I've used mustard, horseradish, vinegar, lemon, limes, potatoes, tomatoes, beef blood, aspirins and many more to get a patina. They all work to different degrees. That day we had sushi takeout for lunch and so the wasabi got used. Here's a surprise for you Charlotte and Thomas, have you tried leaving the blade outside in the dirt? Thanks for the suggestion though. Just using the knife in the kitchen will eventually do it also.