Actually, one practical use is to make your own silver filter for viewing and photographing eclipses. You need to expose the film to a fare-thee-well (at least ten stops overexposed) and develop to full density. Fix, wash and dry as usual. I still have the one the x-ray lab at AGFA made for me in 1975.
Unexposed, unprocessed film cannot be used, as it does not block the U-V portion of the spectrum;- only the fully developed silver will.
