Bronze is a kind of special metal materials and easy go to oxidize in the air, which consisting Sn, Zn, Fe, Pb, P and Cu etc. The bronze used in watch business include CuSn8(C52100), Phosphor Bronze, aluminum bronze. But CuSn8 is more popular and expensive. It's color will change to vintage looking.  

This material is soft then steel and hard to make watches by mold and very difficult to avoid oxidized and finger prints during production.