A basic bleach

I want to examine some pyro negs and compare the stains. Can I just use a potassium ferricyanide based bleach, or does something more delicate / stronger need to be used?

That will do it. Mix a very dilute solution with water.

I'm guessing there would be no need for any other ingredients?

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Use cotton balls to apply it to a wet print. I put the very wet print on a large sheet of glass and use the cotton with the bleach very lightly and slowly. Use a wet cotton ball with water and have a container of water to flood the photograph to quickly stop the bleach.

In order to evaluate the amount of stain you need both potassium ferricyanide and sodium thiosulfate. This is to clear out any silver salt that would obscure the reading. Essentially dilute Farmer's Reducer. Afterwards wash the negatives briefly.

I was quietly assuming I was going to fix. But that is not a big deal. I'm curious how strong I would have to make it. I would imagine farmers reducer would be a little weak, as it is supposed to reduce and not obliterate.

When Farmers is used with prints for selective bleaching the solution usually used has a weak yellow color. I would start with this concentration. If the solution is too weak then it will just take longer to work.

mrred said:

I'm guessing there would be no need for any other ingredients?

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If you use a potassium ferricyanide/potassium bromide rehalogenating bleach, you can compare stains and then redevelop the negatives.

I use bleach/redevelopment (redeveloping in a staining developer) to increase contrast on the occasional negative that really needs it. It's a very useful tool and yields surprisingly satisfying results.

My recipe:
15g potassium ferricyanide
15g potassium bromide
1 liter of water

(Actually, I mix only 500ml at a time, halving the recipe.) This can be stored and used repeatedly till activity decreases noticeably. Bleach with the lights on till the silver image is gone (it has been rehalogenated to silver bromide).

If you don't want to redevelop, fix and wash as usual.

Best,

Doremus

It's interesting that the pot bromide is there. Does this have any function other than replacing the bromide that was removed during development? I ask beceause I have no need to re-develop and that's a lot of bromide for my supply level.

mrred said:

It's interesting that the pot bromide is there. Does this have any function other than replacing the bromide that was removed during development? I ask because I have no need to re-develop and that's a lot of bromide for my supply level.

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The potassium bromide (together with the ferri) reacts with the metallic silver to make silver bromide, a silver halide; hence "rehalogenating" bleach. If you rehalogenate in ambient light, you end up with an exposed emulsion. But, since it is there in the exact same proportions as the original image, you can redevelop it.

If you just want to bleach and get rid of the silver, then use ferri+hypo = Farmers reducer.

Best,

Doremus