Re: Image formation in gum

On Dec 16, 2005, at 8:32 AM, Katharine Thayer wrote:

>
> To my eyes, this shows even more clearly the pigment "stain" in the
> form of excess pigment unevenly deposited across the glass, lighter
> in some places and darker in others in an apparently non-image-related
> way. It also shows how the hardened gum ends neatly at the edges of
> the letters (you can see this especially on the T, U, and second F,
> you can clearly see the hardened gum pulling off along the edge of the
> letters. This is as one would expect, since the gum shouldn't have
> hardened under the letters, which are black on the negative, and the
> unhardened gum would have simply dissolved away from the letters
> themselves, leaving clear glass, letters spelled out in negative
> space, letters made of non-gum. The question of course is why the
> pigment deposits itself so nicely on the non-image areas. I surely
> don't know why that is, although I have a sort of theory, but in this
> case I think this image shows pretty clearly that the gum is not
> involved.

I've just done a series of tests with concentrated lamp black on glass,
to answer a question I was going to ask Tom but decided, taking Judy's
advice, that I could just as well discover the answer for myself.

  From my own experience, I know that an image printed in gum on glass ,
once dried, is not easy to remove; it requires a sharp razor blade
applied with effort behind it, and sometimes more than one blade per
image. So it seemed to me that this would be a perfect way to check to
see whether this tonal reversal is made of hardened gum or just
pigment. Because if this reverse image is really just pigment, as I
suspect, it should wipe off easily from glass; it would be like wiping
soot off glass, because that's essentially what lamp black or carbon
black is, just fine charcoal or soot.

I did several prints and watched the gum float off, watching very
carefully to make sure the gum was floating off the entire glass
including the image areas. In each case, there was a step tablet print
left behind, after the gum was completely gone, much like Tom's, with
the heading printed in black (reverse) but the step numbers also
printed in black, (which is the right way, so the inversion wasn't
complete) also a general pigment tone over the entire tablet rather
than actual steps, and also a general pigment tone (what I would call
"pigment stain") over the entire area where the gum had been coated. I
also coated the other half of the glass, after making the exposure for
the "treatment condition" dried it and washed both at the same time,
to see if I got the same pigment tone left from the unexposed coating
as I did from the exposed coating, just as I had with paper. I did.

What's more, in each case, the image that stayed after the gum left was
indeed simply made of pigment rather than gum, because after being
dried it wiped easily off the glass with a tissue. So that was the
answer to my question, and now I am quite satisfied that this tonal
reversal is indeed just pigment, no matter what fancy theories you guys
come up with for it. 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.

Unfortunately these tests are all wiped off so I can't scan them to
show them to you, and you'll just have to take my word for it, unless I
have the energy to make some more to scan later.
Katharine
Received on Fri Dec 16 15:59:58 2005