It's sometimes used in archival processing of film, the effect is pretty much the same as that of toning a print. The selenium protects the emulsion from atmospheric pollutants. With some films, it was required in the processing sequence for long-term storage....SO-132 dupe film (and it's predeccessor, SO339) was like this. I used to go straight from rapid fix (with hardener) to selenium 1:19, then later 1:9, then did a regular water wash of about 25-30 minutes. Kodak recommended not using a hypo clearing step with this process. The earlier versions of SO dupe films had some lousy storage characteristics, and would change density over time. It's one of the reasons why alot of people quit using this film.....
FWIW, they used to recommend using selenium toners in microfilm processing as well, but recent studies have shown that you needed to use a rather strong dose of selenium to get complete protection. So strong that the contrast could increase too much--now they recommend using sulfide toners, or proprietary ones like IPI Silverlock. Microfilm is the "gold standard" for records storage in archives, and they use polyester based films with sulfide toning.....the polyeseter based films last the longest , and the toner protects the emulsion (along with good PAT enclosures of course).
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