The material you mention is dried brake dust. The stuff could probably double as a reentry heat barrier on teh shuttle its so damned tough to remove... maybe I should offer mine to NASA if they can get it off my car?

Seriously, its a PITA to remove. Its a result of the fact that volvo pads are messy and make lots of dust. I had some success with a knife one time, bu that also damages the finish on the rims and leaves marks. If I had to bet on the best approach I would probably say either:

1. repeated treatment with brake dust cleaner and a serious brush

2. a high pressure washer with a serious nozzle (10-15 degrees). However, I would suggest you do this with the tire off since a pressure washer with a 10 degree nozzle "could" cut a hole in the tire. :)

good luck,
rt