There was an article years ago in Handloader by Don Krout Jr. about cleaning brass called, “Of Wine And Roses”. As a last step the author filled the tumbler with hot water, then added about a teaspoon of cream of tarter – a mild acid. Then tumble for about an hour. The brass sparkled and looked even brighter. I think he called the article Of Wine And Roses because cream of tarter is a byproduct of the wine making process.
But the cleaning method he used was different too. He used about 2 tablespoons of Ivory Liquid, the older white liquid they don’t make anymore. And in with the brass in the tumbler, he added a pound of brass brads, the kind they use to make picture frames. The brads would rub all over and get inside the cases and even in the primer pockets (why I deprime first).
Seems like a lot of trouble, but the cases looked amazing.