Quote Originally Posted by Mr Bill View Post
Regarding Kodak's instructions to periodically discard (in 2-stage fixing), I've never understood why. In my experience, Kodak's info is pretty reliable, but they seem to prefer to err on the conservative (safe for the customer) side.
I remember reading this recommendation in Haist's volumes on photographic processing. The rationale behind it was that fixing can suffer from impurities and processing errors. Note that with a two bath fixer setup every impurity or other problem in bath 1 gets carried over into bath 2 during use, so it will stay in the process forever, unlike in single bath fixing where all used fixer is tossed out regularly. A fresh setup after a certain number of reuses made sure that these issues don't stay around forever and, even more important, don't accumulate up until they become obvious.