Re: Image formation in gum

From: Tom Sobota ^lt;>
Date: 12/16/05-03:36:11 AM Z
Message-id: <>

Katharine, your post raises several interesting points which I'll have to
consider and 'digest' :-)

You're right in that the gum that is floating away is the hardened one.
The unhardened just dissolves and cannot be seen. But then, why ...
but never mind, at this point every answer brings up a new question.

I'll make more tests during the weekend. More later

Tom Sobota
Madrid, Spain

At 02:43 16/12/2005, you wrote:
>This is very interesting. I just skimmed your post fast and looked
>at the images fast, and will want to come back and look at it more
>carefully later and think about it more, before commenting in full..
>But first, a couple of initial thoughts:
>(1) It was the inversion thing, in a different way, that made me
>first wonder if maybe the top-down hardening thing is wrong. Before
>that all my experience had led me to think that the top-down model
>made sense, and the arguments against it didn't, so much. I haven't
>shown my reversal images yet because I put them into my pigment
>stain page and I'm still trying to decide if I think it belongs
>there. But maybe I'll pull it out and show it separately. There's
>something weird about this inversion thing that I sure wish we could
>make sense of.
>(2) The letters and numbers at the top of the tablet should be
>printing in white, or clear in the case of glass. That it's printing
>in black says pigment stain to me. I hadn't seen pigment stain on
>glass before, but I can't say it can't happen, either, and the black
>in other areas on the glass, under the gum, as well as under the
>tablet headings,
>also looks like pigment stain to me. At any rate we still don't
>know, really, WHAT the tone is in the parts that should be clear.
>Pigment stain makes sense to me, but I can't think of a way to prove
>it to myself, and I'm open to whatever our collective observations
>end up showing us, as long as they speak with one voice.
>(3) The fact that the areas around the step tablet, that were well
>exposed, floated off, doesn't mean that the gum wasn't hardened there.
>The fact that it floated off in veils and sheets shows, IME, the
>opposite: it WAS hardened. If it weren't hardened, it would just
>run off like dye into the water, it wouldn't form sheets like that.
>What you see, I would say from my own experience printing on glass,
>is the heartbreaking fact that plain unprepared glass doesn't hold
>hardened gum very well, and usually as soon as it hits the water,
>off it goes. It doesn't mean it wasn't hardened, it just means that
>the hardened gum didn't have enough of a purchase on the glass to
>keep it there. Different people use different means to hold the gum
>to the glass (Bill Winkler has been successful with silane; Galena
>Markova has been successful with two layers of gelatin; what works
>for me is grit in acrylic medium. But I don't know of anyone who has
>been able to make hardened gum stick to unprepared and untreated
>glass. (But I'll be glad to be made a fool of by anyone who can do
>that reliably.)
>More later, but thanks a lot. I think the more we show examples of
>things and talk about them, the more likely we are to come to some
>glimmer of how gum works.
> Katharine
>On Dec 15, 2005, at 2:20 PM, Tom Sobota wrote:
>>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 (
>>) 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 ( )
>>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 (
>>) 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 (
>>) 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 03:36:32 2005

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