Waterlogged wood artifacts may be rescued
by technological advances in drying techniques
When Union ships sunk the
English blockade runner
Modern Greece off the coast of North
Carolina in 1862, Confederates at nearby
Fort Fisher wasted little time in salvaging
munitions from the vessel. Nearly
150 years later, what they left behind is
helping make history in preservation
research.
Gunstocks from the shipwreck were
used in NCPTT-funded research by the
Mariners' Museum in Newport News,
Va. to investigate the use of emerging
technologies to dry waterlogged
archeological wood.
"Waterlogged wood collapses and
shrinks when exposed to air, potentially
destroying significant archeological
data," Eric Schindelholz, the project's
principal investigator, said. "We studied
a method called supercritical drying,
which does not add chemicals to the
wood that could affect its long-term
preservation and the reversibility of the
treatment."
Researchers at St. Andrews University
developed the supercritical drying
technique in the 1990s and the method
promises to reduce treatment time
from months, as with current methods,
to days.
Supercritical fluid is created by
compressing a gas or a liquid at elevated
temperatures. The resulting product
is neither liquid nor gas and its lack of
surface tension prevents pressure damage
to the weakened wood. A similar
method is used in "green" dry cleaning
and in decaffeinating coffee.
The supercritical drying method
involves replacing the water in the
wood, which will not mix with supercritical
carbon dioxide, with methanol,
which will mix in supercritical carbon
dioxide. The methanol in the wood is
replaced with the carbon dioxide in a
pressure chamber. The chamber is then
decompressed and the carbon dioxide
escapes from the wood leaving a dry
specimen.
Researchers compared the physical
effects of supercritical drying on
the wood to air-drying and the popular
freeze-drying method. They had plenty
of material with which to work. The
gunstocks from the Modern Greece
were among hundreds of artifacts
in submerged containment at the
North Carolina Underwater Archeology
Branch. Additionally, researchers
studied samples from an unidentified
shipwreck in Maryland.
The samples were measured using
pins inset in the samples and by 3-D laser
scanning before and after treatment
to measure shrinkage. The microscopic
effects of each treatment were evaluated
as well.
"We found the supercritical drying
was successful in someÐbut not all Ðof
the wood samples," Schindelholz said.
"This project did confirm the potential
of this process and has laid the necessary
groundwork from which to scale
up the supercritical drying technique
for use on larger archeological artifacts."
The study also confirmed the efficacy
of freeze drying for preventing
collapse and shrinkage in the wood.
The Mariners' Museum partnered
with the National Oceanic and Atmospheric
Administration, Old Dominion
University, Clemson University, University
of Minnesota, Maryland Trust and
the NPS Harpers Ferry Center.