ArAGATS researchers are working in concert with scientists Argonne National Laboratory (ANL) outside Chicago to utilize high energy, high intensity synchrotron x-ray diffraction (XRD) and x-ray flourescence (XRF) in the evaluation of early ceramic and metal technologies from the Caucasus. Argonne’s Advanced Photon Source (APS) synchrotron is ideal for the assemblage-based analysis of archaeological materials because it enables (1) non-destructive element composition and microstructure analysis of artifacts, (2) provides those data quickly and efficiently in a single scientific instrument. While synchrotron radiation has previously been used to evaluate archaeological objects, ArAGATS researchers are developing protocols for a more systematic application of synchrotron techniques in order to evaluate stratified object assemblages from the Tsaghkahovit Plain. This assists our goals of a detailed analysis of South Caucasian material regimes, an improved understanding of early Eurasian society and culture, and the preservation of artifacts for future study and public display.
The metalwork portion of the protocol development project involved high-energy synchrotron X-ray diffraction and fluorescence experiments on samples from 10 third millennium B.C. copper and bronze ornaments from a catacomb tomb at Velikent, Daghestan. This work was undertaken at the 1-ID and 5-ID APS beamlines. The Velikent assemblage is important archaeologically because it contains the earliest ‘true’ bronze (tin bronze) in the Caucasus: 1 tin bronze bracelet, 2 copper bracelets, 2 tin bronze rings, 4 arsenic bronze rings, and 1 copper ring (the tin bronzes contain 7-10 wt% Sn, while the arsenic bronzes have 1-3 wt% As).
Experiments at the APS confirmed that synchrotron XRD and XRF techniques may be non-destructively applied to remarkably similar ends as more traditional metallographic techniques, providing a “fingerprint” of the phases present in the metal and identifying the processing method used in the creation of various metal artifacts. For example, the grainy, spotty rings in the diffraction pattern for DP 1 correspond to small, randomly oriented grains due to cooling and minimal to no working after casting. The coarse, choppy rings in DP 3 indicate long annealing or slow cooling. The smooth rings with symmetrically oriented spots in DP 4 indicate a fine, aligned-grained metal which results from cold- or hot-working of the object.
XRF analysis was also able to identify the relative proportions of metal admixtures (Cu, Sn, As) used to prepare the materials in the objects. However, we found that in dealing with ancient copper-based artifacts care should be taken in spectral analysis to select non-overlapping spectral lines, specifically the obscuring overlap of lead Lα and arsenic Kα—confusion may be avoided by selecting lead Mα and arsenic Kβ instead.
Ongoing research with metalwork recovered during Project ArAGATS excavations will use these new techniques developed at the APS to perform a large-scale characterization of copper-based metal assemblages in order to address anthropological questions of variability in the use of materials, the fabrication of different kinds of objects, and the broader, historically particular technology of past communities. Ongoing analyiss of ArAGATS metals at the APS proceed under the direction of project member David Peterson.
Ceramic working techniques such as vessel tempering strategies and firing conditions can also be inferred from microstructural evidence drawn from synchrotron XRD. Like metals, ceramic vessel properties resulting from particular combinations of ingredients and techniques can be inferred from analytical results as a systematic means of identifying ancient technical practices: firing temperature, formational techniques, and tempering strategies, among others. However, the synchrotron analysis of ceramics is less developed than metalwork research due to the more variable microstructures present in ancient ceramics. In order to produce a more reliable suite of synchrotron ceramic analysis techniques, Project ArAGATS researchers, in collaboration with Dr. David Cookson (ANL), have collected 2,403 synchrotron “shots” of 250 ceramic samples from Armenia, China, and Russia at APS Sector 15 using the 15-ID beamline.
We are now working through the resulting images using multivariate statistical analysis and rendering techniques such as GGOBI to define patterns within and between groups in the sample sub-sets. We condense the synchrotron data to a limited set of less than ten 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 should reflect micro- and meso-structural characteristics of ceramics, such as: 1) the character of plastic and non-plastic constituents, representing differential phase states within the vessel, 2) the spatial organization and orientation of clay particles and inclusions, which tells us about techniques of vessel formation, and allows us to distinguish pottery made by wheel and by hand, and 3) water content, porosity, and other aspects of the thermal history of the artifact from which we may derive ancient firing strategies and parameters. Preliminary results indicate significant variability between spatio-temporally distinct potting traditions throughout Eurasia, potentially due to technologically and sociologically differentiated approaches to ceramic production on both a local and regional scale.