New metal artifact preservation method explored

Submitted by Sammy Smith (sammy@thesga.org)

On 27 December 2009, the online version of Charleston’s Post and Courier published a fascinating story by Tony Bartelme titled “Research on Hunley spurs new discoveries.”

The Hunley is of course the H.L. Hunley Confederate Civil War submarine, which sunk near Charleston in February 1864, and was found by a diver in 1995. The approximately forty-foot submarine was raised in 2000. Since then, its preservation has been a major problem.

As Bartelme notes:

Iron and seawater have a complex relationship, one that sometimes resembles a love story with an unhappy ending.
Put a piece of iron, such as a submarine, in the ocean, and iron and water begin to merge, with iron swapping its ions with chloride ions in the seawater. As long as the iron stays under water, this relationship is stable, and the iron stays well preserved.
But if you remove the iron and expose it the air, the romance turns bad; new and often violent reactions begin as the iron oxidizes. After being pulled from the sea, old cannonballs have been known to spontaneously combust.
On the Hunley, metal shavings collected during the removal of some rivets got so hot they burned plastic bags. Had the sub’s conservators removed the Hunley from the sea and left it alone, the sub would be a pile of dust today, Mardikian said.

Conservators are now using a subcritical reactor, which acts like a pressure cooker to super-pressurize water, and improve preservation by reducing corrosion. Despite the name, there is no radioactivity involved in using the subcritical reactor.

Instead, it creates pressures 50 times higher than what might be found in the open air, and this intense pressure causes materials to react differently. The boiling point for water, for instance, shoots from 212 degrees Fahrenheit to 392 degrees.

Read the full story by Bartelme by clicking here.

Where to find it