CSS submarine Hunley begins a five year bath in a caustic solution to remove concretions from 136 years on the floor of Charleston Harbor.

Last Thursday was the first day of approximately 1,825 days the Confederate submarine H. L. Hunleywill spend immersed in a caustic bathof sodium hydroxide and water. This a watershed step in the conservation of the 40-foot iron submarine, a long bath 14 years in the making. The caustic bath will first remove the concretion, the thick layer of hardened shell and sand that formed on the hull of the sub during the 136 years it spent on the floor of Charleston Harbor since it sank the night of February 17th, 1864. The concretion completely covers the outside of the sub, obscuring the damage that might answer the questions about how and why the Hunley sank after successfully torpedoing the Union warship USS Housatonic.

The solution of 99% water and 1% sodium hydroxide should begin to loosen the concretion within a few months, giving conservators their first look at the hull in the 14 years since the submarine was raised from the Atlantic seabed. Once the concretion is loosened, conservators will scrape it off the hull, a painstaking process that will take months.

After the scraping, the Hunley will settle in for the long soak. The sodium hydroxide has another important job to do: removing nearly a century and a half of sea salt from the iron hull. That’s the process that will take five years, with conservators regularly draining the 76,000 gallon tank of its 68,000 gallons of solution (large jugs of gravel and water in the tank displace 8,000 gallons) when it gets too salty.

Via The History Blog