Re: Image formation in gum (2)

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 Tue Dec 20 17:30:05 2005