Under the microscope sat a thin section of the smaller Bamiyan Buddha from Afghanistan which was destroyed in 2001. As polarizing filters were placed in front of and behind the sample creating cross polarization, grains of large quartz, feldspars, chert, and other components sparkled in vivid color on a dark background. This technique is called thin-section petrography. The analysis can help archeologists, historians, preservationists, and conservators know how and perhaps when the objects were made.
Ten participants gathered last week at the National Conservation Training Center in Shepherdstown, West Virginia to learn how to interpret 30 micron slices of mineral, rock, and ceramic under a polarized microscope. The two-day workshop, Petrographic Analysis for Conservation, was hosted by the National Center for Preservation Technology and Training (NCPTT), an office of the National Park Service, and the Center for Historic Architecture and Design (CHAD) in the University of Delaware’s School of Public Policy and Administration. Participants like Erik Gjesfjeld wanted to learn petrography to assist in his Ph.D. work at the University of Washington. Michael Trinkley from Chicora Foundation wanted to gain a better understand of the deterioration of grave markers.
Chandra Reedy, a professor at the University of Delaware, was the instructor for the workshop. She is a leading authority of the use of thin-section petrography for the analysis of cultural materials. Reedy first honed her skills in petrography by looking at differences in clay casting cores for ancient cast bronze objects at the University of California at Los Angeles. In 2003, Reedy received a grant from NCPTT to develop a manuscript that resulted in the book, Thin-section Petrography of Stone and Ceramic Cultural Materials. The previous year, she received a PTT Grant to evaluate analysis software for use in interpreting thins sections under regular and polarized light.
The end of the first day laid a solid foundation for the next morning, when the group began to identify minerals and rock in different types of pottery. First the class learned about low-fired pottery, often called earthenware or terracotta. Dyed epoxy was used as a mounting medium for these thin-sections, which allowed the participants to easily see the pores and density of the clays. Students also learned about tempers (deliberate additives to help with the workability of the clays and to prevent rapid shrinkage and cracking of the object upon firing or cooling). Next, Reedy discussed high fired pottery and showed examples of stoneware, and soft- and hard-paste porcelain. Reedy ended the course by looking at non-pottery clay materials, such as Cuneiform tablets, bricks and tiles.
Participants were pleased with the opportunity to have individual microscopes and ample time looking at so many different types of materials under a microscope. Leica Polarizing microscopes were provided by the McCrone Research Institute and the Campbell Center for Historic Preservation Studies. “Seeing people who had never done thin-section petrography before really get interested in it and really see the potential in the types of research questions that can be addressed by the technique was great,” said Reedy. She added, “They learned a lot in these two days.”
More images from the workshop can be found on the NCPTT Flickr site.