Cast Iron, Salt Air, and 140 Years of Exposure: Cannon at Dry Tortugas

Treatment Methodology: The NPS requirement was for abrasive blasting over other approaches, which were deemed impractrical for this project. Conservation protocols were:

• Mechanically remove delaminating and heavy oxidation using ball peen hammers.

• Mechanically reduce the larger areas of oxidation with hand-operated pneumatic chisels worked along the surface of the cannon at 30-45 degrees at 100 psi.

• Further work the cleaved areas to reduce oxidation using hand-held needle scalers operated at 100 psi.

• Blast all exterior surfaces with 80 mesh garnet medium, using a Lindsay 25 with a 1/4” ceramic nozzle at approximately 100 psi.

Coating System: NPS requirements were for a long-term stable primer and paint system that would withstand the harsh environment for a minimum of five years. In consultation with Keith Lucas, director of the Center for Corrosion Science and Engineering at the Naval Research Laboratory, Amercoat 68HS was selected. This zinc-rich, epoxy-based primer is designed for outstanding resistance in a marine environment.

The paint used is PPG Amercoat PSX® 700, an engineered Siloxane coating designed for weather resistance and corrosion control in a marine environment. A glossy black paint was chosen based on historic photographs and military manuals, both of which suggest that “lacker” applied to the guns would have left a shiny appearance.

Bore Treatment: After removal of debris, the interior of the bore was flushed with compressed air and swabbed with Ship-2-Shore®, a semi-liquid dielectric barrier coating that prevents corrosion in aggressive environments.

Uncovering History: Examples of some of the marks revealed during treatment.

Bore Microclimate: A second level of interior bore protection was introduced in the form of a passive microclimate. Two 12.5-pound Tyvek® bags of conditioned silica gel rest on an interior support. Two ports were built using ¼” Plexiglas and ultraviolet stabilized resin deck plates, installed with a two-part epoxy putty. A Hobo® Pro V2 data logger with external sensors was installed to monitor the interior temperature and relative humidity. The outer port allows for downloading the logger without violating the microclimate. Based on conservation standards the microenvironment should be maintained at or below 15% relative humidity. Monitoring data to date suggests a minimum five-year maintenance cycle.

15-inch Rodman gun, after treatment (EVER 4579)

NPS Photo

Bore microclimate data, 2009-2015 (EVER 4576)

NPS Photo

For More Information: Waterways TV show, episode 265 “Fort Jefferson Preservation”

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