What does KBr do in a bleach formula?
I've seen formulas for ferricyanide bleaching with and without potassium bromide. What exactly does the addition of KBr provide to the bleach solution?
And what is your favorite formula and method of bleaching?
Bromide is needed for rehalogenating the metallic silver - to silver bromide.
I use an old ORWO recipe - but virtually any ferricyanide bleach will also do fine
It is needed to convert silver ferrOcyanide to silver bromide.
Originally Posted by eumenius
As Mr. Rudman has pointed out a bromide, chloride, or iodide
can be used with or after the treatment in ferrIcyanide. And
I'll add; who knows what else? Dan
What happens if just a plain pot ferri/water solution is used? The image still bleaches and has to be redeveloped right?
Originally Posted by PVia
In a nutshell;
When you buy your paper the emulsion contains silver combined with a 'halide' - that is a bromide (silver bromide), a chloride or iodide, or commonly a mix of these 'silver halides'. After exposure the developer converts the exposed silver to silver metal (black) and the fix then removes unused silver halides. You now have a black (& grey) image.
Pot.ferricyanide acts on this metallic silver to convert to silver ferrocyanide - near colourless (straw colour ish). But if a halide (e.g. bromide) is included, it 'rehalogenates' the silver back into a silver halide - i.e. back where you started in the first place, except that it is now only in the image area (the fixer removed it from elsewhere) and it is more stable in room light than the original unexposed sheet was.
Being a silver halide this can now be redeveloped.
This might be in a toner (eg sepia) or in another developer of higher or lower contrst, warmer or cooler tone, lith, and so on.
It can be pretty useful.
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Except that just redeveloping in a "high contrast" developer cannot change the contrast because the amount of image-forming silver has not increased.
Well. it depends on the bleach too. using a dichromate bleach followed by a high contrast developer was a favourite process of mine for some time. The result is intensified blacks with higher contrast to the eye and altered toning properties.
Originally Posted by gainer
Can be very effective with a suitable paper.
You have to be careful with a dichromate bleach with sulfuric acid though. It forms silver sulfate which dissolves in water and therefore removes highlight detail or even the whole image. The Ferricyanide bleaches raise contrast by the same general method, removing highlight detail. This is because a ferricyanide - bromide bleach is a mild blix.
Be careful that there is no ammonium salts in any of these, or you turn them into blixes which of course, destroys the image, at least in part.
Can you elaborate? I would have got it all wrong, and thought bleaching and redeveloping would have kept the same contrast or lowered it. I'm missing something.
Originally Posted by tim rudman
I agree. I use hydrocloric acid
Originally Posted by Photo Engineer