read up on Benzotriazole as a fog restrainer
BTZ, as I refere to it in short is an organic anti foggant that I have used with some reasoanable success on some GAF (rebadged ferrania) slow speed old emulsion B&W that I am still using up. It ws given to me last year. It has been fridged/freezered since its production in the late 60's.
I shot a roll with a black velvet cloth as a background, and a grey card and white card filling most of the frame. I snipped off a bit more than a frame worth of it in the dark, and in total darkness tray processed it. D76 1:1 (500mL worth - same volue as used in my tank when doing 120), running water rinse and TF-4 fixer in trays for the recommended times. A brief wash, sqeegee between my fingers, and dry on a hair dryer set on low to speed the process along. The first batch measured a bit too grey in the highlights, as well as the grey. so I, as expected, had fog.
I make up a 1% solution(it is powerful stuff), and add increments of 5ml to the 500ml of developer that I use to process a roll of 120 on reels. Next shot processed as above showed improvement. So moved up to 10mL of 1% solution per 500mL. That was as good as I tried for, and it made me happy with the results.
The BZT does seem to cut film speed some however. The old film that was once 25asa, and I started shooting as ei12, I now shoot at ei8.
Happy experimenting.