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Use of Computed Radiology During the Archaeological Conservation of LaSalle's Ship - The belle Stuart Kleven Alloyweld Inspection Company, Inc. 796 Maple Lane Bensenville, IL 60106 (630) 595-2145 (630) 595-2128 Fax E-mail: alloyweldinsp @ aol.com Contact

History

The Belle, a French vessel that sank in1686 was discovered in July 1995 in shallow water in Matagorda Bay, Texas. The shipwreck is considered one of the earliest and most important finds in North America. The remains of the wreck were completely excavated in 1996-1997 by the Texas Historical Commission under the auspices of the State Marine Archaeologist. All the artifacts and the hull of the ship were sent to the Nautical Conservation Research Laboratory at Texas A&M University where they will be cleaned and conserved over the next several years.

Rene-Robert Cavelier, Sieur de LaSalle is perhaps best known for his exploit during his previous expedition to the New World four years earlier. At that time he discovered Illinois and the Mississippi River. He followed the Mississippi River all the way down to the Gulf of Mexico at that time. He then claimed all the land bounded by it in the name of France. Two years later he had convinced Louis the XIV, the king of France to commit four ships to an expedition to colonize the new territory and gain a foothold before Spain could do so. In August of 1684 LaSalle left France with four ships, colonist and many tons of supplies and trade goods. The three other ships were lost by various means. One carried crew back to France, another was captured by Spanish pirates and the third ship hit a sandbar and sunk in a channel near Matagorda Bay. The expedition was doomed to failure due to desertion, hostile indians, poor maps that existed and inadequate calculation of latitude and longitude. Finally the Belle was lost due to the captain's drinking and a storm that drove the ship on to a sandbar. LaSalle was eventually assassinated by his own men who could not tolerate his demeanor and direction.

Excavation

Upon discovery of the Belle by use of magnetometer surveys, divers located a bronze cannon. This find confirmed the ship to be the Belle. Since it was such a valuable discovery, a decision was made to excavate the ship and its contents by use of a cofferdam rather than by use of divers. The waters of Matagorda Bay are dark and murky and visibility is virtually zero, even in shallow water. The double walled cofferdam built around the wreck allowed the water to be pumped out and the site was excavated on dry ground. Since the expedition was one of colonization a numerous amount items were found. These included pots and pans, drinking cups, rope, wire, leather objects, muskets, beads, rings, vases, bottles, pins, knives, combs, candles sticks, ladles, and a human skeleton. Months of careful rinsing and sifting through the mud and silt enabled the researchers to recover most of the objects at the site.

Nondestructive Testing

After the discovery of many items, conservation and cleaning is extremely important part of the project. This can be tedious and time consuming. Past historical restorations have made use of nondestructive testing. Such instances include radiography on the Liberty Bell and the Statue of Liberty, and chemical and spectroanalysis on the Shroud of Turin. Radiography has also been used on mummified remains by researchers. The Hunley, a confederate submarine from the 1860's Civil War, one of the first manufactured, has been discovered in the waters off Charleston, South Carolina. This will be recovered in the year 2000 and digital radiological analysis of its contents is planned. The work on the Belle included visual and borescopic inspection, computed tomography and computed radiology as well as standard radiography using film. Many items discovered were encapsulated by marine growth and barnacles and are called concretions. To properly conserve these items the contents and shape of the item inside must be identified. Radiology provided the tool that enabled conservators to save many items by proper removal from the concretions.

A single human skeleton was found in the hull of the ship. The skull was subjected to computed tomography and a digital data set was obtained. From this data a stereo lithographic model was constructed in a plastic resin. Using this model, archaeologists and medical forensic teams were able to reconstruct the face of the man that was lost during the sinking of the Belle. A cross-section of the skull in one tomograph revealed that brain matter was still intact within the skull. Scientists extracted the material and are performing detailed DNA testing of it. The names of the some of the crew onboard the Belle is available. With this information scientists plan to try DNA tests on persons with similar names in France from the area where the ship initiated its journey. This may allow them to link the sailor with his relatives after more than 300 years.

Computed Radiology

Since the development of the phosphor imaging plate, computed radiology is replacing film radiography in a number of applications. This new technology is the ability to digitize the image and store the information in a computer format. The generation of the image follows standard radiographic practice, but an imaging plate is used instead of film. The imaging plate is placed in a standard cassette with lead screens, in a rigid plate cassette, or in a plastic packet. The plate is exposed and processed through the plate reader. The exposed barium fluorohalide, europium doped phosphor plates contain electrons that have been excited into a conduction band. These trapped electrons form a latent image on the plate. The plate is then exposed to a helium-neon laser in the plate reader, which releases the electrons. The recombining of the electrons into their former lower orbit shells causes energy in the form of light to be emitted. This phenomenon is known as PSL or photo-stimulable luminescence. The resultant light energy is collected by a light guide and is quantified by assigning gray scales to the various intensities emitted and converted to an electrical signal. The information is then processed by the reader and transferred to a digital format in the computer. The resultant data can then be viewed on a high resolution monitor, stored on the hard drive, on optical disk, on a CD, imaged on laser film similar to a radiographic image, or on to paper.

The formation of a radiographic image on film is a fairly precise process. A radiographic technique is developed that will produce a specific density and sensitivity, usually within a very narrow range of exposure conditions. Computed radiology allows for a greater range of exposure conditions and still permits an image to be formed that can be adjusted and interpreted. This range or latitude of the imaging plates is a distinct advantage. Expensive reshooting and reprocessing time is not wasted since the range of use is so wide. Since the plates are sensitive to exposure to radiation, the length of time for exposure can be reduced. Typical times for standard film radiography have been cut by one half and some shots have been as low as one third the original exposure time. In addition to the saving of time on exposures, the time involved in processing of film is reduced from a 10-12 minute cycle time in automatic film processors down to less than one minute using computed radiography since the image is available for viewing at the work station almost immediately. A secondary monitor at the plate reader gives instantaneous information on the image formation as it is being displayed. If no image is obtained due to improper technique, a second shot can be made immediately, thereby reducing the time necessary to verify technique or exposure.

The variety of concretions discovered could be a nightmare to a radiographer. There are varying thicknesses, varying amounts of concretion, and a variety of different materials ranging from, wood, iron, bronze, pottery, leather and glass to name a few. This can play havoc with the technique and density required for proper viewing. Computed radiology permits the exposure or over exposure of a wide variety of materials and thicknesses since the image can easily be manipulated digitally. To experiment with material of differing density, thickness and type, a plastic pan was filled with three inches of concrete aggregate and items were strategically placed in the concrete. These items included a brass lock, a wooden handled brush with steel wire bristles, a plastic handled screwdriver, a pair of pliers, a plastic handled paint brush with a metal retainer for the bristles, and a broken shard of pottery. A computed radiographic exposure was made for the thickest, most dense item, a brass lock about one inch (25.4mm) thick. By varying the algorithm used to read the image all the items were interpretable using the over exposure for the brass material.

Once a concretion is exposed and an image is obtained, researchers and conservators can begin to examine the item for detail, make sketches, and formulate a means to extract them from the concretion or mass in which they are contained. Areas that have been corroded away, such as iron leave a pattern or cavity of what the original item was like. This missing area can be recovered by studying the radiological image and then drilling a hole into the cavity. Then an epoxy mixture is poured or injected into the cavity. After the epoxy has hardened, the concretion can then be removed and the shape of the original object is retained in the epoxy that filled the cavity.

Conclusion

The use of nondestructive testing in the field of archaeology and conservation is becoming a standard tool that allows scientists to accurately identify and conserve items that would have been lost or damaged in the past. A new tool, computed radiology using photostimulable phosphor plates, is providing a valuable and effective means for improving archaeological investigation of relics and artifacts. Cost savings are also realized since film and chemicals are eliminated. This is an important factor since most of the funds used for archaeological activities are provided by limited grants or are donations from individuals or foundations. As these new techniques are used, more and more applications are being found for the new technology within the archaeological field.

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