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

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. © 2016 Pendleton et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Open Access *Correspondence: [email protected] 1 Microscopy and Imaging Center, Interdisciplinary Life Sciences Building, Texas A&M University, Mail Stop 2257, College Station, TX 77843-2257, USA Full list of author information is available at the end of the article Page 2 of 7 Pendleton et al. Herit Sci (2016) 4:11 Background The classification of the type of pigments applied to the pottery sherds or vessels recovered at an archeological site is vital to the determination of the cultural and temporal context of the site. The traditional Pueblo pigment types are carbon-based (containing organic compounds, primarily potassium [1], mineral-based (containing primarily iron [2] compounds), or in some cases a mixture of these types. Throughout the Pueblo sequence from the advent of pottery making (c. AD 600 to the contact period, AD 1600), these pigment types appear to correspond to regional cultural groups and temporal changes in these groups. For example, in southern Utah and southwestern Colorado, carbon-based paints characterize the type sequences of western Pueblo black-on-white pottery types in Arizona and in the Mesa Verde/Colorado River/San Juan River area. In contrast, in northwestern and southwestern New Mexico, mineral painted wares dominate the Chaco and Mimbres sequences. These pigment types are typically distinguished in the field and in most lab analyses by visual inspection. The carbonbased pigments are usually characterized by their fuzzy edge and their apparent penetration into the clay matrix while mineral-based pigments appear to have worn or flaked off the clay surface [3]. Researchers [2] have validated the accuracy of these observer-based identifications using scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (SEM–EDS) to identify the elements present in pottery pigments. They determined that the accuracy of visual identifications was 84.2 % for fifteen Mesa Verde White Ware sherds from Wallace Ruin in southwestern Colorado. However, carbon (typically potassium-based) and mineral (typically iron-based) pigment types can be difficult to distinguish if they are combined or “mixed” or if much of the paint is worn [2]. Visual identification is inadequate for these cases. For example, a researcher [3] on the ceramics from the La Plata district, a southern extension of the Mesa Verde carbon painted area, has advocated that thermal and chemical tests should be used to validate visual evaluations of paint pigments. Another researcher [4] also advised that chemical tests be used for sherds from Montezuma Canyon, southeastern Utah. A more precise determination of either mineral or carbon-based pigment than merely visual observation was stipulated by [5] to classify the pottery of the eastern San Juan Basin and the Acoma-Laguna regions of the American Southwest. In order to distinguish mineral pigments from organic pigments in painted Ancestral Puebloan pottery, [2] used scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (SEM–EDS). To prepare the sherds for SEM, they applied a coating of carbon on the surface of their sherds to control SEM charging effects. These effects can reduce the clarity of the image, cause a smearing of the image, and produce bright areas with corresponding loss of detail. While the carbon coating allowed precise mapping of the boundaries of the iron (in the mineral based pigment) in backscatter SEM mode, the dark black carbon coating visually obscured the paint pigment boundaries. To avoid the use of carbon coating to reduce charging, this paper describes alternative methods to control charging such as (1) the addition carbon and copper tape (Fig. 1) to the outer edges of the sherd, (2) the use of ruthenium vapor coating [6] to reduce charging without the visual color change associated with sputtered metal coatings and (3) the use of a lowered SEM vacuum pressure in secondary imaging mode. To demonstrate the effect of ruthenium coating