For anyone interested in corrosion of antique bronze, "Trophy No. 31" at the Washington Navy Yard presents a unique opportunity. Briefly (according to my notes) this rifled Austrian field gun or howitzer was captured in the blockade runner COLUMBIA during the Civil War, and was eventually displayed inside the Museum of the United States Navy. I can't find my photo of it there at the moment, but it was quite an impressive sight decked out primarily in gloss black paint, but with (as I recall) muzzle and breech painted gold. The important point is that the barrel was completely painted and on indoor display.
At some point, perhaps 8 or 10 years ago, a permanent concrete stand for iron guns outside the museum became vacant due to an Army 3-inch ordnance rifle being exchanged with another museum for a Navy weapon. However apparently the weapon received didn't fit the display stand, so the painted Austrian howitzer from inside the museum was stripped of its paint, and put on the outdoor display stand where it is now.
Since the environmental exposure of this piece only began relatively recently, the bronze is still dark and in relatively good condition, although etching by acid rain has begun. You can see the green stain on the concrete where the copper is running off, and the white tin residue is clearly visible over most of the surface of the bronze. The surface has been roughened slightly by the etching, but not seriously, and all of the original markings on gun are still visible, as is the US Navy's marking "Trophy No. 31."
Due to the very limited time of exposure of this piece, it presents a unique opportunity for studying the effects of corrosion on antique bronze displayed outdoors.
If this piece was anything I'd categorize as either rare or fragile, I'd recommend the Navy slap some paint on it to prevent the inevitable, destructive corrosion. However it isn't in that "endangered" category. There are a few more of he same type nearby, apparently with the same provenance which seem to have held up relatively well during their long term of outdoor exposure.
See slideshow linked below.
http://s17.photobucket.com/albums/b62/cannonmn/miscforumsetc/forums36/?action=view¤t=f4edfa00.pbw
