I have recently acquired these for ritual use.  These are antique Japanese. Bronze mirrors were widespread before the advent of glass. In Tibet and other countries they are still used for divination.  In this case, the concave surface is sometimes decorated and the convex outside is polished and plain.  The convex side is shown on the outside when worn.  Shamans may have a coat partially covered with these mirrors – to deflect evil and to concentrate the power of deities :