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Under the briny sea

Photonics Spectra
Dec 2008
Lynn Savage,

In the second century of the common era, three Roman ships sank beneath the waters off the coast of Neapolis, now the site of the modern Italian city of Naples. Fourteen centuries later, off the coast of England, the Mary Rose, beloved warship of King Henry VIII, also met its watery doom. Now, researchers are using a variety of classic and novel optical tools to explore ancient life through these rediscovered relics.

ShipsBell.jpgIn 2004, workers excavated a site intended for the extension of Naples’ subway system. Per government policy, a team of investigators with the Soprintendenza per I Beni Archeologici delle Province di Napoli e Caserta was sent to the excavation site to assure that it was clear of important cultural relics. Instead, they found the largely intact remains of three ancient ships that once plied the waters of coastal Italy.

Lying in proximity were three vessels: two onerariae (boats used for maritime trade along coastal waters) and one horeia, a type of craft used primarily for transferring goods from larger boats to the docks or for fishing within the harbor. Interestingly, the horeia and one of the oneraria apparently were sunk and abandoned on purpose.

Over the years after they were sunk, each vessel was invaded and overrun by sand and silt, and eventually they were buried completely as the harbor of ancient Neapolis filled in and became the land upon which Naples rests. The sand and silt that filled the wrecks, combined with the fact that the ships eventually rested about 11 feet below the city’s water table (and about 44 feet below ground level), formed a nearly perfect anaerobic safe house that protected the wooden vessels and their contents from microbial and fungal attack.

Before the boats could be removed, investigated or displayed to the public, however, researchers had to carefully characterize the wood so that the best preservation methods could be used. To do so, they examined wood fiber samples taken from various locations on each of the ships using bright-field and polarized microscopy. They used a Leitz DMRB and a Leica DM LB2 microscope for this work. According to Gianna Giachi, head of the Italian organization’s analytical laboratory, basic microscopy is sufficient.

Preservation of antiquities found on shipwrecks depends on knowing what they are made of and, just as importantly, how they are corroding due to seawater, bacteria or other influences. Courtesy of the Mary Rose Trust.

“It was possible for us to also use scanning electron microscopy, but [that] would require an excessive amount of time, at least in this case, in comparison with optical technology.” Similarly, they eschewed Fourier transform infrared and near-IR spectroscopy as well.

Even under anaerobic conditions, wood deteriorates. The carbohydrates in the cells of the plant material evacuate, leaving behind mostly lignin to fill the space within the cell walls. Decay results when the cell walls distort and collapse over time. When the wood samples were removed from the boats, the researchers reported that the material felt spongy.

Giachi and her colleagues found that the amount of decay seems to be related to the type of wood, with hardwoods like oak more prone to deterioration than softer woods, such as cypress. They also noted that, despite the anaerobic conditions, carbohydrate-hungry bacteria were the main culprit behind most of the decay.

The Roman ships are now undergoing conservation, and the ongoing characterization of the wood is helping in that process.

“We are trying to use the already characterized material in further research with the general aim of improving the level of knowledge on waterlogged archaeological wood,” Giachi said. Furthermore, they are working on developing the best method of using the resulting quantitative information for a tailored treatment of archaeological finds similar to the three ships.

A more powerful approach

About 1300 miles and 1400 years away, the warship Mary Rose sank during a battle with the French navy in the Solent channel between the British mainland and the Isle of Wight. The Mary Rose capsized and sank in 1545, likely by accident or bad design – not by French gunners. It was found in the 19th century but lost again; it was rediscovered once more in the 1970s. It has since been lifted from the channel floor and its contents removed for study.

Art_84-0064.jpgShips of just about every era, including warships, may have unique cargoes, but also myriad common items used by sailors, passengers and commanders: tools, jewelry, weapons, coins and galleyware, to name a few. All of them – individually and collectively – paint a picture of the lives and customs of people who went to sea. And many of these artifacts are made of copper or an alloy that includes the metal.

According to Mark G. Dowsett of the University of Warwick in Coventry, UK, copper was the metal of choice for centuries, much as stainless steel is today, and he and his colleagues have set to the task of preserving copper items, especially ones removed from the ocean depths.

“In particular, we are testing methods for removing chlorine from copper artifacts and, subsequently, protecting the copper,” he said.

Along with Annemie Adriaens of Ghent University in Belgium and their colleagues at the Synchrotron Radiation Source in Daresbury, UK, and at the European Synchrotron Radiation Facility in Grenoble, France, Dowsett set out to develop a novel way to measure the layers of corrosion that form on antique copper and alloys.

X-ray absorption spectroscopy is a high-energy method of characterizing various materials. The technique, however, requires that samples be very small so that they can be placed in a vacuum chamber. This would be too destructive for preservation activities. The researchers, led by Dowsett and Adriaens, turned to a recently developed technique called optically detected x-ray absorption spectroscopy (ODXAS) to get around the problem. ODXAS takes advantage of the fact that materials undergoing x-ray bombardment emit photons in the 200- to 1000-nm range. These photons carry important chemical information about a sample’s material structure.

Using a proprietary device called an eCell, which aids electrochemical analysis by focusing x-rays onto an electrolytic sample, the group captured emissions from the sample using a Hamamatsu photomultiplier tube. Normally, an x-ray detected would be used to gain spectrographic information, but the photomultiplier tube captured the optical spectra instead. With this information, the group was able to determine the presence and distributions of such corrosion materials as cuprite, nantokite and various isomeric copper hydroxychlorides. In addition, the investigators used a germanium detector from EG&G Ortec Inc. to capture fluorescence data.

The group is generally pleased with its results thus far, though its members report that filtering out some optical wavelengths may improve the data.

“[The technique] gives us the basis for developing a chemical microscope which works directly to image the surface composition of an object in air or any other environment which is transparent to light and x-rays,” Dowsett said.

FeaturesmicrobialMicroscopyoptical toolspolarized microscopySensors & Detectors

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