Nature of the beast, vulcanite oxidizes when in comes in contact with UV light, saliva, or just about anything else. The only way to get rid of it is to remove it mechanically, i.e. sand, rub, polish it off. Its easier to prevent than it is to remove. Mineral oil, obsidian oil, plain chapstick, wax etc. will go a long way to preventing it. Keep you pipes stored away from sunlight will help immensely. When I'm done smoking I wipe a tiny amount of plain chapstick on my bit and polish it up real quick with a soft rag. Or just buy pipes with acrylic bits and never worry about it again.