I just found a pair of bridle or livery buckles wherean 1870s stable stood. They were so brown I assumed they were iron. Scrubbed them and saw a hint of copper or bronze. Watched a number of YouTubevideos and tried EVERYTHING they recommended. Brasso, a mixture of flour, salt and white vinegar, and even ketchup. Yes I'm serious. All of those were recommended. None helped. I scrubbed and polished for hours.
Finally I put them in my tumbler with some gravel, Dawn detergent and white vinegar. They came out beautiful.