were examples of tonal inversion,
which is what I thought we were talking about, but there is almost no
sign of that here, except for the one place where the pigmented gum
either floated or dissolved off steps 20 and 21, forming a resist that
kept the pigment stain from depositing on the numbers, which are the
most exposed areas on all steps but the first one. This is exactly as I
described on my page I showed a couple of days ago, showing the tonal
inversion on paper. But except for that one small thing, what you're
showing here looks to me just like normal gum images (meaning
normally-toned rather than inversely-toned) except for being heavily
pigment-stained (in the ordinary sense of how pigment stain normally
appears and behaves on a normally-toned gum print that's
pigment-stained, which is a different issue than how pigment stain
appears and behaves when it presents as tonal inversion).

The fact that exposing for different times pushes the tones up and down
the step tablet doesn't change the fact that the tones are the same, no
matter where they appear on the tablet, (a particular pigment at a
particular concentration will print with the same tones, as long as it
is sufficiently exposed; exposing it longer will just block more lower
steps and push the discernable steps farther up the tablet, but the
tones printed will be the same tones no matter where they fall on the
tablet) and the tone of pigment stain, the tone that's left by the
deposit of excess pigment in unexposed areas, will be the same whether
the coating is exposed or not exposed.

What I don't understand is why the letters at the top of the step
tablet printed white (with pigment stain of course) if the glass was
painted black and if, as you seem to be claiming, the letters are
printed in gum (which, BTW, I don't see at all). The numbers on the
steps are printing in gum, and they are printing black. If the letters
and numbers on top were printing in gum, they should be printing
black, not white. If they were printing normally (not reversed) they
should print in negative space, meaning no gum. Is the appearance that
the letters are printing white simply an optical illusion?

I think we're going in circles; I don't think you're understanding what
I'm saying, and I don't think you're demonstrating what you seem to
think you're demonstrating, which now that we're not talking about
tonal inversion any more, I'm not sure any more what it is that you're
arguing that you're demonstrating. Whatever it is you think you're
demonstrating, what I'm seeing is pigment stain.

Katharine

On Dec 20, 2005, at 3:29 PM, Tom Sobota wrote:

> I did a second series of experiments to further study the 'tonal
> inversion' and the 'bottom-up' image formation.
>
> The issues that were raised by the last test were:
> 1. How clean was the glass. This time I took extreme care to clean the
> surface until no 'beading' of the water occurred. I guess it could be
> called a 'technical grade' clean glass.
>
> 2. Reflections from the back face. I painted the back face with a
> matte black paint. The effect was of a black mirror, very nice.
> Reflections still formed, to say the truth, but to a much lesser
> grade.
>
> I soon discovered that coating a black glass with a black emulsion in
> a subdued light environment is not for the faint of heart. Looking for
> the proverbial black cat in a coal bin is childrens' game, compared to
> this.
>
> Looking at an angle I could more or less see where I was putting the
> gum, but little else.
>
> Once exposed, in the first 10 minutes of development most all of the
> emulsion just floated away as usual. But the borders of the three
> areas where I coated the gum did not float away, or at least not
> completely, as you can see in an image of the gum still wet:
> http://usuarios.arsystel.com/tksobota/Gum_on_glass_7.jpg
>
> Once dry, the glass shows the rests of the gum, as well as the usual
> images of the Stouffer tablets. The photo has been taken with a light
> coming in a small angle, so you see the gum shining white on the black
> background. The images of the tablets shine also, and this shows
> clearly that they are made up of gum with dispersed pigment, even if
> very thin and delicate:
> http://usuarios.arsystel.com/tksobota/Gum_on_glass_13.jpg
>
> This can also be seen in more detail:
> http://usuarios.arsystel.com/tksobota/Gum_on_glass_11.jpg
>
> and with even more detail here, where you can see the extremely fine
> detail:
> http://usuarios.arsystel.com/tksobota/Gum_on_glass_9.jpg
>
> Here is a detail of the 'inversion' side of the tablet images. A
> difference can be seen between steps 14 and 15, but that is all. The
> fine black lines are just brush marks. The same gum sheen is seen here
> also.
> http://usuarios.arsystel.com/tksobota/Gum_on_glass_12.jpg
>
> In another test I used a piece of frosted glass. Actually it was a
> focusing glass that I made as a spare for a 8" x 10" camera, now I'll
> have to clean it :-). As suspected, the emulsion was much easier to
> apply here and during development dissolved slowly, just as a normal
> gum on paper. The images of the tablets show the same pattern of
> inversion as on paper, and the same linearity:
> http://usuarios.arsystel.com/tksobota/Gum_on_glass_14.jpg
>
> Just in case you didn't notice the wonderful example of inversion:
> http://usuarios.arsystel.com/tksobota/Gum_on_glass_15.jpg
>
> which shows that this effect is clearly dependent on exposure time.
> The rightmost tablet has received 6 minutes of exposure, the middle
> one 12' and the rest, 18'. The actual times are not important, of
> course, but this image shows that perhaps 'pigment stain' and 'tonal
> inversion' are two different effects. Both are probably related to
> pigment concentration, but inversion is clearly also dependent upon
> exposure. Or so it seems, I don't think that I'm ready to make
> inferences with the scant data available.
>
> Tom Sobota
> Madrid, Spain
>
>
Received on Wed Dec 21 00:23:03 2005