Research on Hunley spurs new discoveries
By Tony Bartelme
The Post and Courier
Sunday, December 27, 2009
Clemson Professor Michael Drews explains how the subcritical reactor at the Warren Lasch Conservation Center works. Conservators working on the H.L. Hunley have discovered a new way to preserve iron artifacts using this new technology.
The complex reaction between iron and seawater has always plagued marine conservators. The new preservation technology could ease their frustration. At right is an X-ray imaging of before and after treatment.
Water is just water, right? Not at the Warren Lasch Conservation Center in North Charleston, where researchers with Clemson University and conservators working on the H.L. Hunley use super-pressurized water in ways that could transform the preservation of metal artifacts, increase the durability of offshore windmills and even make paint cling better to ship hulls.
The secret of the water's transformation is tucked in a corner of the Lasch lab, in a room next to the Hunley and a pair of cannons from the Confederate raider Alabama.
"This is the big one," said Michael Drews, director of the Clemson Conservation Center at the Lasch lab, pointing to a panel of levers and pumps next to a waist-high metal cylinder.
Called a "subcritical reactor," the contraption is a sophisticated cousin of the pressure cooker and has nothing to do with radioactivity.
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.
Just below this higher boiling point, water becomes "subcritical" and behaves like methanol and other solvents.
Drews and other Clemson researchers have been investigating whether subcritical water could preserve iron artifacts. They've tested the process on several of the Hunley's rivets, and so far, the subcritical technique has successfully preserved these pieces 10 times faster than other processes.
Paul Mardikian, the Hunley's senior conservator, has described the process as a potential "turning point in the history of archaeology and conservation."
This work also has led to new looks at how metal can be protected from corrosion, research that bolstered Clemson Restoration Institute's successful proposal to the federal government for a national wind turbine drive train test lab.
The $98 million wind lab will be built next to Lasch lab.
"Metal and the sea. It's all connected," Drews said. "The Hunley showed what happens to metal in the long term, and we're using what we're learning from the Hunley and applying it to modern metals."
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.
Marine conservators always have wrestled with how to preserve iron once it was removed from the sea. They boiled artifacts, heated them and painted over them, but none of those techniques worked.
For the Hunley, conservators decided the safest course was to soak the sub in a caustic bath and zap it with electricity for a number of years to remove the chlorides.
By chance, the Hunley conservators discovered a new option. In the fall of 2001, Mardikian had a meeting at Clemson, and Drews, a professor in the university's material science and engineering school, happened to sit in.
Drews had been working with subcritical water and its effect in wastewater systems. As he listened to Mardikian, Drews wondered: Could subcritical water work on the Hunley? If it did, Drews thought, conservators might be able to preserve artifacts in a matter of weeks or months instead of years.
During the past several years, Drews and the Hunley's chemist, Nestor Gonzalez, tested subcritical water's effects on thimble-sized iron artifacts using a scuba-tank-sized reactor nicknamed "Felipe 1."
They studied how these artifacts did compared with artifacts preserved through slower methods. So far, Drews said, the subcritical technique has worked.
Useful for paint
Now they're poised to begin testing the process on artifacts in the new large subcritical reactor, which is big enough to preserve cannonballs.
Drews said someday the subcritical technique could be used on even bigger objects, such as cannons.
How about the Hunley?
That's not likely, Drews said. Years of tests on large objects still need to be done. "The Hunley is unique, and the risk (of something happening to it) is unacceptable." Besides, the need for a sub-sized pressure chamber probably would be limited to a few artifacts, making the construction of a large subcritical apparatus a tough investment to justify.
Research into the effects of subcritical water on iron artifacts also has led to other discoveries.
Drews said researchers learned that the process has an etching effect on metal that can be seen only with powerful electron microscopes. Drews said he didn't confirm this until the lab acquired such a microscope a year or so ago.
Researchers are now studying whether this etching effect could help paint cling to metal more securely, an important issue in maritime circles. Work on the Hunley also spurred research into new ways to prevent barnacles and other sea life from accumulating on ship hulls and foundations for offshore wind turbines.
Partly because of this metals research, the International Conference on Historic Metal Conservation will meet in Charleston in October.
About 300 conservators, museum officials and scientists from 50 countries are expected to attend. "The vision of what we wanted to do here is starting to fall into place," Drews said.