X-ray absorption spectroscopy

An article that may be of interest (Stanford Report, February 21,
2002)

    X-ray analysis of shipwreck may help
    conservators save waterlogged artifacts
    By Dawn Levy

    In 1628, the Swedish warship Vasa sank a mere 4,265 feet (1,300
    meters) into her maiden voyage. Whereas merchant vessels of the
    day were stabilized by the weight of cargo in their holds,
    warships carried their cargo--heavy guns--higher up. Though 122
    tons of stone had been stored low in the Vasa as ballast, it was
    not enough to counter the weight of the ship's upper hull, three
    masts, 10 sails and 64 guns. The ship leaned too far, and water
    poured in through her open gun ports. She capsized and sank to
    the bottom of Stockholm harbor, where she remained for 333
    years.

    In 1961, the Vasa was raised in good condition from her brackish
    grave. After extensive treatment to stabilize and dry her
    timbers, in 1990 she was put on display at the Vasa Museum in
    Stockholm. Ten years later, museum conservators noticed that
    powdery salts were rapidly forming on her surfaces and that the
    wood in her holds was growing soft and acidic. In short, the
    Vasa was disintegrating.

    Scientists at Stanford and in Sweden used x-ray absorption
    spectroscopy to study the chemistry of decay of the wooden beams
    of the Vasa.

    The conservators called an emergency meeting with colleagues and
    chemists from Sweden, Denmark and Australia to seek solutions to
    a problem that threatens the Vasa and other famous finds
    including the Skuldelev Viking ships, the Bremen Cog, the Mary
    Rose and the Batavia.

    Collaborators in Sweden and at Stanford have analyzed the
    chemistry of wood decay in the Vasa using a new technique--x-ray
    absorption spectroscopy. It employs high-intensity synchrotron
    beams, produced when electrons accelerate around rings, to make
    chemical "fingerprints" that identify different oxidation states
    in a sample. Their findings, published in the Feb. 21 issue of
    Nature, may help conservators worldwide preserve wooden
    artifacts retrieved from the deep, many of which are displayed
    in museums.

    The findings explain "the important role that scientists can
    have in keeping our historical treasures, which are indeed our
    connection to the old days," said researcher Farideh Jalilehvand
    of the Stanford Synchrotron Radiation Laboratory (SSRL). "We are
    now analyzing the samples from other historical ships, and it
    seems that the Vasa problem is in fact a general problem for
    most of them."

    That problem is formation of highly corrosive sulfuric acid
    inside wood beams. The Vasa had sunk to a depth of 105 feet (32
    meters), and the dearth of oxygen there inhibited
    wood-metabolizing microbes. But this environment favors bacteria
    that convert sulfate ions in seawater to hydrogen sulfide. In
    the hundreds of years that the Vasa was submerged, hydrogen
    sulfide penetrated into the deepest layers of the wood. Under
    the sea, chemical reactions turn hydrogen sulfide into elemental
    sulfur or pyrite, depending on the amount of available iron
    ions.

    The Swedish and Stanford researchers found evidence that sulfur
    had accumulated within the beams. The presence of many sulfur
    intermediates indicated stepwise oxidation toward a corrosive
    end product of sulfuric acid. In the air of the museum, iron
    species in the wood were catalyzing oxidation of the accumulated
    sulfur.

    Almost half of the sulfur has already been oxidized, according
    to the researchers. Full oxidation of the remaining elemental
    sulfur could produce up to 11,000 pounds (5,000 kilograms) of
    sulfuric acid. Oxidation was facilitated by rust released from
    the completely corroded original bolts, as well as from new iron
    bolts inserted after salvage.

       "The work is important because it draws attention to a
        previously unknown danger to recovered wooden
        marine-archaeological artifacts that are on display or in
        storage in museums all around the world--that is, the
        destruction of the wood by way of sulfuric acid produced
        when sulfur embedded in the wood is oxidized in air," says
        SSRL chemist Patrick Frank. "Museum curators world-wide will
        have to pay attention to the threat revealed by this work."

    Saving the past for the future

    To learn the anatomy of a shipwreck, scientists applied present
    technologies to past relics. They used x-ray spectroscopic
    techniques to bombard samples with high-energy x-rays that
    produce energy "fingerprints" identifying a sample's chemical
    composition. One technique, x-ray powder diffraction (XRD), was
    used to chemically identify products of oxidation on wooden
    surfaces. Another technique, x-ray absorption near-edge
    structure (XANES), allowed identification of sulfur compounds
    inside the wood by analysis of ground-up core samples. Yet
    another technique, x-ray photoelectron spectroscopy (XPS),
    allowed scientists to determine the amounts of all significant
    chemical elements, except hydrogen, in the wood.

    Based on their findings, the researchers recommend strict
    regulation of a wooden marine artifact's environment. A relative
    humidity of 55 percent and a temperature of less than 68 degrees
    Fahrenheit (20 degrees Celsius) will slow migration of water and
    oxygen into wood and ensuing mechanical and chemical damage. The
    researchers also recommended replacing the 8,500 iron bolts
    holding the boat together--inserted to replace the completely
    corroded original bolts--with bolts made of an inert material.
    Eventually the infused iron in the wood might be rendered
    electrochemically inert with a chelating agent, and even
    extractable in an alkaline solution, but more work is needed to
    investigate this option.

    The lead author on the Nature paper was Professor Magnus
    Sandstrom of Stockholm University. The other Swedish
    collaborators were Professor Ulrik Gelius of Uppsala University;
    Professor Ingmar Persson of the Swedish University of
    Agricultural Sciences and Chief Conservator Ingrid Hall-Roth of
    the Vasa Museum.

    Jalilehvand has worked at the SSRL, an arm of the Stanford
    Linear Accelerator Center, since August 2000. Before that, she
    studied with Sandstrom at the Royal Institute of Technology in
    Stockholm, where she had the chance to visit the Vasa Museum
    many times.

       "Just six months after I arrived here at Stanford, I learned
        about the acid problem in Vasa, and I asked Professor
        Sandstrom to send me a core sample of Vasa if possible,"
        Jalilehvand said. "Last year on Feb. 16, he brought a
        10-centimeter-long core sample, and I ran the X-ray
        absorption spectrum of the core sample at different depths
        in SSRL. This was the first time that the sulfur x-ray
        absorption spectrum was measured on wood."

    Said Frank, who taught Jalilehvand how to analyze the spectra
    she collected: "X-ray spectroscopy is almost uniquely suited to
    evaluating the chemistry of elements in situ in these artifacts.
    There is no doubt that the information we obtained would not
    have been available by any other method."

    This research was supported in part by grants from the Knut and
    Alice Wallenberg Foundation, Sweden, and the U.S. Department of
    Energy.


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                  Conservation DistList Instance 15:60
                  Distributed: Tuesday, March 5, 2002
                       Message Id: cdl-15-60-012
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