From: Richard Knoppow (dickburk@ix.netcom.com)
Date: 02/17/03-07:01:09 PM Z
----- Original Message -----
From: "Philippe Monnoyer" <philippe.monnoyer@skynet.be>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Monday, February 17, 2003 3:36 PM
Subject: Re: Bleaches
> Hi Richard,
>
> Well, a rehalogenating bleach won't work at all for
reversal. These bleaches
> contain either bromides, chlorides or iodides.
> In the reversal process, the purpose of the bleach is to
remove the black
> silver image WITHOUT replacing it by silver halides
(rehalogenating bleach).
> If you do so, you transform the image back to what it was
before
> developpment, namely silver halides. So, you end up with
an homogeneous
> film/paper, like unexposed. If afterwards you developp it,
everything will
> turn black.
> An adapted single bleach would have removed the first
silver image, leaving
> nothing else in the film/paper. That way, the second
developpment will make
> all but the initial image turn black. That's the reversal
process.
>
> I think copper sulfate with a touch of sulfuric acid, like
for dichromate
> bleach, or permangante bleach will work OK. Try different
concentrations.
> Rinse thouroughly to eliminate silver sulfate.
>
> And keep us aware of the result,
>
> Cheers,
>
> Philippe
The above is true where a conventional reversal process
is used. There the final image is silver. If a sulfiding
solution is used things are a little different. The metallic
silver negative image does not have to be removed to develop
the silver sulfide. Afther this second development there
film will contain a negative image of metallic silver and a
positive image of silver sulfide. If the film is now treated
with Farmer's reducer the metallic silver image will be
removed leaving the silver sulfide postive image. Farmer's
reducer, is of course, a mixture of a rehalogenating bleach
and hypo. The bleaching and fixing processes can be
separated as two steps or combined as in regular Farmer's or
as done in color "blix". The Ferricyanide bleach in Farmer's
will not affect the silver sulfide.
There may be other problems with this procedure and, of
course, a sepia image must be acceptable, but it does work.
I believe Permanganate bleach will also remove the
sulfide.
--- Richard Knoppow Los Angeles, CA, USA dickburk@ix.netcom.com
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