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

[Tomas Sobota <tom@sobota.net>]

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.

Tom

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?????

Judy