Try coating the copper with the cheapest ketchup you can find. Let it sit for a few minutes, then wash normally.
I clean my copper pots with tomato sauce. I used to use brasso but one day I was making tomato soup and in spilled over a big mess, so after I wash the pan I noticed that where ever the tomato soup spilled over the copper was shiny. So I took out my whole set and put tomato sauce all over them and let it set for like 10 min. and washed them they looked great. So that is my cleaner for copper now.
I'm guessing you've tried regular copper cleaner? You could also ask the guys at Safety Clean. They do clean up after fires and probably have some ideas.
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