Detecting iron-based pigments on ruthenium-coated ancestral Pueblo pottery using variable pressure scanning electron microscopy

Abstract Background Ancestral Puebloan black-on-white ceramics of the American Southwest can be classified as containing pigments within their painted designs containing high levels of organic-based elements such as potassium, or mineral-based elements such as iron, or a mixture of these elements. The identification of pigment elements of the pottery of a site is fundamental in determining the site’s cultural and temporal context. This paper will concentrate only on the analysis of mineral based pigment which was shown by previous researchers to exhibit greater concentrations of iron than organic based pigment. Although the visual discrimination of these pigments can be difficult if the pigment is a mixture of both pigment types or if the pigment is worn, this paper will describe a sherd sample previously shown to contain only mineral pigment. For the present study, a Tescan variable pressure scanning electron microscope, a JEOL 6400 scanning electron microscope, and a Hitachi S-3400 N scanning electron microscope were used with the same sherd. This sherd was coated with ruthenium to reduce charging without the visual color change associated with sputtered metal coatings. A reduction in microscope chamber vacuum also greatly reduced charging of unpainted areas. An energy dispersive spectrometry detector produced a map of the iron present in the sherd. Areas of iron in the sherd were identified using a backscatter electron detector. Iron as well as other elements present in the paint pigment was also detected using micro-X-ray fluorescence on the same sherd. Results The images and maps produced by the Tescan variable pressure scanning electron microscope did not always show well-defined iron-based pigmented areas on the sherd. Although the secondary image taken with a high vacuum did not show clear boundaries of the pigment on the sherd, a secondary image taken at a low vacuum of the same area showed well defined pigment boundaries. Other images taken with this microscope such as the backscatter image showed boundaries of sections of the pigment and the energy dispersive spectroscopic map showed a green colored pattern corresponding in general to the pigment area of the sherd containing iron. Using micro-X-ray fluorescence, the Hitachi S-3400 N scanning electron microscope mapped the following elements: iron, aluminum, potassium, calcium, sulfur, and silicon at a high vacuum with excellent resolution primarily for iron in the paint pigment on the sherd. Conclusions The best resolved image of iron-based pigment for the ruthenium coated sherd was obtained using the low vacuum secondary detector in the Tescan Vega 3 XMU. Excellent resolution for the energy dispersive spectrometry maps for iron was obtained by the micro-X-ray fluorescence detector on the Hitachi S-3400 N scanning electron microscope.