From: Liam Lawless (liam.lawless@blueyonder.co.uk)
Date: 06/22/02-02:13:27 PM Z
Sam,
With hydrochloric acid, dichromate bleaches silver to silver chloride, which
is insoluble and redevelopable (as in chromium intensification). With
sulphuric acid, the product is soluble silver sulphate that just goes into
the solution, which is why it's used for reversal. Pot. ferri. bleaches to
silver ferrocyanide, which can also be redeveloped, but it's very slow; it's
usual to add some pot. bromide to speed things up... the result is then
(insoluble) silver bromide. Any bleach giving an insoluble silver salt
should be usable for negative reduction by bleaching and redevelopment.
After bleaching, you need to redevelop or fix (or tone) for a stable result,
since silver chloride, bromide, etc., are light-sensitive. A good way of
reducing dense negs is the process known as *harmonising*, whereby you
bleach fully in (e.g.) pot. ferri + pot. bromide, redevelop less than
completely so that you get full shadow detail with reduced highlights, and
then fix. The image will lighten more or less in the fix, and it takes a
little experience to judge just how far redevelopment should go. For
redevelopment, a print developer is probably best... conventional wisdom is
that developers containing a high proportion of sulphite (e.g. D-76) may
dissolve some of the silver salts before they can be redeveloped, but I
can't say I've noticed it.
Liam
-----Original Message-----
From: S Wang [mailto:stwang@direcTVinternet.com]
Sent: 22 June 2002 19:31
To: alt-photo-process-l@sask.usask.ca
Subject: photographic bleach
I have a question about the various types of photographic bleach. In
general terms, are the dichromate bleaches reversible while
ferricyanide ones not, that is, the dichromate changes the state of
the silver, whereas ferricyanide removes it?
The reason for asking is that I am looking for a way to bleach and
redevelop some over-developed (in pyro) negatives. My tentative
trials of using the bleach from Kodak Sepia Toner seem to be
promising without using anything else, that is, without clearing,
redevelopment, or toning. I suspect the image not to be very stable,
but would like to hear a knowledgeable opinion. What bleach do you
use for bleach-and-redevelop? It appears that the chromium
intensifier formula posted by Judy may be a good one to try. I assume
that you can control and hold back redevelopment, so as to make it a
reductive instead of an intensifying process.
Anyone with chemistry background still around this time of year?
Thanks in advance.
Sam Wang
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