Do any of you carbon steel knife owners ever "force patina" any of your blades? I just did to one of my tool steel Case knives. This one dates back to 1974. I used several acidic applications, i.e., onion and lemon juice, and a ketchup/mustard mixture to achieve a sort of color case hardening effect.
It was given to me (because I admired it) by a friend who had recently inherited it from his grandfather. We were kids at the time. At the time, it already had a nice iridescent, deep blue, naturally acquired
patina, but back then I thought a blade with a patina was just a blade that needed a good polishing, and did so as soon as I got it home, bringing it back (after lots of elbow grease) to looking brand/shiny new. Kept it that way for years with careful applications of oil after every use.
When I realized many years later how highly prized such natural patinas are on carbon steel blades, I made an effort at restoring it using a "forced patina" process. Not quite as nice as the natural one that was on it when I first acquired it, but still nice I think. Vinegar works well, too, I hear.
PS I've since offered to give it back to my friend, assuming he didn't quite understand at the time the significance of owning his grandfather's pocket knife, but he refuses to take it back, insisting a gift is a gift.