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Ferricyanide and RC Paper

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Old October 1st 09, 08:29 PM posted to rec.photo.darkroom
Richard Knoppow
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Posts: 752
Default Ferricyanide and RC Paper

"Lew" wrote in message
I plan to experiment with cleaning up print highlights this
via ferricyanide solution & refixing, so I'm googling
around for
whatever tips I can find. A thread on APUG suggests that
this won't
work on RC prints, yet another assures that it will.
Any comments on this dilemma?

I intented to answer directly about RC vs: Fiber in my
other reply but didn't so here goes...
It isn't the support but the kind of emulsions often
found on "modern" papers, especially variable contrast
paper. These are more complex than those found in
conventional, graded, papers and may be harder to bleach and
to tone using indirect (bleach and redevelop) toners. I am
not sure of the exact reason but it may have to do with the
presense of silver iodide in the emulson although that is
found in much larger quantity in film. In any case the same
bleaches, including ferricyanide, will work but may have to
be stronger or used for longer. Kodak encountered this
problem with their Sepia Toner, a bleach and redevelop type
using a ferricyanide bleach, and came out with a new toner,
Kodak Sepia Toner II, which differs mainly in the
composition of the bleach. It is still a ferricyanide bleach
but is stronger. My experience with the older toner on
variable contrast paper is that it never quite completely
bleached out the shadows, which remained black where older
paper was bleached to the point where the image was a yellow
color even in the densest parts. While a reducer is not
usualy required to bleach to this extent they may take
longer to work on variable contrast emulsions. This may not
be a bad thing when bleaching for the purpose of removing
highlight veiling.
Note that the bleach used for reduction is somewhat
different from that used for toning or intensification. The
former converts the silver to a form that can be removed by
hypo while the second converts the silver to a silver halide
which can be redeveloped in sulfide or in a conventional
developer. The halide can also be removed by hypo, of
course, but if that is what is desired the first type works
as well and the formula is simpler since it does not require
a halide (typically bromide) to be present.
As far as the emulsions go there is no difference
between emulsions suitable for coating on conventional
"fiber" (untreated paper) support and on resin coated paper
support but typical RC papers may have newer types of
Most of the photographic solutions are pretty old.
While developers and fixers may work about the same way on
both old and new papers some formulas, such as the bleaches
used for toners, reducers, intensifiers, etc, may not and
need some modification when used on modern materials.

Richard Knoppow
Los Angeles, CA, USA


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