There are several ways to use ferricyanide. The following are things I have used before. I'm sure there are more too.
Farmers reducer and other similar bleaches combine the ferricyanide with either sodium or ammonium thiosulfate (standard and rapid fix main ingredients respectively). These bleaches effectively "fix" the silver that is being acted upon by the ferri. This kind of bleaching is not reversible. Bleaches of this type tend to work rather rapidly. There are lots of possibilities here for varying the proportions and thus the activity of the bleach. The effect of this type of bleach is immediately visible.
Ferricyanide can be used alone as a bleach. Alone, it tends to act rather slowly. I'm not sure of the silver compound that is produced with this procedure. Unless the ferri is completely rinsed out of the emulsion, the carried-over bleach will be "activated" when the print is transferred to the fixer and continue bleaching till the ferri is exhausted. Some workers apply a bit of ferri, rinse a bit (not completely) and then transfer the print to a tray of fixer, where the majority of the bleaching takes place, but is limited by the amount of carried-over ferri. This is essentially a two-bath Farmers Reducer, and a good way to control the bleaching activity. The downside is you don't know how much you've bleached until the print is in the fix and it's too late. Therefore, working with a rather weak ferri solution and in many steps is a good idea.
There are also rehalogenating bleaches in which ferricyanide is combined with a halogen, most commonly potassium bromide. This bleach converts the bleached silver back into a silver halide, which can be redeveloped and is, therefore, reversible to an extent. The redeveloped image, however is usually of a different image tone and tones differently from the rest of the unbleached print (many use this for precisely this effect). The effect of this bleach is also immediately visible. Bleach/redevelop steps are used in some toning processes and for increasing contrast in negatives as well.
In all of these cases, the bleached print needs to be fixed and washed to remove the bleached silver compounds that remain in the print.
Thanks for this conscise explanation. Others have said it, but you've laid it out expertly.
to convert metallic silver into a soluble form, it has to be oxidized. Potassiumferricyanide = potassiumhexacyanoferrate(III) is an oxidizing agent. While the silver is oxidized the threevalent iron(III) is reduced to bivalent iron (II) = potassiumhexacyanoferrate(II) = potassiumferrocyanide. The sodiumthiosulfate converts the silverferrocyanide into water soluble complexes like it does in the fixer. These complexes are so stable that the equilibrium is shifted and the oxidation is accelerated (law of mass action of Guldberg and Wage).
Tom, on Point Pelee, Canada
Ansel Adams had the Zone System... I'm working on the points
system. First I points it here, and then I points it there...