Traditionally bronze is an alloy of copper and tin, while brass is comprised of copper and zinc. Nowadays there is a wide spectrum of alloys and I am not aware of any component that specifically distinguishes bronze from brass. However, there is a strong tradition of bronze being used in bells and outdoor sculpture because of its durability, as well as its ability to produce a strong clear tone.
True "bell metal" is an alloy of 80% copper with 20% tin. It is still used in large church bells and tuned hand bells. Unfortunately, bell metal is very brittle and would not be as appropriate for our wind and door bells. We use "Silicon Bronze" which is 4% silicon, 1% manganese and the balance copper. This alloy is used almost universally in outdoor sculpture, marine hardware and navy bells. It works across a good range of forms from very thin sectioned to heavy, and indeed, it rings very well.