Re: Image formation in gum

On Dec 17, 2005, at 12:35 PM, Tom Sobota wrote:

> I'm also glad that you found this new example, since that is what I
> see in
> my test: soot, as you say, but soot imbedded in a very thin matrix of
> hardened
> gum. It can be removed rubbing with something but without having to
> scrape
> it with a knife.

If it can be removed by rubbing with something, I am skeptical that
it's hardened gum, and would say that just the fact that you can rub it
off seems more a confirmation of my observation than disconfirmation.
But then you get into all the questions about who's using what kind of
dichromate (recent research with dichromated PVA has shown that it's
not just the difference in dilution, but that there is an actual
improvement in the stability of an (holographic) image made using
ammonium vs potassium dichromate, at ratios which provide the same
amount of Cr(VI)/PVA) and at what concentration, and the sizing, and
how that might affect the stubbornness of the gum image. As we keep
saying, there are just too many variables running loose to be able to
pin down a lot of things in gum.

But as I said the other day, if I get actual hardened gum stuck to
glass, no matter how thick or thin, it doesn't rub off with anything.
On the jpeg I showed yesterday, you can see at the lower right where I
started to scrape the gum off the glass with a razor blade. The blade
in the holder was not new, and when I looked for a fresh one, I found
that there weren't any more in the box. It was so difficult to scrape
the gum off with the used blade that after scraping that much (about an
inch by half an inch) I was winded and decided I wouldn't try to finish
the job until I get some more new razor blades. That's the way it is
for me, removing hardened gum from glass; it's a REAL hard job. So the
idea that you can have a layer that you are sure is hardened gum that's
adhered to the glass, but that can be removed by rubbing it with
something, doesn't work very well for me.

> Yesterday I was somewhat surprised with your observation that the
> image was
> just carbon black sitting on the glass. That implied all the gum
> dissolving around
> the pigment particles without carrying it along?

Yes, that's what I've been saying all along; that my new broadened
definition of pigment stain says that pigment stain is exactly that:
the gum goes away and leaves pigment behind, whether it presents as an
actual indelible stain or not.

  This happens of course only when excess pigment is used, beyond the
gum's ability to hold the pigment in suspension, and as Dave said so
quietly and sensibly way back in the beginning of the discussion about
tonal reversal, the way to make it stop is to use the right amount of
pigment. I think maybe the fundamental cause of the phenonemon has been
lost in all this speculating: if you use too much pigment, weird things
happen. The way to make it stop is to use less pigment. End of story.

There's a misperception among some quarters that the more pigment you
cram into the emulsion, the better. But this is true only to the point
of gum's ability to hold the pigment in suspension. Believe me, you can
get a a good dark tone (as dark as the pigment itself is capable of)
without overloading the gum with pigment, and when you do overload it,
you get this weird stuff going on.

As I've said many times throughout this discussion, when you've got
excess pigment, you're going to get pigment left on the substrate
whether or not the gum is exposed, as I demonstrated by cutting the
coated paper in half and exposing half and not exposing half and
finding that I had the same pigment tone on both halves of the paper
after development, as long as I used a hyper-pigmented emulsion. I did
the same thing with the glass, as I described, confirming that both on
glass and on paper, the excess pigment thing is an extra-photographic
phenomenon. It's a function of pigment, pigment concentration, sizing
and paper, and not a function of exposure.

>
> But I agree with you that it _is_ a great looking image. Even if as it
> is it's not too
> strongly adherent, perhaps you could attempt to fix it with some
> fixing spray,
> egg white, or a coat of gelatin, no?

Any of these things, I'm sure, would disturb the carbon image as surely
as wiping it with a tissue would; I don't like it *that* much that I am
willing to spend any time trying to create it and preserve it. The
only time I got the pigment stain on glass that I couldn't remove,
remember, was when it was just an overall stain, not when it was a
tonal reversal creating an actual, though tonally reversed, image.
Those reversed images (black letters on glass) were all made of pure
carbon, as far as I could tell; at least they all wiped off easily with
a tissue or a finger, as I described before, and I don't see any reason
to suppose that they contained any gum at all.

Katharine
>

> At 19:25 17/12/2005, you wrote:
>
>> On Dec 16, 2005, at 1:59 PM, Katharine Thayer wrote:
>>>
>>> But I also think it's not just an either-or thing, it's probably a
>>> continuum, so there are probably intermediary steps where there is
>>> some gum involved. I don't know, I'm just guessing.
>>
>> I'm glad I said this last, because I've just encountered an example
>> of something that probably fits this in-between category. It's lamp
>> black on glass, where some of the hardened gum came off. It doesn't
>> represent a tonal inversion kind of deal; the letters didn't print in
>> black, for example. there is a definite pigment stain left behind
>> where the hardened pigment floated off, but in this case, the pigment
>> tone is very definitely not removable. If you look closely you can
>> see a couple of places where I scratched it with a fingernail, but
>> it's definitely not easily wipable like the pigment I was wiping off
>> yesterday where there was a definite tonal inversion effect. (One
>> possible confound is that this is on etched glass while the ones I
>> did yesterday were on plain window glass). But at any rate, I
>> think this is probably an example showing that this phenomenon is a
>> continuum, but whether or how the gum is or is not involved in this
>> case, I'm at a loss to guess. It seems to me that there isnt any gum
>> there, but then why? as Tom said yesterday about something else. I
>> think this is fascinating, but difficult to fully explain in all its
>> possible variations.
>> Katharine
>
>
Received on Sun Dec 18 09:58:42 2005