Nondestructive Evaluation of the Mineralogy of Rock Temper in Ceramics Using X-Radiography
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NONDESTRUCTIVE EVALUATION OF THE MINERALOGY OF ROCK TEMPER IN CERAMICS USING X-RADIOGRAPHY CHRISTOPHER CARR AND JEAN-CHRISTOPHE KOMOROWSKI *Arizona State University, Department of Anthropology, Tempe, AZ 85287 *Arizona State University, Department of Geology, Tempe, AZ 85287 ABSTRACT Industrial and medical x-radiography can be used in a new manner analogous to back-scattered electron microscopy to identify the approximate mineralogy of rock temper particles in ceramics, but without their destruction by thin-sectioning and at very low cost. Mineralogical traits similar to those applied in petrography to identify mineralogy are visible in a magnified radiograph. They include particle radiographic grey level contrast, which varies with specific density and mineralogy; morphology; cleavage; and internal texture. Blind tests that evaluate the specificity and accuracy of the method are presented. OLD AND NEW METHODS OF TEMPER MINERAL IDENTIFICATION Archaeologists have used the mineralogy of rock temper in prehistoric ceramics in a number of important ways: to identify vessels that differ in their engineering, function, date of manufacture, and/or source of manufacture [1,2,3,4,5,6]. Traditionally, temper mineralogy has been identified using the methods of geological petrography, occasionally supplemented with information provided by the electron microprobe or x-ray diffraction [7]. Although quite useful, these methods are disadvantaged in several ways. (1) They are destructive. (2) They are costly and time consuming. A single thin section costs 6 to 12 American dollars to prepare. Thus, typically, only a small number of sherds or vessels can be analyzed for their temper mineralogy. This restricts behavioral reconstructions to a qualitative rather than quantitative scale. For example, it may be feasible to determine only the presence or absence of tradewares in an assemblage, rather than their proportions. (3) Traditional methods sample only a small volume of each ceramic specimen. A typical thin section, which is cut perpendicular to the vessel's wall, is less than 5 cm long, as wide as the vessel's wall, and only 30 microns thick. Based on our experience, the temper contained in this small volume will not represent the full range of kinds of temper and their percentages in the vessel at large, if the temper is coarse and heterogeneous. To help overcome these difficulties, certain industrial radiographic and medical radiographic methods can be used. These allow one to document temper particles in intact ceramic samples of large size. It is possible to image the entire volume of multiple sherds simultaneously on an 8 x 10 inch sheet of film. The cost is approximately 13 € per sherd. Certain specific radiographic procedures and materials must be used to optimize image contrast and resolution because some rock temper particles may vary only slightly from their clay matrices in their elemental compositions, x-ray absorption coefficients, and the radiographic image grey levels that they create, and because radiographs must be
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