Ceramic Surface Paintings and Pigments from the Aguada Culture (Argentina): XRD and SEM-EDX Arxhaeometric Studies

S. BERTOLINO; V. GALVAN; G. CASTELLANO

Paints: Types, Components and Applications

NOVA publishers

Lugar: N. Y.; Año: 2010; p. 169 - 211

Scanning electron microscopy (SEM) combined with energy-dispersive spectrometry (EDX) and X-ray diffraction (XRD) nondestructive analytical techniques are well established in materials characterization and, recently, in archaeometric investigations, since their simultaneous application allows mineralogical, chemical and topographic analysis. There is still a lack of detailed information regarding their suitability for studying samples such as thin paint layers on pottery or micro-granular phases, where standard procedures are not always appropriate. This work focuses on the use of X-ray and SEM techniques for the mineralogical and chemical characterization of pigments, and surface treatments and paintings in pottery from Ambato and Portezuelo styles of the Aguada Culture (Catamarca, Argentina, ca. 600-1000 AD). An image-treatment software was developed to solve the difficulties for paint discrimination, which implements a new methodology to process backscattered electron images. This software brings to evidence small mean atomic number contrasts among paintings and the sherd body with a minor detail loss. Quantitative mineral compositions were obtained by Rietveld refinement of XRD patterns. In Tricolor pottery, reddish paint resembles the paste due to the presence of hematite or Fe-clays; sometimes the white paint contains Pb-phases. Black paint has scarce Mn-minerals but often, like in the case of Black Incised type sherds, no particular phase is identified as a color source, suggesting possible organic pigments or resulting from the firing technique. The polychrome paints in sherds of Aguada Portezuelo style were made over a white Ca-rich base and they contain Fe-Mn (black), Fe-Mn-Ca (dark reddish) and Fe-Ca (reddish). The white ones correspond to gehlenite, a firing product (possibly above 900-1000 C); but calcite and CaO also occur (above 900 C). White and reddish pigments found at Piedras Blancas (Aguada Ambato) were also characterized. Due to their scarcity a new methodology was developed complementing XRD Rietveld refinements for mineral quantification with quantitative elemental analysis by SEM-EDX spectra, which proved to be consistent. A special sample-holder for few milligrams was developed; surface charge accumulation effects were considered by determining the Duane-Hunt limit to assess the effective incident energy, which remarkably improved the sets of concentrations obtained. The Rietveld method combined with the new EDX procedure has proven to be a suitable method for routine quantitative analysis of samples in small amounts. The mineralogical and chemical differences found between Ambato and Portezuelo styles suggest that they are two distinctive entities not only on their designs but also on the materials chosen and the technology used.