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