I have had godd success with the Lyman liquid re-charge liquid. Just a little cap full in a full bowl, tumble for an hour and let dry. The corn cob is black but it buffs the brass well.
Also because of the mess with tumbling Iam always adding a bit more new corn media to the mix so topping up the mix with fresh liquid is not always needed.
Most stores carry the liquid. $10 a bottle and it will last you for years.