Nothing magic here. Take any old pencil and rub it over the rusted or darkened area on an old knife blead, especially the stamped areas. Then just wipe the area with a clean cloth or paper towel. You do not remove any metal and it cleans the area up a lot. I guess the physical rubbing and some action of the rust with the graphite in the pencil lead does the trick. You keep the aged patina of the blade but make stamps more readable and blades cleaner.