Re: Image formation in gum

Hi Tom,

I'm impress by how little is known about gum in general. Here you have made
an experiment trying to get an answer on something specific and instead of
finding this answer I think we can say that we have more questions now then
we started with, incredible.

I've inserted some observation and or comments below.

Thanks
Yves

----- Original Message -----
From: "Tom Sobota" <tsobota@teleline.es>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Thursday, December 15, 2005 5:20 PM
Subject: 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!

Can I assume this first image as been exposed for 6 min.???

It seems like the gum kind of layered, maybe if I say it seem like something
happened near the glass and something (else ???) happened some distance
above the glass and this last layer doesn't seems to show any much evidence
that it was exposed under a tablet though I know it was??? It looks a bit
like when we use a chemical paint stripper or something like that.

>
> 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.

I'm not sure at all I would call the top layer "unhardened" gum???
If you look between the O and U of Stouffer you'll see like if the o and u
where holding this layer firmly on the glass but between the letters the
layer kinds of float. I'm sorry if this doesn't make much sense, I don't
have the words to describe this correctly.

>
> 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...
>

Yes, this one is really weird.

> 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'm not sure about inversion here, this effect or difference may be caused
by different contrast in different areas.
I notice at the top of the U and at about 10 o'clock on the O a tiny shaded
line like if the letter where in a depression or made of compressed material
surrounded by less dense or "fluffy" material???

> 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?

I wouldn't bet to much on this bottom-up concept I think this can be easily
explained away by the observation I just made above. When you exposed the
gum it was dry and the layer you put on was thin, maybe it hardened for
these letters at least and stayed the same thickness as when dry but the
"less" exposed areas surrounding the letter swell a bit with water you
introduce at dev time and/or the bound to the glass wasn't strong enough
and the whole layer lifted and broke apart afterwards. Someone mentioned the
glass may be still dirty???

>
> 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...

Yes, yes, lots of questions and not to many answers.

>
> No need to say that any opinions will be appreciated.
>
> Tom Sobota
> Madrid, Spain
Received on Thu Dec 15 19:22:59 2005