1) You use P. Ferricyanide with P. Bromide if you want to convert the silver back to silver bromide. This is done in preperation for certain types of toning, such as Sepia. Google for "rehalogenating bleach".
2) You use P. Ferricyanide with S. Thiosulfate for removing the silver when you want to lighten an area or clear highlights. Rapid fixer/Am. Thiosulfate can result in very fast bleaching that can get away from you. This bleach will go bad rather quickly once mixed up, after it has lost its bright yellow color it should be discarded. Google for "Farmers reducer".
The problem with (2) is that the process is not reversible - if you go too far there is nothing you can do to get the image back.
As a result some use (1) to get the right amount of visual bleaching - and if the process goes too far then the negative/print can be put back into the developer and one can try again. After the bleached area looks right the negative/print is put into fixer (S. Thiosulfate) to remove the silver. This bleach-ooops-redevelop-bleach-again process can cause unsightly split-toning.
If you don't thoroughly refix the print after bleaching then any silver left behind will cause staining.
Some people use a combination of (1) & (2) (P. Ferri + P. Brom + S. Thio) - I haven't found any advantage to doing so.
You can not bleach after toning.
Any bits of Ferricyanide left behind will eventually turn to bright 'Prussian' blue. Prussian blue can not be removed once it sets - be sure to clean up carefully after making bleach.
Ferricyanide is not poisonous, as long as you keep it away from strong acids. The used bleach turns to Prussian blue in the drains and is environmentally benign in the small quantities used in the home darkroom.
Last edited by Nicholas Lindan; 09-15-2011 at 03:30 PM. Click to view previous post history.
I use Potassium Ferricyanide and Potassium bromide as a toner. I bleach a print that is slightly darker than normal, rinse it with water, dry it the best I can with a paper towel and I re-expose the print in sunlight. Some of the bleached areas come back. The print end up warmer and higher in contrast. It looks a lot like a lith print. It's totally unpredictable which I love.
I recently took half of a print back to nearly paper-white with a mixture of Potassium Ferricyanide and water alone. I then washed and redeveloped that half of the print. In comparing the original print half to the half that was bleached, I could find no evidence that the print had been bleached. So, a question . . . If the bleaching is in fact "re-halogenating the silver back to silver bromide", shouldn't I be able to re-expose the bleached print area with less exposure and redevelop to get a different level of contrast in that area? Does that make sense?
Originally Posted by Ian Grant
You can bleach in a re-halogenating bleach and after re-exposure redevelop in a different developer. You can't be precise enough with the re-expose unless it's done mechanically (this was once done for reversal processing as an additional control).
Re-developing with a different developer gives greater control, you can choose a contrast or soft working developer or even a staining developer. Ilfords IT-8 Toner works this way.
Also using Chloride or Iodide instead of Bromide can make a difference, Chloride has the gretaer effect.
With a plain Ferricyanide bleach there's a risk of some density loss as the silver/ferricyanide complexs aren't totally insoluble in water unlike the halides.