Re: Image formation in gum

Tom,

Isolating the possible reflectance of the glass and background might tell a
lot. I am wondering if the process is very different than I always assumed.
I think most of us thought that the light would diminish in strength as it
diminish the gum layer. Some of the light being absorbed by the pigment and
the gum. But maybe that is a minor effect and most of the light is reaching
the base. If so the reflection of the light could create strong exposure at
the back of the layer. If these ideas are combined, the light is diminishing
as it penetrates, and it is reflecting off the base, there could be more
hardening happening at both the front and back of the gum than in the middle
of the layer. I'm not sure what that has to do with the reversal effect but
it could explain a lot of confusing things we see commonly in gum printing.
For instance how can a hardened layer float off an image layer.

These are thoughts I had last night when I first saw your images but was too
tired to remember to write in my first post. Perhaps your trying to limit
the reflectance will tell us more...

Jack

> From: Tom Sobota <tsobota@teleline.es>
> Reply-To: alt-photo-process-l@sask.usask.ca
> Date: Fri, 16 Dec 2005 10:22:11 +0100
> To: alt-photo-process-l@sask.usask.ca
> Subject: Re: Image formation in gum
>
> Jack,
>
> The background under the glass was the matte black paint of my contact press,
> which is pretty antireflecting.
>
> However, now that you mention anti-halation backing, there surely could have
> been some kind of reflection off the back side of the glass, which was glossy.
> The exposition sure looks as if it had been from behind.
>
> Thanks for reminding me. Next time I will blacken the back side more
> carefully.
>
> Tom Sobota
> Madrid, Spain
>
> At 01:24 16/12/2005, you wrote:
>> Tom,
>>
>> How interesting! I remember having (many. many years ago some Kodak paper
>> that was made for copying printed matter by putting it face down on the
>> original and exposing through the back. The emulsion was very high in
>> contrast. The light went through the paper and reflected off the original.
>> The paper recorded the difference of reflected light. I wonder is gum is in
>> some way combining the light going in the front and the light reflecting off
>> the base. In your test on glass was there a white background under the glass
>> or could the light have reflected off the gloss of the glass. Remember in
>> film the anti-halation backing is there to prevent this effect so its a real
>> possibility.
>>
>> Jack
>>
>>> From: Tom Sobota <tsobota@teleline.es>
>>> Reply-To: alt-photo-process-l@sask.usask.ca
>>> Date: Thu, 15 Dec 2005 23:20:39 +0100
>>> To: alt-photo-process-l@sask.usask.ca
>>> 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!
>>>
>>> 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 Fri Dec 16 10:09:53 2005