Originally Posted by Michael R 1974
Hi, I guess the lack of responses so far (Doremus aside) implies that there is not a wealth of information out there on this sort of thing. I don't have personal knowledge on this, nor any good tech papers, nor am I a chemist. My main experience has been with color neg/print paper, in very high-volume operations.
Keeping in mind my limitations, my understanding is that with B&W materials the gelatin is the easiest part to wash. After this, there is some tendency for thiosulfate (and related) ions to adsorb to silver grains; I'm not sure how difficult or necessary it is to completely remove this. I take it that various wash aids might tend to bump off and replace this thiosulfate with their own ions. This is my own personal interpretation, not supported with hard literature references.
Regarding the paper base (not RC paper), Mason, in Photographic Processing Chemistry (1966) says, "...the removal of further thiosulphate from the base is a lengthy process by washing alone. The reason for this retention is that the thiosulphate ions have to pass through the cell walls of the paper fibres, which are often protected by sizing materials, etc." There is no mention of pH in this respect, except mentioning that "washing is slowed down appreciably if an aluminium hardening fixing bath has been used." (we know that such a hardening fixer has a fairly low pH.)
Mason mentions wash aids, saying, "The phenomenon was first fully investigated in 1956 by Crabtree, Henn, and King who showed that the introduction of a short soak in certain salt solutions between fixation and washing greatly increased the rate of the subsequent removal of thiosulphate and silver complexes." But again, pH is not mentioned.
I don't know if there IS any modern research on this, other than occasional articles by photographers who have done their own tests for residual thiosulphate.
To throw one further complication in there, about 10 years ago there where some internet articles to the general effect that it may be desirable to maintain some low level of residual thiosulfate - that is, to limit the washing. A couple of technical papers were referenced, of which I read one. It was regarding micro-film, where (per my fuzzy recollection) small "redox" spots tended to form on certain films, but not others. And that a tiny amount of residual thiosulfate and iodide existed in the unaffected films, but not in the others. (Don't put too much stock in my exact account of this.) I didn't investigate any further, as my day job didn't involve B&W by this time, only color, but you can probably find further info via a web search. My point is that it may be possible to over-wash.
Perhaps someone will have some better information on the relation of fixer pH to washing. (I mean with some sort of solid basis other than "it's well-known," or that sort of thing.)
Last edited by Mr Bill; 05-09-2013 at 06:06 AM. Click to view previous post history.