Basically selenium treatment of negatives adds density to highlights that are too flat due to underdevelopment. It does next to nothing to shadow areas. If the negatives are underexposed, selenium will have little effect.(posted 8405 days ago)The procedure is to dilute selenium with water at 1:2. Immerse the negatives for 10 minutes, agitating 5 sec / 30 sec. Wash and dry negatives as usual.
You can expect to gain roughly one paper grade of contrast, with no apparent penalty. It's a highly effective treatment. I've used it a number of times with very satisfying results--even on 35mm negs. It's so good, I've even considered underdeveloping 35mm deliberately (to keep grain at a minimum), then treating with toner, since selenium does not increase apparent grain.