Hi there Sjixxxy,
Good timing on this post...I just selenium intensified a problematic (thin) negative a couple days ago and got great results.
I always doubted the examples shown in Ansel's, The Negative, thinking that the difference shown must be an exaggeration in the printing of the book...but it's not! I had two negatives of the same scene (one compositionally weak, the other one of my best...both images screwed up in the same way I'm embarassed to say). I tested the procedure a tad differently than Ansel described it, refixing in TF-3 alkaline fixer, then toning for five minutes in a 1:2 solution (using water). I made prints of both negatives prior to toning at max-black time, then re-printed the test negative at max-black time after toning. There was a marked difference, but not enough for the other negative. I gave it 6 minutes...it now prints beautifully (for a work print) at normal contrast settings at max-black time.
One thing I was surprised about was how the shadows were effected. While there was no real difference in the print values around I and II, the higher values within the shadow textures were raised...lifted...revealed...opened.
Murray