Re: Color of dichromate stain (Was: Re: Gum Sizing

dan jones wrote:
>
> katherine
> can the brown stain be removed with a clearing bath?

Not after a certain amount of excessive overexposure. I don't know where
the cutoff point is, but somewhere between 5 minutes and 3 hours in the
sun. You can leave this way-overexposed brown gum sitting in bisulfite
all day without any effect. This may be a case where sulfuric acid is
advised, but I don't have any so can't test that. I suspect that the
reason that bisulfite doesn't work on it is that there's nothing left to
reduce. According to Duncalf & Dunn, the chromium keeps reducing after
the colloid is rendered insoluble (after 4 minutes of exposure, their
PVA was completely insoluble, and 42% of the dichromate remained in the
dichromate form. After 120 minutes of exposure, the colloid of course
was still insoluble, but only 20% of the dichromate remained as CrVI.
They stopped at that point, so I don't know if it goes to zero or
reaches an asymptote somewhere above zero).

At any rate, the point here isn't the brown stain so much as the fact
that I couldn't ever make green stain, contrary to conventional wisdom
which holds that when most or all of the CrVI is turned into CrIII, the
stain should be green instead of brown. Same popular belief says that
the brown of dichromate stain must be due to a mix of green CrIII and
yellow-orange CrVI, but my observations do not seem to support this
belief, because (a) the yellow dichromate is very soluble and completely
dissolves out of the gum into the wash water IME, so couldn't contribute
to a stain that remains after the water bath, and (b) gum that is
over-exposed to the point that here's no evidence that it contains much
if any unreduced CrVI is still stained brown not green. Normally-exposed
gum, or gum that's overexposed a "reasonable" amount, runs bright
yellow unreduced dichromate when put in water, but this way-overexposed
gum gives off nothing whatever into water, further suggesting that
there's no unreduced dichromate left there to dissolve.

When this extra-overexposed stained gum is dried and scraped off the
support, it comes off as a dark brown granular powder (the color of
finely-ground roast coffee, even though in the film it looks a much
lighter brown), very different from normally-exposed gum, which scrapes
off in thin plates, kind of like very thinly shaved paraffin wax. It's
not waxy but it has a similar translucent whitish appearance (although
in the intact film the gum appears clear, colorless and transparent) and
it shaves off in somewhat the same way as paraffin, in thin sheets
rather than in a granular form.

I attribute the crystallization to superfluous crosslinking, though of
course that's speculation. Gum that's crosslinked what I consider the
right amount will withstand dilute acid as well as boiling water poured
with force, besides not re-dissolving in cool water, so I can't think
of any reason I'd need more crosslinking than that.
     
kt

> --- Katharine Thayer <kthayer@pacifier.com> wrote:
>
> > Katharine Thayer wrote: (Sept 20)
> > >
> > > Loris Medici wrote:
> > > >
> > > Actually I liked the combination of cyanotype
> > > > with the warm background (which is tanned gum -
> > probably plus the dichromate
> > > > stain too)
> > >
> > > Yes, if there is dichromate stain in the hardened
> > gum it will impart a
> > > warm tan or brownish color, unless it is so
> > heavily overexposed that it
> > > actually turns green, but hardened gum by itself
> > is colorless.
> >
> >
> > Retraction:
> >
> > Well, I thought I'd seen green dichromate stain once
> > or twice after
> > egregious accidental overexposure, but after trying
> > unsuccessfully to
> > produce green stain in unpigmented gum, I am forced
> > to consider that my
> > earlier perceptions of green may have been
> > influenced by the color of
> > the pigment. At any rate, in my experiments with
> > unpigmented gum, even
> > after six or eight hours in direct sun the only
> > color I've seen is
> > brown. Once it gets to a certain brown, which
> > doesn't take more than
> > five minutes or so in the sun, the color stays the
> > same brown shade no
> > matter how much excess exposure is applied.
> >
> >
> > Katharine Thayer
> >
Received on Tue Sep 28 12:50:49 2004