Re: Rethinking pigment stain

I like the way all this sound Tom and I like the distinction you make
between fog and pigment stain. One other possible way to view fog would be
to borrow the signal to noise ratio concept used in other disciplines and
associate fog with noise. Using this analogy fog is then any
insolubilisation of gum that wasn't caused by the "normal" exposure under a
negative. Pigment stain is caused by pigment particules being physically
trapped in paper fiber. Since to a large extent dichromate stain can be
removed after the fact we could dump those residuals on the back of fog and
compensate for any tone or color (temp) shift with a warmer or cooler
pigment depending on ones preference. (Based on test I've seen, cleared
dichromate is almost clear but not quite)

I know this doesn't give a satisfactory explanation as to what causes
dichromate stains. Katherine as refered us to a PVA dichromate study from
which she seems to infer that overexposure is the cause of these stain
because an excessive amount of activated dichromate molecules is not bounded
to colloid molecules (in this study all colloids where made insoluble thus
where bounded with dichro). I suppose one could say just increase the number
of available colloid molecules such that you don't have a surplus of
dichromate. I'd be curious to here ear from those who suggest to use a 2:1
gum / dichro ratio, about their dichromate stain??? Assuming this simple
adjustment could resolve the dichromate stain problem, then the exposure
scale could be largely extented without causing those infamous stains and
leave us with just beautiful clear gum and some pigment of course but I
don't know if this is true so lets forget about it here.

Yves

----- Original Message -----
From: "Tom Sobota" <tsobota@teleline.es>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Wednesday, December 14, 2005 5:55 AM
Subject: Re: Rethinking pigment stain

> It all depends on how you define 'fog'. To me, fog is a global
> hardening or insolubilization of the gum-dichromate layer which
> does not contribute to the formation of an image.
>
> It can be produced by any (or a sum of) several causes, among which
> accidental exposure to light too, of course. Others are heat, exposure
> to chemicals, and the simple passing of time. Surely others.
>
> The normal exposure to light of a layer of dichromated gum under
> a negative produces a hardening which is not physically different
> from a fog-producing exposure to light, but it produces an usable image
> since it is not global but rather differential. So we don't call it fog.
>
> This loose definition of fog could be applied to silver-gelatin emulsions
> too, actually.
>
> Pigment stain is different, and perhaps unique to gum, carbon and
> other processes using finely ground particles of pigment.
>
> But dichromate stain has a sort-of analogue in silver-gelatin emulsions
> when you develop with pyrogallol or pyrocatechol, for example, which
> also produce stain. This stain contributes to the image, however, because
> it is in the negative and not in the positive.
>
> If you want to see fog in gum, coat a paper and keep it in the dark for a
> week or two in a warm place. Then develop side by side with a
> just-prepared emulsion. The relative darkening of the older coat should
> be evident.
>
> Tom Sobota
> Madrid, Spain
>
>
> At 03:38 14/12/2005, you wrote:
> >I don't know all the details regarding that past discussion, but I think
> >"fog" is a misleading term. I consider fog to be accidental exposure,
e.g.
> >someone opens the back of a 35mm camera and the film gets "fogged".
Perhaps
> >gum paper can get fogged from ambient light levels being too high in the
> >darkroom, but I've never had that problem.
> >
> >Dave Rose
> >Powell, Wyoming
> >
> >----- Original Message -----
> >From: "Katharine Thayer" <kthayer@pacifier.com>
> >To: "alt photo" <alt-photo-process-l@sask.usask.ca>
> >Sent: Tuesday, December 13, 2005 12:16 PM
> >Subject: Rethinking pigment stain
> >
> >
> > > Hi gum printers,
> > > Because for most of my gum printing career I've printed on unsized
> > > paper, I developed a definition of pigment stain that I realize now
> > > probably applies mainly, if not exclusively, to printing on unsized
> > > paper. My definition required that in order for something to be
called
> > > pigment stain, the pigment had to penetrate the paper and be
indelible.
> > > And in my experience, when pigment stain occurred, it occurred
> > > immediately when the coating was applied; if the pigment was
excessive
> > > in relation to the gum, it would stain the paper immediately on
> > > application.
> > >
> > > The tonal inversion thing made me rethink that idea, as I said a
couple
> > > of days ago, and after doing some experiments with sized and unsized
> > > paper, I've decided I need a more inclusive definition that
> > > incorporates what happens on sized paper, or maybe two different
terms;
> > > I haven't decided yet for sure.
> > >
> > > A more inclusive definition for "pigment stain" would say that pigment
> > > stain is whenever you get pigment in places where it shouldn't be,
> > > such as in unexposed areas of an image or step print. Whether or not
> > > that out-of-place pigment forms an indelible *stain* will be a
function
> > > of how well the paper is sized. On sized paper, this "stain" will
wipe
> > > off easily, whereas on unsized paper it will be indelible, but in
> > > either case, you've got pigment you don't want in areas that should be
> > > very light or paper white, hence: stain.
> > >
> > > One problem with this more inclusive definition is that it doesn't
> > > distinguish between stain and fog. Someone referred recently to a
> > > discussion from last summer where Mark showed a gum test print where
> > > there was color on areas where the print should have been paper white.
> > > I called that stain, and was told that it was fog. I conceded the
> > > point; when told that it could be wiped off the paper I assumed (given
> > > my then understanding of stain) that it couldn't possibly be stain and
> > > must be fog, although I didn't have a clear understanding of what
could
> > > have caused the fog. And when that was brought up recently, I
> > > acknowledged I'd been wrong when I'd called it stain. But now that I
> > > have seen for myself that pigment stain can also be easily wiped off
> > > sized paper, (while still wet, of course) I'm not sure I know how to
> > > tell the difference between stain and fog on sized paper.
> > >
> > > They are of course different in substance, because what I would call
> > > "pigment stain" is just pigment, since it occurs in areas where no
> > > exposure, and therefore no formation of crosslinked gum, has occurred,
> > > whereas fog, in my opinion, would involve the formation of crosslinked
> > > gum.
> > >
> > > On unsized paper, excess pigment impregnates the paper as stain, and
> > > that's why it stays with the paper rather than dissolving away with
the
> > > dichromate and soluble gum from unexposed areas. But on sized paper,
> > > even though the pigment isn't held in the paper as stain, or in
> > > crosslinked gum as "tone" it still remains on the paper in unexposed
> > > areas, as seen in the examples of "tonal inversion." This is
> > > interesting, but puzzling, to me. At any rate, I've satisfied myself,
> > > by cutting coated papers in half and exposing one side and putting the
> > > other side directly into water, that the "pigment stain" is the same
> > > on unexposed areas of exposed coatings as it is on completely
unexposed
> > > paper, whether sized or unsized, which makes me even more confident
> > > that the effect has nothing to do with exposure, heat or anything
else
> > > related to the exposure itself, but is simply pigment stain.
> > >
> > > Thoughts, anyone? I will soon be revising my page on stain, lord
> > > willing and the creek don't rise, to reflect the evolution of my
> > > thinking on this topic.
> > > Katharine
> > >
> > >
>
Received on Wed Dec 14 10:08:51 2005