Re: Modifying bleaches

"The bleaching agent seems to need to be potassium ferricyanide, or maybe
Potassium permanganate, or a dichromate."

Actually, there are a great many oxidizers that can do the job - from copper
sulfate to ceric sulfate (not to mention nasties like bromine and the like).

"Unless I am told I really need something else, just from a safety
standpoint, ferricyanide looks like my choice."

Ferricyanide can be replaced by Fe-EDTA or ferric sulfate, which do less
harm to the environment.
E.g.:

EDTA (2Na) 30g
Fe(III) Sulfate 30g
Potassium Bromide 30g
Sodium Hydrogen Sulfate Crystals 30g
Water to 1000ml

Note, this formula applies to ultra-fine grain (Lippmann) emulsions. For
coarser grains you might have to increase the concentration of the bleaching
solution.

Martin

>

----- Original Message -----
From: "Charlie Goodwin" <cgoodwin@mcttelecom.com>
To: "AltPhoto List" <alt-photo-process-l@sask.usask.ca>
Sent: Monday, November 29, 2004 6:38 AM
Subject: Modifying bleaches

> I have become interested in the, new to me, subject of rehalogenating
bleaches, both for the flexibility they appear to offer for toning, and for
the potential to "turn" a paper of whatever flavor, bromide, chlorobromide
or chloride, into a bromide paper or to a chloride paper, or an iodide paper
etc.and to redevelop it as such, and also to be able start with a paper as
whatever it is, and then to tone it as whatever else I might want it to be.
Seems awfully adaptable.
>
> I have no knowledge of the chemistry of bleaches, and wish to proceed
safely. I am hoping more knowledgeable persons could tell me that either
my guesses are correct, or if I am off base, to tell me what I need to know.
>
> From browsing the web and scanning through "The Darkroom Cookbook" it
appears that the keys to rehalogenating bleaches are a bleaching agent and a
source of a halogen, chlorine, bromine or iodine.
>
> The bleaching agent seems to need to be potassium ferricyanide, or maybe
Potassium permanganate, or a dichromate. Unless I am told I really need
something else, just from a safety standpoint, ferricyanide looks like my
choice.
>
> And then a halide source, potassium bromide, potassium iodide or sodium
chloride
>
> I found in "The Darkroom Cookbook" a bleach called a rehalogenating Bleach
and several very similar looking bleaches like the DuPont 6-T Toning System,
which deploys three different bleach baths for varied effects:
>
> Varigam Toning Bleach 6B-1
> Potassium Ferricyanide 22g
> Potassium Bromide 25g
> Water to make 1000ml
>
> Varigam Toning Bleach 6B-2
> Potassium Ferricyanide 22g
> Potassium Iodide 10g
> Water to make 1000ml
>
> Varigam Toning Bleach 6B-3
> Potassium Ferricyanide 22g
> Sodium Chloride 35g
> Nitric acid 15ml
> Water to make 1000ml
>
> These are ordinarily followed by toning baths.
>
> I presume these Varigam Toning Bleaches are all rehalogenation bleaches
which could be used to reduce a print back to a silver halide and followed
by a simple redevelopment in one or another common developer.
>
> My initial question is that I wonder if chloride rehalogenating bleaches
like Varigam Toning Bleach 6B-3 could be modified by using hydrochloric acid
rather than nitric acid.
>
> That quickly becomes several related questions. Would it be safe to do
so? Would it work well? I am hoping to use, wherever practicable, common
household chemicals or commonly available items like muriatic acid. If I
can get along well without ever needing nitric or sulfuric acids and super
concentrated acids I would be glad. But, if I do need to use nitric or
sulfuric acids, can they and should they be obtained in lower
concentrations? Would that create other issues? Cost is less an issue
than safe handling.
>
> Reading about ferricyanide bleaches, the precautions I see say that
ferricyanide is generally not a dangerous source of cyanide gas unless it is
mixed with strong acids. I am guessing that the 15ml of nitric isn't
enough to create that hazard. I hope I can use hydrochloric in a similarly
safe manner (or safer). I don't want to mess around with safety on this
one.
>
> Thanks in advance!
>
> C
Received on Mon Nov 29 06:20:41 2004