Marion, tumbling (without abrasive media) will oxidize brass and
copper to a very deep almost indestructible finish. I discovered
this by accident when I tumbled about 500 small brass beads to
polish them up and they turned out greenish black with a satin
finish (I was using porcelain beads as media). It wasn’t what I
wanted at the time, but I liked the “patina” so much I’ve
repeated it several times with different brass alloys and copper.
I asked around about why this happened and was told that the
tumbling process provides a super-oxygenating atmosphere which is
enhanced by the oxygen in the water (similar principle to an
aerator in an acquarium).
If you don’t want this to happen, I think you might be able to
get some kind of solution from Rio or other places which will
prevent it and give you the high brass color you want. I asked
someone in the tech department at Rio and he suggested one of
their products, but I never followed up on it because I liked the
Hopes this helps solve your puzzle.