By David Morgan
Much of the information needed for effective archeological preservation is hard to see. For objects, sometimes that information is encased by layers of corrosion. For whole sites, sometimes it is buried beneath centuries of sediment. And sometimes that information is inaccessible simply because it has not been shared with others. In 2009, NCPTT focused on looking “beneath the surface.” It focused on using technology to see what was hidden, to see what preservation clues could be revealed, and, critically, how those discovery methods could be taught to other professionals.
One of NCPTT’s major tasks was exploring the application of new portable X-ray fluorescence (pXRF) devices to archeology preservation problems. One facet of the task involved looking at the use of a Tracer pXRF to characterize the elemental composition of 800-year-old copper artifacts from the Gahagan mound site in Louisiana. This will help the Louisiana State Exhibit Museum better understand the artifacts’ conservation needs.
Working with some materials with a pXRF is relatively easy—glass or metals, for example. Working with textured, composite materials can be quite hard. Another facet of our experimentation with pXRF consisted of working with Bruker Corporation to design an appropriate method to assess heterogeneous materials like pottery. This is important for understanding what kinds of clays particular pots are made, and what sorts of temper ingredients were added. That information helps researchers understand how to conserve the artifacts and it can lead to important discoveries about the artifacts’ place of origin. The protocols were devised this year, and the analysis of pottery excavated from colonial period sites in Louisiana and the Caribbean—a proof of concept project—is underway.
NCPTT continued to help people learn how to use cutting-edge technology to see beneath the ground. Geophysical technologies, like ground penetrating radar, can be used to create three-dimensional images of buried archeological features. Best of all, such techniques are non-destructive, can cover large areas of ground, and can supplement traditional ways of digging. The difficulty with this approach is understanding how the geophysics imagery relates to the genuine—but invisible—archeological features. NCPTT co-hosted two workshops on this topic. “Prospection in Depth,” was held in partnership with The Presidio Trust at El Presidio de San Francisco. The other, “Current Archeological Prospection Advances,” was held in partnership with NPS’ Midwest Archeological Center at Los Adaes, Louisiana.
At El Presidio, the course participants used geophysics to identify pre-World War II building features and to identify several remnant Spanish colonial period features that had, on the basis of traditional archeological methods, been overlooked. The discovery clearly illustrated the benefits of geophysical prospection in site management and planning. Similarly, at Los Adaes, participants plainly identified major buried features of the colonial fort, and the results showed that one of the wall lines at the state park was interpreted and exhibited incorrectly. This revelation also highlights the potential impact of archeological geophysics on the accuracy of education and interpretation programs.
NCPTT grantees also used geophysics to view the hidden past in the service of preservation planning. Dr. Christopher Fennell of the University of Illinois concluded his research on the utility of aerial thermal infrared to identify infrastructural features at historic period archeological sites (PDF link). Working at the 19th century town of New Philadelphia—the first town platted and legally registered by an African American in the United States—Fennell found that the technique holds promise for detecting buried structural foundations.
Likewise, this year Cultural Heritage Imaging, a non-profit preservation group, used a grant from NCPTT to host a workshop on reflectance transformation imaging (RTI) in San Francisco. RTI uses low-cost digital imaging techniques to highlight surface features of art and objects that would otherwise be hard to see, if not invisible. One outcome of the workshop is a series of do-it-yourself RTI guides soon to be available on the Internet from Cultural Heritage Imaging and NCPTT.