Image formation in gum

This afternoon, instead of working as I should have been ;-) I was
testing what we loosely have called 'tonal inversion' in gum and a
failed experiment produced nonetheless a series of images that I find
interesting so I'm sharing them with the rest of 'gummists' for what
it's worth.

The idea was (still is) to isolate the gum from the substrate, i.e.
paper, to see what would happen with the inversion. So I decided to
use glass instead, to minimize porosity effects and such.

Now I have relatively little experience coating glass with gum, more
so when I don't want to use any substrate such as egg white which
could produce a better adherence but also possibly the same effects
of pigment 'absortion' as paper. So I used a piece of clean window pane.

I first tried with a layer which was too thin, so I washed it and
made another with twice the amount of powder carbon black I'd use
normally for paper, some 0.5g for 10cc of gum. This emulsion was not
overly difficult to extend on the glass.

Once exposed and during development, this 'dark' coat just started to
slide off the glass. OK, I thought, this is what is expected from a
layer too dark where the exposing light could not penetrate and
harden the deeper layers of gum so they just floated away.

After all, mostly all the descriptions of the gum process tell us
that the hardening (insolubilization, crosslinking) goes from the
surface to the bottom of the layer of gum. Right? Yes, I have
heard/read people doubting this theory but I have never seen any hard
evidence to the contrary.

Well, what we see in my images could be this evidence.

Image 1 ( http://usuarios.arsystel.com/tksobota/Gum_on_glass_1.jpg )
shows the general situation: a layer of gum on glass that is
disintegrating. The glass is perfectly smooth, the marks you see are
on the other side. The gum has been exposed under three Stouffer
tablets for 6, 12 and 18 minutes.

What strikes you immediately is that the exposed gum (the heading of
the tablets) is _under_ the gum that is floating away. What is more,
the area just besides the tablets, which conceivably received more
light, has not been hardened and is also floating away!

Image 2 (http://usuarios.arsystel.com/tksobota/Gum_on_glass_2.jpg )
is a detail of the above, an in camera magnification. You see how
well the text 'Stouffer graphic arts ...' is holding to the glass,
but you also notice that the unhardened gum is above of this text. It
looks as it were exposed from behind, but it was not. It was exposed
from above, just as we look at it.

Image 3 ( http://usuarios.arsystel.com/tksobota/Gum_on_glass_3.jpg )
is another region of the glass, and another of the tablets. I fairly
clearly see that the image is at the _bottom_ of the drifting
unhardened gum, and is very thin. Also, in several of the letters I
perceive what seems to be an inversion, which is better seen in...

Image 4 ( http://usuarios.arsystel.com/tksobota/Gum_on_glass_4.jpg )
where the letters T,O,U are clearly inverted, or at least different
from the other letters where the excess of gum has already gone away.

I find these images interesting, because they seem to show that the
image is somehow formed bottom-up, as if only the gum adhered to a
substrate is capable of hardening. But this needs more verification,
I think. And why the well illuminated areas around the tablets were
not hardened?

Also, this is glass. All the usual surface irregularities that
supposedly fix the exposed gum to the substrate are missing. However,
where an image has formed it is very clean and sticks to the glass
perfectly. Even now, that it has dried.

Questions, questions...

No need to say that any opinions will be appreciated.

Tom Sobota
Madrid, Spain
Received on Thu Dec 15 16:21:49 2005