Characterization and Comparative Analysis of Ancient Earthen Plasters from the American Southwest
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Characterization and Comparative Analysis of Ancient Earthen Plasters from the American Southwest Angelyn Bass1, Douglas Porter2, Mike Spilde3, Matthew Guebard4, Katherine Shaum1, and Nicholas Ferriola1 1
University of New Mexico, Department of Anthropology, Albuquerque, New Mexico 87131 University of Vermont, School of Engineering, Burlington, Vermont 05405 3 University of New Mexico, Institute of Meteoritics, Albuquerque, New Mexico 87131 4 National Park Service, Southern Arizona Office, Phoenix, Arizona 85012
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ABSTRACT We have studied earthen plasters and wall materials from three ancient Native American sites in the American Southwest that include a twelfth-century, subterranean kiva used for textile production in Natural Bridges National Monument (Utah), a thirteenth-century, defensive, cliff dwelling at Montezuma Castle (Arizona), and a mid-fourteenth-century, puddled earth Great House at Casa Grande National Monument (Arizona). In each case, the data collected has been used to develop long-term preservation strategies and monitoring plans for each site. To understand the conservation issues, earthen materials were analyzed. Characterization included examination of BSE-SEM micrographs of polished thin sections prepared from a total of 36 samples from the three sites, and XRD of twelve samples. Research goals included: 1) determining the microstructure, micro-composition, porosity, mineralogy of aggregates and phases in the binding matrix for each sample; 2) reconstructing plaster technologies and their variation within and between sites, including material selection, preparation and application sequences, and; 3) identifying principal deterioration conditions and processes. Our findings support the idea that plaster materials were collected locally and manipulated to optimize their performance to suit the unique site conditions and needs of the ancient people using the structures. INTRODUCTION Earthen plasters and mortars were used extensively in the construction of ancient southwestern architecture for forming, leveling and finishing walls, as colored washes and grounds for incised or painted embellishments, as the closing courses in roof construction, for pointing unit masonry in the margins at the tops of walls, as a multi-coat floor surfaces, and to form built-in architectural features to process food, fabric, and perform other utilitarian activities. Wall and floor plasters frequently have multiple layers and were often renewed as a ritual and maintenance practice [1, 2, 3, 4, 5]. Although these plasters differ in terms of material components, they share characteristics that provide clues about how building materials were selected and processed, as well as evidence about sites were used and changed over time. There are no written accounts of ancient plaster preparation or application techniques, so our knowledge of them is based on field observations and architectural survey, in combination with instrumental analysis to determine the mineralogical, microstructural and chemical characteristics of the m
