From: Richard Knoppow (dickburk@ix.netcom.com)
Date: 06/22/02-04:28:45 PM Z
At 02:31 PM 06/22/2002 -0400, you wrote:
>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
>
Do you want to bleach the silver or the Pyro stain image?
Any reducer can be used to remove part of the silver image. Different
formulas affect the resulting image contrast a little differently. Some
increase contrast, some reduce it, some just reduce overall density.
The reducer is a combination of bleach to reconvert the silver to a
halide and hypo to remove the halide. The halide image can also be
re-developed. If a slow re-developer is used the degree of redevelopment
can be controlled. High solvent developers, like D-76, D-23, D-25, etc.,
will dissolve some of the very fine grain silver halide produced by the
bleach resulting in a a cutting type of reducing action (shadows are
cleared).
Farmer's Reducer, when the bleach and hypo are combined, is a cutting
reducer which clears low density areas faster than high density ones. When
the bleach and hypo are used separately Farmer's becomes a proportional
reducer for lowering overall contrast.
Persulfate reducers (Kodak R-15) substantially reduce contrast.
None of these bleaches has any effect on the Pyro stain image. In fact,
the silver can be removed completely leaving only the stain.
It is also possible to bleach out the Pyro stain. I can't find the
specific formula I have for this but, by memory, its a Potassium
permanganate type bleach, used with redevelopment in a non staining
developer. I will look for the specific instructions about removing Pyro
stain images. I just don't remember where it is at the moment.
Pyro develpers which produce a stain image (not all do) are effectively a
developer with a built-in intensifier.
Reducer formulas are reasonably easy to find. If not I will either post
them to the list or to you privately.
---- Richard Knoppow Los Angeles, CA, USA dickburk@ix.netcom.com
This archive was generated by hypermail 2b30 : 07/02/02-10:33:22 AM Z CST