Bronze will oxidise when exposed to air, developing a patina coating. A brown, black, red or blue to Verdigris on Bronze is a sign of normal, harmless corrosion. This outer layer protects the inner layers of the bronze object from deteriorating. This coating is often considered attractive or desirable.
Like brass, Bronze is made of metal alloys, meaning they are a combination of two or more different metals. Bronze is an alloy of copper and tin, sometimes with other elements such as phosphorous or aluminium added.
Aluminium does not rust or degrade over time, making it a highly durable material.