Blast from the past

Former student assists in restoration
of hot shot furnace at Fort Jefferson

A furnace, once used to heat cannon balls hot enough to set ships ablaze, was recently rehabilitated by a National Park Service (NPS) historic preservation team headed by Paul Neidinger (BED)’86, (MARCH) ’89, a professional fellow with Texas A&M’s Historic Resources Imaging Lab and an NPS historical architect.

The furnace, known as a hot shot furnace, is located at Fort Jefferson in the Dry Tortugas National Park, a group of seven islands, approximately 70 miles west of Key West, Fla. Europeans first became aware of the islands when Ponce de Leon found them in 1513. The area is best known for its coral reefs, which once claimed many ships, and for Fort Jefferson, one of the largest coastal forts ever constructed.

Construction of the fort began in 1846 on the island of Garden Key. It was supposed to protect shipping access to the Gulf of Mexico, but it never served this purpose because it was not completed. The structure was later used to house Civil War criminals and in 1935, it was declared a national monument.

The use of “hot shot” actually predates gunpowder, Neidinger said. The British first used heated projectiles in 54 BC to fend off the Romans. When hot shot was first used in cannons, the cannon balls were heated over an open grate, but this proved dangerous and inefficient. The first hot shot furnace was constructed and successfully used in 1794 at the mouth of the Rhone River.

The furnace at Fort Jefferson is the largest hot shot furnace ever constructed. It was capable of heating a 24-pound ball to red hot in 25 minutes.

The harsh marine environment, tourism, and aging eventually led to the deterioration of much of Fort Jefferson and the giant hot shot furnace. To protect the fort and surrounding islands from further deterioration, the area was declared a national park in 1992.

In 2001, Neidinger and a team of NPS preservation specialists began the complex task of stabilizing and restoring the Fort Jefferson furnace. The three-year effort included disassembling the furnace, removing the original internal iron components, refabricating the components in corrosion-resistant bronze, and then reassembling the furnace.

Like all of the iron objects used in the fort’s construction and armament, the furnace’s internal and external ironwork had oxidized, become infiltrated with salt, and as a result, had expanded, displacing the surrounding brickwork.

To guide the restoration, prior to disassembling the furnace, the conservation team made detailed photographs and sketches of the structure. It took about a month to carefully take it apart. During that time, two preservation masons, a historical architect, and a photographer — all under the direction of the exhibit specialist — worked side-by-side gathering as much information as possible from the furnace as it was slowly dismantled. As the masons removed brickwork to expose new features, the architect rendered them on paper, and the photographer captured them on film. Each brick was inspected and set aside for possible use during the reassembly. The remnants of each piece of iron were measured and drawn by the architect to serve as a guide during replication. When all of the ironwork and loose brick had been removed, the furnace was cordoned off, reusable materials were stored in the adjacent bastion, and the process of replicating the ironwork began.

It was soon determined that the most suitable non-reactive metal for the restoration was an alloy called silicon-bronze — combination of copper, manganese, and silicon that is highly resistant to the corrosion caused by a marine environment.

The ironwork replication was done by Pyrotech Services of Townville, Pennsylvania. The ironworkers used the architectural drawings and photographs to produce wooden patterns and recast each piece exactly as was in iron. After each new piece was cast it took on the slightly greenish color, characteristic of cast bronze. To achieve a finish closer to the black finish of the original iron, each new piece was placed in an acidic bath.

The reassembly of the furnace’s masonry and bronze components began in June 2003 and was completed in fall 2003. Restoration of the slate roof and wood rafter system was completed in March 2004.

The roof system design was developed through building forensics using extant historic fabric, ghost lines, historic photos, and building typology. Photos, drawings, and notes collected during the disassembly guided masons in the placement of the bricks, stones, and bronze into their original positions. New bricks were used to replace those that were fractured by the iron expansion. Damaged stones were carefully repaired using epoxy adhesives and stainless-steel pins.

The restored roof system was constructed from Dade County Pine, milled from salvaged historic timber. The pine, specific to southern Florida, has a high resin content and is highly resistant to decay. Remnants of slate found within the coral ballast of the shot furnace were used to match slate in color, dimension, and thickness.

Though restored to nearly complete working order, the furnace can no longer withstand the high temperatures it was originally designed for. However, the restoration allows Fort Jefferson visitors to view an intriguing, accurately rendered component from the fort’s history.

The NPS’s Historic Preservation Projects program is headquartered in Santa Fe, New Mexico and is part of the Division of Facilities Management of the service’s Intermountain Support Office. The Historic Preservation Projects program employees architects, carpenters, exhibit specialists, and masons who work in partnership with parks, other agencies, partners, and contractors, to help preserve the important buildings and structures located throughout the United States.

Read more about hot shot furnaces at the National Park Service's Popular Study Series Web site.

To learn more about Fort Jefferson and the Dry Tortugas National Park.

Editor’s Note: The above information was gathered from National Park Service Web pages, the USDOI NPS Project Brief written by Glenn Simpson, NPS exhibit specialist, and updated by Paul Neidinger, NPS historical architect. Photos are courtesy of Paul Neidinger.

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