[pure-silver] Re: Best fixer with ABC Pyro for maximum stain?
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Sun, 15 Nov 2009 21:20:49 -0800
----- Original Message -----
From: "richard lahrson" <gtripspud@xxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Sunday, November 15, 2009 7:13 PM
Subject: [pure-silver] Re: Best fixer with ABC Pyro for
maximum stain?
Hi! Will pyro stained negs scan ok? Does the stain
matter when scanned or
do scanners compensate? Best, Rich
On Sep 29, 2009 5:09 PM, "Richard Knoppow"
<dickburk@xxxxxxxxxxxxx> wrote:
I am not an expert at scanning but here are a couple of
things of possible interest.
The stain image produced by Pyro and some other developers
is exactly proportional to the silver image. The stain
blocks blue light. When used with blue sensitive materials
it intensifies the image, that is, the image prints with
higher contrast than the silver image alone produces. When
printed onto a color sensitive material the effect of the
pyro stain is reduced. If printed onto a panchromatic
material with an amber or red filter the stain image has no
effect.
Pyro negatives became popular mostly because the
yellowish stain can act as an imagewise low contrast filter
for variable contrast paper. It has the effect of reducing
highlight contrast which some find advantageous when using
films like Kodak T-Max which are capable of very high
highlight density.
Since a scanner can be adjusted to be sensitive to any
color it can be set up to scan using only the transmitted
blue light. When used this way it should show the conrast
increase produced by the stain image. Its possible that a
scanner or image editor could be set up to duplicate the
effect of variable contrast paper but I don't know how to do
that and, in any case, an image editor program can be set up
to apply a curve to the scanned image and so give the same
effect.
Supposedly the acid in an acid fixer will bleach out
part of the stain image. I have never tested this and don't
know if its actually true. If you are concerned with this
you can use "plain" fixer. A plain fixer contains no acid,
it has thiosulfate and enough sulfite to protect the
thiosulfate from oxidadation from the air. The acid in acid
fixers is there for two reasons:
1, to provide an acid environment for the common white alum
hardener used, and 2, to prevent carried over developer from
continuing to develop in the fixing bath.
When a plain fixing bath is used the film must be
washed between developing and fixing. An acid stop bath can
be used to stop the development quickly but it must be
followed by a long rinse to remove both the developer and
stop bath before fixing. The sulfite will to some extent
provide protection of the thiosulfate from acid, which
otherwise will decompose it. For a plain bath where there is
no acid stop about 5 grams per liter of sodium sulfite is
enough. If acid is expected this should be increased to
about 15 grams per liter, the same amount that is used in
acid fixing baths. Thiosulfate itself is not sensive to the
pH of the solution and will fixe equally well in acid,
neutral, or alkaline baths. A plain bath is probably
slightly alkaline depending on how much sulfite is in it.
By a wash between developing and fixing I mean perhaps
a minute or two in running water. That should be enough to
remove the developer and to stop development quickly.
Note that pyro negatives should have _no_ stain, or at
least very little stain, in highlight areas. The often
recommended practice of treating fixed negatives in
developer to increase the stain really only produces an
overall stain which is not desirable.
ABC pyro is a very old formula. It became standardized
and one finds exactly the same one published by Kodak,
Agfa/Ansco, DuPont, etc. Some older formulas have
substantial differences such as the use of acetone to
generate the alkali. Since acetone is quite volitile the
activity of such developers can vary widely. That is one
reason that pyro developers got a reputation for unreliable
in the old days. ABC is probably pretty predictable and
reliable. However, because the amount of stain varies with
the film its difficult to predict final gamma without making
tests. Also, because the _effective_ gamma depends on the
spectral sensitivity of the printing material one must
duplicate it by using a filter in the densitometer to get
any sort of meaningful measurement. Usually, one relies on
cut-and-try.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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