Don't wanna come across as a knowitall but I would seriously caution against using ANY metal brush except maybe for wearing down a crusty (and then really carefully at that). Never mind what the seller says, soft brass, light as a feather silver, etc.... these will all absolutely, positively, unequivocally scratch your coin whether the visible to the naked eye or not. The bronze alloys used in Roman coins, and particularly in the late Roman period, have a high content of lead and tin (along with a witch's brew of other chemicals) which all conspire to make the metal very soft.
Ras