I just read a book that mentioned this! If I remember correctly, you brush it with oil, then etch it in dry acid mix from a pool supply store? After that, I believe you wash the acid off in a baking soda and water mix to neutralize the acid. I'm not quite sure. Please correct me if I'm wrong. The book was titled The Skills of a Blacksmith, Volume 1: Mastering the Fundamentals of Blacksmithing. Mark Aspery