Role of Sodium thiosulfate in Farmer's reducer

[Alex Benjamin]

Found many answers to my questions regarding bleach in various Phototrio threads, except those regarding sodium thiosulfate.

I do understand the role of the potassium ferricyanide, of course, and also understand that the addition of potassium bromide makes the bleach a rehalogenating bleach, meaning I can re-develop the print.

What I'm not sure I understand is the role played by sodium thiosulfate in Farmer's reducer. What, exactly is its impact? Can I still re-develop if my solution contains both potassium bromide and sodium thiosulfate, or is that cancelled because of the sodium thiosulfate? Do I still need to fix the print afterwards?

I use this formula from the latest edition of The Darkroom Cookbook:

 

[revdoc]

Sodium thiosulfate is fixer.

The reducer does this:

Ferricyanide and potassium bromide convert silver to insoluble silver bromide.
Sodium thiosulfate forms complexes with silver bromide and effectively dissolves it, removing it from the emulsion.

The second step is just what fixer does with film and paper normally, and makes this process irreversible.

You shouldn't have to fix after reducing, but I guess a lot of people will, anyway.

 

[koraks]

What I'm not sure I understand is the role played by sodium thiosulfate in Farmer's reducer. What, exactly is its impact? Can I still re-develop if my solution contains both potassium bromide and sodium thiosulfate, or is that cancelled because of the sodium thiosulfate? Do I still need to fix the print afterwards?

The thiosulfate does two things: (1) it accelerates the bleaching action and (2) it renders it irreversible. The thiosulfate acts as a fixer just like in a regular B&W process. What's fixed out, cannot be redeveloped later. Hence, no fixing is needed after Farmer's reducer, no redevelopment is possible, but the print of course should be washed at the end of the process.

 

[Alex Benjamin]

Thanks @revdoc and @koraks

Answers my questions perfectly.