The other day I took a disappointingly weak 18x24cm negative and decided to boost the contrast a bit.
Bleached in 10/10 Potassium Hexacyanoferrate(III) (AKA Pot. Ferri) / Potassium Bromide until only a faint trace of image could be seen.
I then redeveloped it (in daylight) in a mixture of half a teaspoon of Pyrogallol and one teaspoon Sodium carbonate in one liter of water.
Washed and hung to dry.
Comparing "before" and "after" van Dyke prints, the main effect of this seems to have been lifting the midtones up. Glare highlights are still glare highlights, and well separated from other highlights. The midtones (which were rather dark and unappealing) are now clear and brilliant, while still being very clearly separated from the highlights. Shadow areas are still shadows, but the printable detail seems to have increased a little.
All in all, it looks about like increasing the gamma on a d*gital image: Deepest shadows stay in place, midtones lighten dramatically, highlights a little lighter.
Less-than-complete bleaching is the next thing to try, although I think I'll make a few step wedge negatives first...