Re: Image formation in gum

I sent this one to myself by mistake, (something I haven't mastered
about my new mail program) so I'm sending you a copy:

On Dec 17, 2005, at 6:28 AM, Katharine Thayer wrote:

> See, this is an example of where thinking of gum printing as being
> about GUM helps one understand what's going on. It seems a given
> amount of gum can only hold a given amount of pigment in suspension (a
> number that may vary by pigment, for all I know). If you go beyond
> that pigment/gum ratio, the excess pigment is going to settle out, in
> one form or another. One form is as an indelible stain, another form
> is as this "pigment image" you can get in reverse. In either case, it
> seems to me it's simply a matter of excess pigment, pigment that
> exceeds the gum's ability to keep it in suspension, finding something
> to do with itself.
>
> And it also seems that unlike gum, something about pigment (or maybe
> some pigments) makes it stick around on the substrate rather than
> going off in the water, even if it's not trapped there as a permanent
> stain, On glass, with a fine pigment like lamp black, I can see
> electrostatic forces holding the pigment filaments to the glass. But
> it's harder to understand with paper and an earthier pigment like
> burnt umber.
>
> I was voting for the pigment image being more permanent, yesterday
> when I was printing carbon on glass, because it's a great looking
> image. It looks, not like something sitting on top of the glass as a
> gum image does, but it looks like it's IN the glass, like it's a part
> of the glass. It looks really cool, and I wanted to exploit this
> effect further. So in a way I was disappointed to find that I was
> right and it was just pigment and could be wiped off by running a
> finger across it, or a tissue.
> Katharine
>
> On Dec 16, 2005, at 1:59 PM, Katharine Thayer wrote:
>>
>>
>> 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 Sat Dec 17 08:42:03 2005