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What's happening is pretty clear
Posted By: John Morris (70.108.201.45)
In Response To: [Weapons] Deteriorating bronze cannons in gov't museums (John Morris)
Date: Tuesday, 9 June 2009 1330 hrs. EDT
If you look at a few of the videos I've posted links to, you'll probably see the same thing I've been seeing, the peculiar decay process of this old bronze.
Most of the guns are covered with a chalky, light green substance. Where the stampings or engraving is deep and sharp, especially large letters, you often see a pure white outline in the deepest part of the character, which looks like chalk dust. I'm quite sure this is the tin or a tin compound remaining after the copper has been leached out of the bronze by the acid rain.
The videos clearly highlight the bright blue-green stain left on the concrete pedestals under the cannons after rainwater leaches the copper out (or whatever soluble copper compound is formed, such as copper sulfate.) I'm not a chemist other than what I learned in basic college chemistry, but that seems to my marginally-educated brain to be a logical explanation of what I'm seeing.
The one cannon, "El Alano," which has been conserved shows a great deal of honeycombing on the surface, where tiny pedestals of material are left standing but the metal around them has been eaten away. This is glaringly obvious on the cannon mentioned, but I believe the same situation exists on most of the other cannons in Leutze Park, only the powdery tin or stannous oxide compound is still caked on the surface, giving it a deceptively smooth appearance. Until these cannons are conserved, no one will know how deep beneath the powdery surface the corrosion extends.
-- John Morris (70.108.202.14) -- Sunday, 14 June 2009 1141 hrs. EDT
-- John Morris (70.108.229.52) -- Monday, 15 June 2009 2045 hrs. EDT
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