A Potassium Ferricyanide/Potassium Bromide bleach is a re halogenating bleach, more usually used for toners.
Originally Posted by RalphLambrecht
For artwork and complex reduction it's sometimes more useful than Farmers.
Using Potassium Ferricyanide on it's own followed by fixing is quite different in effect to the action of Farmers Reducer.
If you add ferricyanide to commercial fixer it's an agressive bleach particulary if the fixer is ammonium thiosulphate based. However Farmers Reducer uses plain Sodium Thiosulphate and the action is far more gentle and it's useful for lightening shadow details and much slower working.