Rudeofus:This is why one must expose for shadow detail well into the middle of the characteristic curve, because the fog occupies the first half of that curve! With heavily age-fogged film the threshold is in the middle of that curve.
This is why some fast, but age-fogged, films like Kodak 2484 and Easman 4x, will then have highlight detail that is rather contracted and not nicely separated. The highlights have to be 'forced and compacted' in order to allow shadow detail to be registered. That is one of the trade-offs to getting such films to 'perform'.
Gerald: Thank you: so the enigma is really not so arcane: it's only acid that is needed to cause the fixer to become a reducer. (Apparently, there is nothing so special about the 'citric' type?) I will never forget an incident that happened back in the 60s when I was a teen-ager, about 15. I used #2 Kodabromide for paper and was very naive about darkroom matters. I processed the print but it was a tiny bit too dark. My mother called me for lunch and I simply left that print in the full strength fixer (Kodak Fixer powder did not differentiate between film and paper fixer: they were the same strength). After lunch I went down to the basement to my darkroom and was flabbergasted to find a print in the fixer that had the most beautiful tones that I had ever seen. The acid fixer had 'reduced' the density and did so in a 'cutting' fashion that yielded such beautiful tonality and rich contrast. I will never forget that print. - David Lyga