CHARLESTON, S.C. - After months of testing, technology using supercritical fluids shows promise for preserving the Confederate submarine H.L. Hunley, which sat encased in sand beneath the ocean for almost 140 years.

But months of more tests and studies must be done before scientists settle on the best way to remove the corrosive salts from the hand-cranked sub.

"So far, so good. But we still have a long way to go experimentally before we can sit back and say we will use this process," said Michael Drews, the materials scientist heading the Clemson University research team helping with the Hunley conservation.

The 40-foot Hunley became the first sub in history to sink an enemy warship when it rammed a spar with a black powder charge into the Union blockade ship Housatonic on Feb. 17, 1864.

The Hunley never returned and was finally located off Sullivans Island nine years ago. It was raised in 2000 and brought to a conservation lab at the old Charleston Naval Base where it sits in a tank of chilled water.

The Hunley's eight-man crew was buried earlier this year in a funeral that attracted thousands and has been called the last Confederate funeral.

Scientists also have considered using cold plasma technology and traditional electrolysis to preserve the Hunley.

In supercritical fluid technology - in this case the fluid is water - fluids take on the characteristics of both a gas and a liquid under intense heat and pressure and have unique dissolving characteristics.

While the technology is used in some commercial applications such as decaffeinating coffee and processing hops for brewing beer, it has never been used to preserve marine artifacts.

Drews' team has conducted preliminary experiments using two small chambers - the largest about the size of a pint container - to treat rivets removed from the Hunley when sections of the hull were opened to allow the excavation of the silt and the crew's remains.

Results of that work must still be analyzed and, if they are positive, scientists would then construct a larger chamber - perhaps about the size of a garbage can - to treat larger pieces of metal, Drews said.

But even if the technique shows promise, there still is a major question. Can it be used to treat the entire Hunley?

Scientists hope to find a method that does not require taking large sections of the sub apart.

"If everything was go, we would still have do to an engineering assessment on the Hunley to see if it were compatible," Drews said. "We know the Hunley was not designed to be put into a big pressure cooker.

"We may have a good process for removing the salts but it may not necessarily be a good process for the Hunley."

In cold plasma technology, hydrogen gas is blown over an artifact in a sealed container and the plasma formed pulls impurities out of the artifact as a gas. However, results of initial tests were not encouraging and "we have pretty much eliminated it," Drews said.

The traditional method of conserving large marine artifacts, and one that takes years to complete, is electrolysis.

In that method, a slight electrical current is applied to remove the corrosive salts from metal artifacts in a tank of water. However, the electrical field often doesn't always penetrate everywhere.

"With electrolysis, there is some evidence that it doesn't work particularly well for things that are riveted together," Drews said. So that technique would again require taking apart much of the sub.

So when will scientists know just how they will proceed?

"If you asked the scientist in me the answer would be several more years of research," Drews said. "We are probably a year away from being able to make a realistic assessment."

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On the Net: Friends of the Hunley: www.hunley.org