The irony about discussing 'patina' on stainless steel is, the 'patina' formed will be essentially invisible anyway. When stainless steel oxidizes, it's oxide is chromium oxide, which is essentially colorless (clear), unlike the iron oxide (red rust or black oxide) formed in the oxidation of non-stainless steels. This is why stainless steel never seems to be altered in color, when exposed to and reacting to the things that ordinarily would rust (or truly 'patinate') a non-stainless steel. The darkening seen on 'patinated' stainless blades is only a result of fairly deep etching of the steel. It's not really the oxide making it look that way; the effect comes about in the shadows created by the microscopic pits in the etched surface of the steel. And because it's etching, and not an oxide, that implies something far more chemically aggressive is needed to do it. Kitchen-use vinegar (5% concentration) won't generally be strong enough to do it. This is why more aggressive caustics, like ferric chloride, are usually suggested for 'patinating' (etching) stainless steel.
David
