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 Sun Nov 28 23:38:52 2004