What experiment are you doing? Can you explain it in a bit more detail. Are you setting up an electrochemical cell using a salt bridge? Bronze is made of (mostly) Copper with (a little) Tin added. According to Wikipedia (88% copper 12% Tin). And they said, "modern bronze" which I thought was called Brass, is made of 90% Copper 10% Zinc. You can definitely use them in an electrochemical cell, but just throwing a lump of either metal in water won't do anything visibly in any short amount of time.