I am cleaning a bell that sits outside the front of our church. The bottom and top look like what I would consider normal patina. Moderately dark bluish green. The middle portion of the bell is almost solidly pale bluish green. There are areas where the smooth darker color of the bell meet the light portions and the light portions are somewhat thick (maybe the thickness of 4 sheets of tablet paper)and look like a layer of some material rather than a coloration of the metal bell. I cleaned the bell with a solution of 1 TBSP of salt in 1 gallon of warm water. The rags I used turned dark green quickly and after rinsing with clean water, there was very little color on the towel I used to dry the bell. I'm just concerned about the large light portion of the bell that looks like a solid layer of some material. Is this really natural patina or heavy corrosion that should be removed? The writing on the bell is almost perfect and there is not a large amount of pitting anywhere on the bell. How can I tell if I should use something stronger to clean the bell or if this is the way it should look? The bell is McNeely, West Troy, New York, 1884.