When i removed my heatsink to change CPUs, I had to wipe off the old thermal paste from it with paper towels. However I misplaced the paper towel on a the carpet and some of the silver paste got on it. This was very hard to see at the time, since silver does not contrast well in light pink. Now over a 2 week period, the area where there was paste has turned to black (from oxidization presumably). How can i clean it off?