Bleach then fix

[Robert C. McColloch]

There must be some very knowledgeable photo formula compounders out there that understand the chemistry and reasons why we are instructed to follow certain procedures. I have two questions”

1 After bleaching a print (local or otherwise), why do the instructions say to periodically remove the print from the bleaching process and insert in fixer (typically 30-60 seconds)(they are usually talking about a typical acid fixer)?

2
Same question but regarding use of and alkaline fixer?

I guess my problem is that I don’t really understand the chemistry of the process. If someone could explain, it would be of intellectual interest to me.

Thanks in advance.

[Andrew O'Neill]

By periodically removing the print from the bleach (or local bleaching) and placing it in a fixing bath, the action of the bleach is reduced to nil in a few seconds. This is a good way to control the bleaching action so that it doesn't get away from you. It also helps prevent staining. I've always used a plain hypo bath (no hardener). I've never tried bleaching with alkaline fixer. I'm sure someone will pipe in and explain better than me.

[Joe Smigiel]

I'm not a chemist but I believe a standard potassium ferricyanide bleach rehalogenates the silver to form a light-sensitive compound such as silver bromide. If this silver halogen salt is not removed by a fixer, the compound eventually reacts with light and darkens over time. (I think the standard ferricyanide bleach found in sepia toner also contains potasium bromide to facilitate the rehalogenation process which is then followed by a sulphiding agent, sodium sulphide, in that process.) I assume the bleached areas of a print would either react to light or sulphide due to atmospheric gasses over time if the bleached print was not fixed afterward.

You could probably achieve the same results in a monobath by using a weak solution of Farmer's Reducer which is simply potassium ferricyanide mixed with fixer.