potassium ferricyanide

[presspass]

I know this can be mixed with hypo to make Farmer's reducer. Sometimes I read that a print can be bleached with a solution of potassium ferrocyanide alone. Is this possible and how is it mixed and used?
Thanks,

 

[Doubleday]

My procedure is the following: I remove the print from the fix and rinse it. Pour a tiny amount of ferricyanide in a glass of water (the solution is light yellow, not too deep). Put the print on a sheet of plastic and, using a brush, 'paint' with the solution the areas of the print that I want lighten. After few seconds rinse the print under running water and put it in the fix for almost thirty seconds. Remove the print from the fix and inspect it. You can repeat the process again as many time as necessary to reach the desired tone

 

[holmburgers]

Should one discard the fixer after this operation?

 

[Doubleday]

Should one discard the fixer after this operation?

I usually don't discard it. I think that the amount of ferricynide on the print after the rinse is very small and it shouldn't significantly contaminate the fix.

 

[john_s]

The OP needs to be aware that we use ferricyanide, not ferrOcyanide which is a different chemical.

 

[henpe]

The recipes I have seen on Farmers Reducer always include plain hypo, i.e sodium thiosulfate. The same goes for all procedures I have been able to find regarding for local bleaaching (as decribed above e.g.). How about using amoniumthiosulfate instead? What will be the difference, if any? This is unclear to me!

 

[John Austin]

No ferri in the Fix

I usually don't discard it. I think that the amount of ferricynide on the print after the rinse is very small and it shouldn't significantly contaminate the fix.

Ferricyanide in the fix is a contaminent - Any amount, especially as it first appears to have no effect and subsequent ferricyanided prints are added - Also, unless you are ferricyaniding a washed and dried print, there is usually enough hypo' left in the emulsion to do the deed - So rinse your print well before the second fix bath so you don't end up with ongoing contamination when the second bath gets slid along the bench to become the first bath

The only good thing about ferricyanide in the fixing bath is that heady aroma of HCN given off as the ferricyanide is broken down by the acid in the fixer, one of the great sensory deights of printing - In sufficient quantities this gives a boost to the heart rate, too big a boost, unless, of course, you want your children to sell that day's prints as your last ones for huge amounts of money - The Surgeon General will suggest you don't try this!

John

 

[dburian]

My bottle of powder from Photographer's Formulary is labelled as "Potassium Ferricyanide" with an "i"; did I buy the wrong stuff? misprint? ferrocyanide has it's uses, too???
If the proof is in the pudding, it bleaches prints.

D

 

[jeffreyg]

You bought the correct material. The one with the "o" does have a variety of uses but not in photography. My advice about bleaching is to do it slowly and carefully as described by doubleday above. It is best to practice on prints that would otherwise be discarded until you get an idea as to how your mix works. It is very easy to over do it and ruin a print.

http://www.jeffreyglasser.com/

 

[presspass]

Sorry about the spelling error. I did mean it with an 'i'.

 

[Doremus Scudder]

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.

Best,

Doremus Scudder
www.DoremusScudder.com

 

[holmburgers]

Thanks for this conscise explanation. Others have said it, but you've laid it out expertly.

 

[jochen]

Hello,
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).

 

[Toffle]

Well said, Doremus.