From: martinm (martinm@SoftHome.net)
Date: 06/22/02-11:42:40 PM Z
> 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
Principally, a bleach containing halide (e.g. potassium bromide, sodium
chloride, potassium iodide) is a rehalogenating bleach. A reversal bleach
substantially removes "material" (developed black silver) from the emulsion.
So re-developing a reversal bleached emulsion results in a reduced density
compared with the original one (or the one bleached with a rehalogenating
bleach).
Martin
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