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College of Arts & Sciences
Maritime Research Division

Deconcreting the HL Hunley Submarine

By Johanna Rivera, H.L. Hunley Project, Warren Lasch Conservator Center, Clemson University

For the past several months we have been hard at work removing the concretion covering the H.L.Hunley submarine’s hull.  Until recently, the submarine was completely covered by concretion, an encrusted layer of sand, sediment and shells that built up slowly over time.  The concretion masked the original surface of the vessel, hiding many features.  But how does our daily deconcretion routine go?

Well, the work on the Hunley tank starts the night before we are schedule to work in the submarine. The tank gets drained 3 times a week (Tuesdays, Wednesdays and Thursdays) with the work in the submarine lasting the whole day. Conservators will set the drainage sequence in the computer the night before, the pumps for drainage get prepared and valves are turned so the thousands of gallons of chemical that fills the Hunley tank can be safely transferred into holding tanks were the solution will remain until it’s time to fill the tank up, which is late in the afternoon.

The Hunley tank’s draining sequence starts at 4am. By the time we arrive in the morning, the tank is drained and ready for us to work in it. Since the sodium hydroxide solution that the Hunley is sitting on it’s a high pH chemical, it require for us to rinse the submarine with water before we can start with the deconcretion work. A conservator will go down the tank wearing a special Tyvek suit and a full mask respirator (Image 1). This will provide protection against any potential splashing of the solution while rinsing the chemical from the surface of the submarine with a hose.

 Once the submarine is rinsed, we suit up wearing jumpsuits, rubber boots, googles, full face mask, gloves, dust mask, ear protection and lights. Though it seems like a lot gear, working in a high pH environment requires personal protective equipment besides our regular deconcretion equipment and tools (Image 2).

As we go down the tank carrying our equipment we head to our assigned deconcretion area to work on. We take pictures of the area to be deconcreted before, during and after, noting any chances or any new features uncovered. In addition, all the concretion that’s removed from the surface is collected, bagged and sampled (image 3). The concretion and sedimentation will be later studied by the archaeological team.

To remove the concretion we have been using small hand tools, pneumatic chisels and hammers to break away the encrusted shells and sediment, which in some places is stronger than the iron it covers.   As we revealed more sections of the submarine, the archaeological team documents and study all new areas of the hull by measuring plates, drawing sketches of features uncovered as well as photographing areas of interested as well as using 3D scanning (Image 4 and 5).

Nearly 80% of the submarine’s exterior has been revealed (Image 6). The last areas remaining are being called “forensic hot spots” (Image 7),  indicating areas where we think there may be evidence that could help us understand why the Hunley vanished after sinking the USS Housatonic.

As we finish up the work for the day, we bag our samples and photograph the hull; we prepare the submarine and the tank to be filled back up again. Valves are turned, pumps are set, and the computer begins filling the submarine with sodium hydroxide once again. It will take about 3 hours to fill the tank and once that’s done, conservators will check remotely that everything is working fine. Then the draining sequence for next day work is set.


The last areas left to complete on the outside of the submarine are mainly the sections made of cast iron, specifically sections of the bow, conning tower, and propeller. As these sensitive areas of “forensic hotspots” are uncovered we are hopeful to find evidence that will teach us more about the mysterious events surrounding the Hunley’s disappearance.