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

[Judy Seigel <jseigel@panix.com>]

On Thu, 8 Oct 2009, Tomas Sobota wrote:

Oh good grief Tom, you've got me speaking beyond my area of expertise 
again... Have a heart ! (Especially since I've never printed on glass.) 
BUT....

> 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.
I don't visualize this.... If the pigment is mixed with the gum, how (all) 
the gum, but not (all) the pigment is washed away...

> Could be
> some form of electrostatic binding, I don't really know.
I don't understand why no gum would remain in the *color* left on the 
glass, or how you could tell if it did or didn't. Certainly if you tried 
to paint with just pigment on glass, no medium (ie gum), you wouldn't get 
much image, unless you roughed up the glass (as far as I, the lay person 
in this respect, understand anyway).

> 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
But you think it's pigment remaining on the glass when the gum has washed 
away.  For all I know you could be right... but the ordinary mind may not 
grasp it, not right off, anyway.

J.


> 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