I’ve seen many photos of bronze artifacts and they always have this green and brown patina which from my experience with bronze jewelry could probably be removed. Is it just a want to preserve the state they were found in? Is it that there’s not enough artifacts of this type and they might be damaged? Can we learn something about the artifact from the patina?
Bronze, copper, brass and related alloys are highly reactive to oxygen. This is called oxidation. Oxidation causes copper in these alloys to become weak and crumble.
If an object completely patinas over its entire surface evenly, then the core of the object is no longer exposed to oxygen and cannot oxidize further; it is preserved. An object with an even and undisturbed patina is stable.
An object that is polished exposes the inner surface, and allows oxygen to work on the object again. Keeping an artifact polished essentially allows it to be worn away a little at a time.
This is also why copper-alloy artifacts should be handled as little as possible. If any cracks develop in such an object, it allows oxygen to penetrate along the crack, oxidizing the object's interior and weakening it.