Reversal question

[glbeas]

Doing a little research on reversal processes and Ive seen clearing baths with sodium sulfite alone and baths with sodium metabisulfite. What would be the reason to pick one over the other?

 

[138S]

"The clearing solution removes any yellow staining caused by bleaching. This is to clear away all traces of the powerful bleaching bath, and the slight stain it leaves behind."

If using a permanganate bleach often it is recommended to use metabisulfite or bisulfite in the clearing. Just use the clearing bath that's specified in the recipe you follow.

If a yellow stain is left after the complete processing then you may blame the clearing bath that is not suitable, but if no stain remains then your clearing bath should be ok.

Most common bleachings are permanganate or dichromate...

 

[revdoc]

I'm not 100% convinced that the clearing bath just removes the bleach stain. If you skip it, or don't clear long enough, the result will be fog in the end product; consequently, I think the clearing bath might also remove the bleached halides.

However, I'm no chemist!

 

[glbeas]

Thanks, that helps a bit. I have both permanganate and dichromate bleaches available, thinking of trying the dichromate first.

 

[138S]

Thanks, that helps a bit. I have both permanganate and dichromate bleaches available, thinking of trying the dichromate first.

Dichromate is very toxic, before disposal mix it with used developer, it will change from orange color to green, to a way less toxic state.

 

[koraks]

I think the clearing bath might also remove the bleached halides.

Not really. Otherwise sulfite could be used as a fixer on its own. You actually don't want a reversal clearing bath to remove any halides as it will remove halides that are needed for the positive image.

 

[Raghu Kuvempunagar]

Bleaching not only removes silver but also has some densensitising effect on the remaining halides and according to Yurow's Unblinkingeye article, the effect varies with bleach. Sulphite clear apparently does damage control by increasing the sensitivity of the desensitised halides. In this context I wonder whether sulphite clear is more important in reversal processes where a second light exposure is involved as opposed to processes that employ a fogging redeveloper like dithionite or thiourea.

 

[Rudeofus]

Potassium Permanganate bleach has a tendency to deposit insoluble MnO2, unless you keep pH down. In this case I'd suggest you stick with Sodium Metabisulfite.

Whether you use Potassium Permanganate or Potassium/Sodium Bichromate depends very much on where you are. There are countries, in which possession of Permanganate gets you into legal trouble. At the same time Bichromate is said to be very toxic and a known carcinogen. I'd say if you can get Permanganate, then use it, otherwise read up on safe handling of Bichromate and use that.