Re: gum "stain" with zero exposure... etc.
That's very interesting, because I too have only seen it with lamp
black, although that's the only pigment I did those tests with.
On Oct 8, 2009, at 5:38 AM, Keith Gerling wrote:
Regarding Tom's observation regarding the gum-free pigment: I see
the same thing when I print on gessoed surfaces. In printing with
lamp black, for instance the highlight areas are left with a thin
coating of pigment. It can easily be removed with water spray or a
brush, but no amount of mere still water soaking will budge it. It
is certainly pigment without gum, but I've only seen it with black.
On Wed, Oct 7, 2009 at 11:08 PM, Katharine Thayer
<kthayer@pacifier.com> wrote:
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.
Yes?
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.
Katharine
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.
katharine
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.
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
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