Soap & water and a lot of “elbow grease” with a little help from a bronze wire brush will do the trick. Do not use a steel wire brush as it will score the bronze of your bell. If the bell is in the bellfry, be careful to take precautions not to fall and dress in protective clothing with an N95 mask, goggles and gloves as the bellfry is probably crawling with any number of infectious organisms. Pigeon droppings are usually quite rich in Histoplasmosis spores. Be careful.
Harry Long, MD