U of S | Mailing List Archive | alt-photo-process-l | Re: gum "stain" with zero exposure... etc.

Re: gum "stain" with zero exposure... etc.

Judy wrote:

I meant to add but forgot that AFAIK everything we call "paint" is pigment mixed with some kind of medium, gum arabic or casein, for instance, but you (Tom) seem to be saying that the gum print on glass removes the medium, leaving only the pigment.

Tom of course can answer for himself when he wakes up, but I'll jump in here and say that if I'm understanding this right, I think there may be a misperception. I'll stick to my own observations and let Tom clarify his; I'm not saying that the gum print on glass is pigment only; what I'm saying is that pigment stain on glass is pigment only. In my own observations of tonal inversion or of any kind of pigment stain on glass, yes, the pigment stain that is deposited on the glass is pure pigment. Hardened gum, encasing pigment, adhered to the glass and dried, is at least as tenacious as paint that gets on a window when you're painting; the only way you can get the gum print itself off the glass is by scraping it off with a razor blade..

But where there is pigment stain (pigment deposited in areas where there is no exposure) it is pure pigment, and as both Tom and I described, it can easily be wiped off the glass after developing and drying. I've only seen this with lamp black, so I can only describe what lamp black looks like deposited on glass as pigment stain: it looks exactly like powdered lamp black, like fine dry black soot, even though it started as paint with gum added. I have a vague thought that I might have posted one of these once, but I'm not sure.

We do know that unhardened gum dissolves in water, of course; that's one of the fundamental principles of gum printing. So it's not surprising that in the unexposed areas the gum will easily dissolve in development. Why occasionally it leaves the pigment behind is something of a mystery, but that's what pigment stain is, after all, pigment left behind in unexposed areas after the soluble gum has gone off in the water.

On Oct 7, 2009, at 5:18 PM, Katharine Thayer wrote:

Tom, I call it stain without the quotation marks, but in order to do that I broaden the definition of stain to mean a condition in which pigment is deposited where it's not wanted, not just a condition where pigment has soaked into an absorbent surface (usually paper) and left a permanent color change where it's not wanted. Maybe a different word would be better, but that one works for me. Like Tom, I've seen stain, as well as tonal inversion (which I consider a special case of stain) on glass, on yupo, and on other hard surfaces, as well as on well-sized paper. I agree with Tom that it's pigment all by itself, that it's hard to understand what's holding it to the glass surface since it can be easily brushed away, especially if it's lamp black, which is nothing but very fine filaments (soot, actually) it can be wiped off the glass with a fingertip or the edge of a tissue. It seems like it has to be some sort of electrostatic attraction, or something, but I don't know.


On Oct 7, 2009, at 4:27 PM, Tomas Sobota wrote:

Judy, far from me to claim that I understand what happens. I was commenting on your citing of Mike Ware: "The less viscous the emulsion...the more it soaks into the paper, hence the more stain". In the case of glass, where this tonal inversion or "stain" happens also, there's no "soaking into the paper" for any viscosity you might have, so the "stain" must have some other origin. According to what I have noticed, the pigment causing the "stain" just sits there on the paper or glass surface, apparently free of any gum. It is also very easy to remove manually (i.e. with a thin brush), since it doesn't seem to be bound to the surface in any obvious way.
So what seems to happen is that the gum in these regions, as expected, is washed away but for some reason a little of the pigment stays put. Could be some form of electrostatic binding, I don't really know.
BTW I call it "stain" because I don't think that it is stain in the usual sense, i.e. pigment getting stuck between the paper fibers that will not clear.

On Wed, Oct 7, 2009 at 8:22 PM, Judy Seigel <jseigel@panix.com> wrote:

On Wed, 7 Oct 2009, Tomas Sobota wrote:
Two, three years ago, during a loooong thread on this inversion effect, I
posted on the list that I observed this effect with gum on glass. I posted
some pictures, even. So, the explanation by Mike Ware, with all respects due
to him, is not enough for me. Also, the method of hardening is irrelevant in
this case, since glass does not need any hardened gelatin coat. I used
depolished glass.

Tom Sobota
Madrid, Spain

Admittedly Tom, parts of my memory have been eaten by worms... and 2 to 3 years ago I didn't have the browser I have now so wouldn't have seen your example... HOWEVER, to a non-chemist/ trial & error "scientist," I'm not sure that working on glass rather than paper couldn't have the same explanation (or "explanation"). After all, if the emulsion hardens enough to keep the pigment in place, but not all of it hardens, there would still have to be gradations between viscous, slightly viscous and free dissolving..... No?????