Potassium Ferricyanide use

You need to make a 10% solution of Potassium Ferricyanide and Potassium bromide, then dilute this to use as a bleach, when you've bleached the area wash & refix the negative. You will need to use a lot of care to ensure that the bleach doesn't ffect the rest of the image when you wash the negative.

The other alternative is to use Farmers reducer which is similar but with Sodium Thiosulphate added, I prefer the first bleach because if you make a mistake you can wash & redevelop.

Ian

 

Hi, a little different question, but do I have to refix after bleachin a paper too??
Tanks
- OM

 

Yes, because the bleach is re-halogenating the silver back to silver bromide, if you don't fix & wash it will slowly darken again.

Ian

 

Thanks for your quick reply!!
- OM

 

If you are taking the paper back to base white in the areas you are removing the image, you may get yellowish staining with ferricyanide bleaches.

If you do find this happening, the alternative would be to use an Iodine/Iodide bleach. I don't have access to the formulae at the moment, but if you want to know I shall post them when I get home.

 

Steve, the Ilford formula is quite simple.

IR-4 Iodine bleach
Potassium Iodide 16 g
Iodine 4 g
Water to 1 litre

To use mix 1+19 with water. After use re-fix in 20% plain hypo solution (Sodium Thiosulphate).

I used this bleach extensively back in the late 70's & 80's, I found it often stained the emulsion and that a quick dip in developer before re-fixing completely eliminated the staining.

Ian

 

Good tip. I've never had any problems with it myself, but it is good to know how to handle them if they crop up.

 

Please excuse my lack of chemistry knowledge, but what would this mean in terms of grams of each that I'd need to add to a liter of water?

 

A 10% solution is 100g in 1 litre (1000ml).

Ian

 

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.

 

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?

 

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.

Ian