Digital radiographic (DR) and X-ray computed tomographic (XCT) analyses at Argonne's Nuclear Engineering Division (NED) are being utilized to provide non-destructive views of macro- and meso-structural object features and evidence of pottery formation techniques and map the surfaces of metal objects. Micro-scale structures, important to defining cultural preferences for materials and the preparation of raw ingredients, are being investigated by X-ray fluorescence (XRF) and X-ray diffraction (XRD) beamlines at Argonne's Advanced Photon Source synchrotron (APS) and via scanning electron microscopy (SEM) at ANL's Electron Microscopy Center (EMC). This assemblage-based, programmatic research via the Argonne APS represents one of the first large-scale synchrotron analyses of archaeological materials. Finally, compositional analysis of Eurasian ceramics with portable X-ray fluorescence (pXRF) are conducted at the Field Museum of Natural History's Elemental Analysis Facility (EAF).
Previous applications of X-ray imaging technology in archaeology have focused on the use of analog (film) radiographic imaging methods to assess internal, macro-scale features of archaeological materials, including ceramic fabrics, human and animal bones, textiles, and light metals. Our research pushes the programmatic transition from analog to digital radiographic imaging methods using the NED's experimental X-ray facility, a spatially unrestricted and tunable instrument that can achieve up to 420 kVp in X-ray beam energy for penetrating particularly large of thick potsherds. Adjustments in technique are needed due to the differences between analog film and the photostimulated luminescence material used in digital detectors, but the physical principles remain essentially unchanged, resulting in digital images displaying the micro- and macro-scale structural features of archaeological materials. Digital imagery of ceramics enables the rapid, high volume collection of assemblage-based data as well as the ability to post-process and manipulate imagery through our Sherd Image Visualization and Analysis (ShIVA) software.
X-ray computed tomography, the production of three-dimensional tomographic volumes and two-dimensional tomograms, provides MAE researchers with the ability to nondestructively section and examine ceramic artifacts with resolutions as fine as dozens of microns. A computed combination of radiographs, XCT can most easily be described as a "moving" radiograph, in the same way that motion pictures are simply a sequence of individual photographs. While radiography takes a single two-dimensional image of each sample, data acquisition for XCT involves collecting many radiographs of the sample taken from unique angles as the sample stage rotates 360 degrees. Following data acquisition, these radiographs are "computed" into a three-dimensional volume through a process called "back projection" and made available for a variety of qualitative and quantitative manipulations, including slicing, porosity estimation, void mapping, etc.
XCT data collection at ANL is conducted with both linear (CMOS) and area detectors, utilizing custom data acquisition softwares, which MAE has augmented for the particular vicissitudes of archaeological materials analysis. Working in concert with ANL scientists, MAE researchers now back project XCT data on the CentOS computing platform at the University of Chicago's Department of Anthropology. As a multi-threaded application running on a 64-processor server, back projection and post-processing are now quick and agile.
Synchrotron X-ray analysis provides nano-scale analysis of artifact structures that are inaccessible through standard radiographic and tomographic techniques. Because of their extremely high energies, synchrotron X-rays allow non-destructive, bulk analysis of artifact elemental composition and microstructure, making these techniques highly attractive to our assemblage-based approach and consonant with the preservation goals of archaeology.
For research on ceramic materials, MAE utilizes small- and wide-angle X-ray scattering (SAXS/WAXS), performed at the APS ChemMatCARS beamline. SAXS/WAXS utilizes transmission geometry, where the amount of sample measured only depends on the specimen thickness, which can be controlled by closely monitoring the beam attenuation with the CCD X-ray camera. Our assemblage-based work with several hundred samples from Armenia, China, and Russia represents, to our knowledge, the single largest sample of archaeological materials ever examined with a synchrotron. SAXS/WAXS data are condensed to a limited set of numerical parameters including such quantities as: position and relative intensity of the most dominant Debye-Scherrer lines, azimuthal distribution of line intensities or 'spottiness', and the relative proportion of crystalline versus amorphous scattering. These quantities reflect micro- and meso-structural characteristics of the potsherds, including the shapes and reflectance capacities of plastic and non-plastic constituents, the spatial organization and orientation of clay particles and inclusions, water content, porosity, etc.
Synchrotron techniques for the analysis of metal artifacts, undertaken at the APS Sector 1 beamline, include high-energy transmission mode XRD for the non-destructive measurement of polycrystal grain sizes and textures. It is simultaneously possible to carry out grain XRF measurements, enabling us to learn about the samples' elemental composition. This analysis applies an expanding conical beam to capture micro structural and compositional data from at least 1000 crystals allowing us to non-destructively discriminate between metal groups (copper, tin bronze, arsenic bronze, silver, and gold) and identify the relative proportion of alloy components in our samples. MAE researchers quantify the XRD results in order to systematically identify crystalline microstructures by utilizing similar data to that of ceramics (Debye-Scherrer patterning, line intensities, and scattering), identifying the phasing, and metalworking techniques through which the objects were formed (casting, annealing, forging followed by annealing, and intensity of cold-working).
Our approach to scanning electron microscopy (SEM) is one that combines the enhanced imaging capacities of SEM in the investigation of the microstructure of objects with the qualitative and quantitative analysis of the elemental composition of those structures by electron-dispersed spectrometry (EDS). The Hitachi S-4700-2 FESEM at the EMC combines high resolution SEM with EDS and is the preferred instrument for MAE. An environmental SEM (ESEM) is also available as needed, and can be operated without a vacuum to accommodate the dirty samples that often confront archaeologists. To date, our SEM-EDS analysis has centered on ancient Eurasian gold work, but has also examined ceramics in order to distinguish wheel thrown pots from those turned by hand, relative firing temperatures based on vitrification, and patterns of exchange indicated by EDS. The investigation of pyrotechnology through SEM constitutes one of the primary means of interrogating links between ceramic and metal technologies.
pXRF analysis at the Field Museum combines the illuminating discrimination of constituent elemental compositions with the high volume, non-destructive, rapid data collection available in a portable analytical device. MAE researchers are working with collaborators at the Field Museum (EAF) to develop protocols for high volume pXRF data collection and corroboration with high resolution techniques such as instrumental neutron activation (NAA) and synchrotron-based XRF.