Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

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

Detecting iron-based pigments on ruthenium-coated ancestral Pueblo pottery using variable... Background: Ancestral Puebloan black-on-white ceramics of the American Southwest can be classified as contain- ing 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 concentra- tions 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 previ- ously 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 charg- ing 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 spec- trometry maps for iron was obtained by the micro-X-ray fluorescence detector on the Hitachi S-3400 N scanning electron microscope. *Correspondence: mikep@tamu.edu 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 © 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. Pendleton et al. Herit Sci (2016) 4:11 Page 2 of 7 Keywords: Iron, Pigment, Archeological pottery, Scanning electron microscopy Background used scanning electron microscopy (SEM) coupled with The classification of the type of pigments applied to the energy dispersive spectroscopy (SEM–EDS). To prepare pottery sherds or vessels recovered at an archeological the sherds for SEM, they applied a coating of carbon site is vital to the determination of the cultural and tem- on the surface of their sherds to control SEM charging poral context of the site. The traditional Pueblo pigment effects. These effects can reduce the clarity of the image, types are carbon-based (containing organic compounds, cause a smearing of the image, and produce bright areas primarily potassium [1], mineral-based (containing pri- with corresponding loss of detail. While the carbon coat- marily iron [2] compounds), or in some cases a mixture ing allowed precise mapping of the boundaries of the of these types. Throughout the Pueblo sequence from iron (in the mineral based pigment) in backscatter SEM the advent of pottery making (c. AD 600 to the contact mode, the dark black carbon coating visually obscured period, AD 1600), these pigment types appear to corre- the paint pigment boundaries. To avoid the use of carbon spond to regional cultural groups and temporal changes coating to reduce charging, this paper describes alterna- in these groups. For example, in southern Utah and tive methods to control charging such as (1) the addi- southwestern Colorado, carbon-based paints character- tion carbon and copper tape (Fig.  1) to the outer edges ize the type sequences of western Pueblo black-on-white of the sherd, (2) the use of ruthenium vapor coating [6] pottery types in Arizona and in the Mesa Verde/Colorado to reduce charging without the visual color change asso- River/San Juan River area. In contrast, in northwestern ciated with sputtered metal coatings and (3) the use of and southwestern New Mexico, mineral painted wares a lowered SEM vacuum pressure in secondary imaging dominate the Chaco and Mimbres sequences. These pig - mode. To demonstrate the effect of ruthenium coating ment types are typically distinguished in the field and in most lab analyses by visual inspection. The carbon- based 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 vali - dated the accuracy of these observer-based identifica - tions 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 min- eral (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 inade - quate 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 vis- ual 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 Fig. 1 Proscope digital camera image of sherd with area of interest than merely visual observation was stipulated by [5] to outlined by black carbon tape and copper tape. Tabs of carbon tape classify the pottery of the eastern San Juan Basin and the (arrows) added to locate area of sample. The green dotted lines denote the area of the sherd shown in Figs. 2 and 6. The yellow dashed lines Acoma-Laguna regions of the American Southwest. denote the area of the sherd shown in Figs. 3, 4 and 5. The blue solid In order to distinguish mineral pigments from organic lines denote the area of the sherd shown in Fig. 7. Scale 5 mm pigments in painted Ancestral Puebloan pottery, [2] Pendleton et al. Herit Sci (2016) 4:11 Page 3 of 7 as an aid to reduce charging, a secondary electron image charging was present in the unpainted areas of the sherd (Fig.  2) of the sherd before coating was applied taken (Fig.  2) but the same sherd (following ruthenium coat- (using 30 keV) utilizing a JEOL 6400 SEM at a chamber ing) had far less charging present in the unpainted areas vacuum of 5.0e−3 Pa. Another secondary electron image of the sherd (Fig. 3). The painted areas were grounded by of the same sherd (Fig.  3) following ruthenium coating the iron present in the pigment which provided a path was obtained (30  keV) at a similar vacuum (2.1e−2  Pa) to ground. At a reduced vacuum (60  Pa), but the same with the Tescan Vega SEM. For the uncoated sherd, beam energy (30  keV), the charging effects were greatly reduced on the unpainted areas of the same sherd sur- face so that the darker painted areas could be easily iden- tified (Fig.  4). In this image, the caliche deposit on the sherd surface was easily resolved with the Tescan Vega 3 XMU SEM using the low vacuum Tescan secondary detector (LVTSD) with a built-in turbo molecular pump. An Oxford Energy Dispersive Spectrometry detector produced a map of the location of iron as an overlay in green (Fig.  5) using a vacuum of 2.3e−2  Pa over a sec- ondary image (30 keV) of the same sherd by the Tescan Vega 3 XMU SEM secondary detector. Another image of the same sherd (Fig. 6) was obtained (using 30 keV at 2.2e−2  Pa vacuum) on the Tescan Vega 3 XMU using the conductive annular mono-crystal scintillator-type (retractable) backscatter electron detector which dem- onstrated compositional contrast which allowed the identification of iron present in the brighter portions Fig. 2 JEOL 6400 SEM image of un-coated (no ruthenium) sherd at 30 keV and a chamber vacuum of 5.0e−3 Pa. A JEOL Everhart–Thorn- of the painted pigment area. Using a Hitachi S-3400  N ley secondary detector is used for this image. White arrow carbon SEM (at 5.0e−3  Pa vacuum), micro-X-ray fluorescence tape, C area of charging, P area of paint pigment. Scale 1 mm was used to map (Fig. 7) several elements including iron in the same sherd. Fig. 3 Tescan Vega 3 XMU SEM secondary electron image of ruthe- Fig. 4 Tescan Vege 3 XMU SEM secondary image of ruthenium nium coated sherd at a vacuum of 2.1e−2 Pa. A Tescan Everhart– coated sherd at 60 Pa vacuum. Tabs of carbon tape (arrows) added Thornley secondary detector (without a turbo-molecular pump) is to locate area of sample. Low vacuum secondary Tescan detector used for this image. Tabs of carbon tape (arrows) added to locate area (LVSTD) is used which has a turbo-molecular vacuum pump inside of sample. Bright areas on image are caused by charging. Scale 2 mm the detector. Almost all charging effect is eliminated. Scale 2 mm Pendleton et al. Herit Sci (2016) 4:11 Page 4 of 7 Results and discussion The SEM images used in this paper demonstrate differ - ent techniques used to reduce or eliminate the effects of charging without applying a carbon coating on the pot- tery sample which visually obscures the painted pottery designs. The image shown in Fig.  4 demonstrate that high resolution details of iron-based pigment with lit- tle or no charging can be best produced using the low pressure secondary detector at 60 Pa vacuum. The other images (Figs.  2, 3, 4, 5 and 6) were compared using the same primary beam energy as used in Fig. 4 (30 keV) but with a different vacuum setting, a different detector, or an absence of coating for the same Ancestral Pueblo black- on-white pottery sherd. SEM secondary image production begins with a pri- mary beam of negatively charged electrons that penetrate the surface of the sample to induce the production of secondary electrons which are attracted to the positively charged grid in front of the secondary detector. These Fig. 5 Tescan Vega 3 XMU SEM secondary image (30 kV ) of ruthe- secondary electrons are used by the secondary detector nium coated sherd at 2.3e−2 Pa vacuum with an overlay using an to produce an image. However, if too many negatively Oxford Energy Dispersive Spectrometry detector to demonstrate the charged electrons do not penetrate the sample surface location of iron as a green color with Aztec software. Tabs of carbon tape (arrows) added to locate area of sample. Scale 2 mm and cannot find a pathway to a positive ground (along the ruthenium coating to the conducting carbon and cop- per tape attached to ground), then charging can occur with the accumulation of electrons on the sample. These accumulated electrons can be attracted to the secondary detector by the positively charged grid mounted over the scintillator producing bright areas on the image (charg- ing) which do not reflect the topography of the sample surface. The backscatter detector is typically mounted around the objective lens of the SEM and is divided into four quadrants. No positively charged grid is needed for attracting the electrons for this detector. The backscat - tered electrons were originally part of the incident beam but some may have lost energy. The backscatter detector will only detect electrons which have interacted elasti- cally or quasi-elastically. Depending on how the signals from the four quadrants of the detector are combined, areas of the sample’s atomic number, surface topography, and surface crystallography can be analyzed. The Ancestral Pueblo pottery sherd is shown in Fig.  1 with carbon and copper tape surrounding the periph- ery to reduce charging. The pigment is visible as a black vertical band and triangular sections of black conduc- Fig. 6 Tescan Vega 3 XMU SEM backscatter image of ruthenium tive tape (white arrows) were added to the unpainted coated sherd at 2.3e−2 Pa vacuum. Tab of carbon tape (arrow) added area to facilitate locating the pigment areas in different to locate area of sample. Bright areas of compositional contrast are imaging modes. The green dotted lines denote the area due to iron in paint pigment which is correlated to painted pattern of the sherd shown in Figs.  2, 6. The yellow dashed lines shown in Fig. 1. Scale 1 mm denote the area of the sherd shown in Figs. 3, 4. The blue Pendleton et al. Herit Sci (2016) 4:11 Page 5 of 7 Fig. 7 Micro-XRF-SEM map showing areas of the sherd containing iron, aluminum, potassium, calcium, sulfur, and silicon. Tabs of carbon tape (arrows) added to locate areas of sample. A Hitachi S-3400 N SEM equipped with an X-ray source (IXRF Systems 10 μm X-beam Micro-XRF using an X-ray source monocapillary optic Rh tube operating at 50 kV and 1000 μA) and an IXRF Systems (30 mm) EDS detector was used to produce the Micro-XRF-SEM elemental maps Scale 5 mm solid lines denote the area of the sherd shown in Fig.  7. and of aluminum, potassium, calcium, sulfur, and sili- Figures  2, 3, 4, 5 and 6 are all images of the ruthenium- con (Fig.  7). The greater number of elements detected coated sherd obtained with the Vega 3 SEM at 30  kV. by Micro-XRF-SEM is due to the better signal to back- Figure  2 image (produced with a JEOL 6400 SEM at ground ratio of XRF compared to SEM–EDS. The Mini - 30  keV and a chamber vacuum of 5.0e−3  Pa) was not mum Detection Limits (MDL) of an electron beam are ruthenium-coated in order to demonstrate the increase about 1000 ppm (or 0.1 weight percent) but the MDL of in charging produced with no ruthenium coating present. an XRF beam is less than 100 ppm [7]. The painted area was grounded by the iron pigment so that only the unpainted area of the sherd demonstrated Experimental the bright areas of charging. Another secondary elec- An Ancestral Pueblo pottery sherd from the American tron image of the ruthenium coated sherd (Fig.  3) was Southwest (Fig. 1) was used for this study. The dark black obtained at a similar vacuum (2.1e−2  Pa) with the Tes- painted designs on the sherd matched the visual criteria can Vega 3 XMU to demonstrate the reduction of charg- used [2] to define mineral (or iron-based) paint pigment. ing due to the ruthenium coating. The image taken with a The pottery piece was vapor coated with a thin coating vacuum of 2.1e−2 Pa in secondary mode (Fig. 3) using an of ruthenium as described in [6] to help reduce charg- Everhart–Thornley detector did not show clear pigment ing effects (blurred images from the SEM). Along with boundaries on the sherd. However, the image of the same the ruthenium coating, carbon and copper tape were sherd taken at a lower vacuum of 60  Pa (Fig.  4) showed applied to the outside edges of the sherd (except for the very well resolved pigment boundaries using the other section to be observed) and grounded to the specimen Everhart–Thornley secondary Tescan detector (with a holder to further reduce the effects of SEM charging. The turbo-molecular vacuum pump inside the detector). image of the sherd prepared for SEM in Fig. 1 was taken An Oxford Energy Dispersive Spectrometry detec- with a Proscope camera mounted to a dissection light tor (using Aztec software) produced a map of the loca- microscope. tion of iron as an overlay in green (Fig. 5) using a vacuum A Tescan VEGA 3 XMU variable pressure scanning of 2.3e−2  Pa over another image of the sherd made by electron microscope (VP-SEM) was used for this study the Tescan Everhart–Thornley secondary detector at equipped with two Everhart–Thornley type second - 2.1e−2  Pa with the Tescan Vega 3 XMU SEM. Figure  6 ary detectors: a low vacuum secondary Tescan detec- demonstrates the compositional contrast (brighter areas tor (LVSTD) and another Everhart–Thornley secondary of iron) obtained with the Tescan Vega 3 XMU SEM at a detector for use with vacuum settings near 2.3e−2  Pa. vacuum of 2.3e−2 Pa. The LVSTD produces a secondary image by using a turbo While the SEM–EDS detector (Oxford Inca X-act) on molecular pump to keep the detector components vac- the Tescan Vega 3 produced an elemental map of iron uum (as low as 1000 Pa) while an electron porous barrier (Fig.  6), the Micro-XRF-SEM detector on the Hitachi isolates the detector from the higher vacuum (2.3e−2 Pa) S-3400  N was able to produce elemental maps of iron SEM chamber. The low vacuum mode of the Tescan Vega Pendleton et al. Herit Sci (2016) 4:11 Page 6 of 7 3 serves to reduce the tendency of the sherd sample to the pigment boundaries of the pottery paint due to the charge in the SEM chamber (Fig. 4). A conductive annu- lower SEM chamber vacuum and vapor coating which lar mono-crystal scintillator-type (retractable) backscat- reduced charging effects (Fig.  4). The same Micro-XRF- ter detector is also used in the Tescan Vega 3 VP-SEM EDS data presented in [9] was included in the present to analyze the pigment present on the sherd (Fig.  6). paper for comparison with the SEM–EDS data obtained An Oxford X-act energy dispersive spectrometer (EDS) by VP-SEM. A more detailed explanation of the different (using Aztec software) is used on the Tescan Vega 3 to detector designs and function of the SEM can be found in produce SEM–EDS (high vacuum mode) the elemental introductory sources such as [10]. map of the location of iron in green on the sherd overlain by a secondary electron image (Fig. 5). Methods A Hitachi S-3400  N SEM equipped with an X-ray In order to prepare samples for vapor coating, the sherd source (IXRF Systems 10  μm X-beam Micro-XRF using was attached to an SEM mounting stub with carbon an X-ray source monocapillary optic Rh tube operating tape and copper tape applied to the edges of the sample at 50  kV and 1000  μA) and an IXRF Systems (30  mm) (Fig. 1) and placed in the vapor chamber. This ruthenium EDS detector was used to produce the Micro-XRF-SEM vapor protocol must be conducted in a properly func- elemental maps (Fig.  7) of the same Ancestral Pueblo tioning fume hood with a minimum flow rate of 100  ft/ pottery sherd analyzed with the Tescan Vega 3. The same min. Although osmium vapor coating has been used in ruthenium coating along with conductive carbon and some cases, only ruthenium vapors should be used for copper tape were utilized during Micro-XRF-EDS analy- this type of SEM–EDS study since the osmium X-ray sig- sis in order to provide similar experimental sample con- nal can interfere with the iron X-ray signal in the spectra. ditions for a comparison with SEM–EDS. A plastic bottle cap was used to hold the vapor solution of 1  ml of 10  % (wt/vol.) sodium hypochlorite to 0.02  g Conclusions ruthenium chloride (RuCl ·3H O). Ruthenium chloride 3 2 The SEM images used in this paper demonstrate differ - and 10  % sodium hypochlorite were purchased from ent techniques used to reduce or eliminate the effects of Sigma Chemical Co., St. Louis, MO. Once these chemi- charging without applying a carbon coating on the pot- cals are combined, the reaction is rapid. A beaker of hot tery sample which visually obscures the painted pottery water on the vapor chamber expedites the vapor coating designs. For this study, ruthenium vapor coating was of the sample [6]. After the reaction, the sample (attached applied to the sherd to reduce charging because the coat- to the stub) can be placed in the SEM for observation ing did not visually obscure the painted designs of the and analysis. Unlike the appearance of a sherd following sherd. Figure  2 shows a greater degree of charging prior carbon coating, the ruthenium-coated sherd does not to vapor coating while Fig. 3 shows the sherd with vapor appear darker after the application of vapor coating. coating exhibiting a lesser degree of charging than Fig. 2. The image shown in Fig.  4 demonstrate that high resolu- Abbreviations tion details of iron-based pigment with little or no charg- SEM: scanning electron microscope; EDS: energy dispersive spectrometer; ing can be best produced (with ruthenium vapor coating) SEM–EDS: energy dispersive spectrometry produced by electron beam in SEM; VP-SEM: variable pressure-scanning electron microscope; Micro-XRF-SEM: using the low pressure secondary detector at 60  Pa vac- micro-X-ray fluorescence produced by X-rays detected by EDS in SEM; LVSTD: uum. The images (Figs  2, 3, 4, 5, 6) were produced using low vacuum secondary Tescan detector; MDL: minimum detection limits; XRF: the same primary beam energy (30  keV) but with a dif- X-ray fluorescence. ferent vacuum setting, a different detector, or an absence Authors’ contributions of coating for the same Ancestral Pueblo black-on-white MWP completed technical operation of VP-SEM and EDS, DKW provided pottery sherd. Micro-XRF-SEM is more effective than the archeological context, EAE assisted in determination of optimal sample preparation, and BBP contributed to project conception and presentation of SEM–EDS in locating trace elements (aluminum, potas- scientific data. All authors read and approved the final manuscript. sium, calcium, sulfur, and silicon in addition to iron) (Fig. 7) in the paint and matrix of Ancestral Pueblo black- Author details Microscopy and Imaging Center, Interdisciplinary Life Sciences Building, on-white pottery for the sherd sample tested. A previ- Texas A&M University, Mail Stop 2257, College Station, TX 77843-2257, USA. ous study [8] examined a piece of the sherd used in the 2 American Section, University Museum, University of Pennsylvania, Philadel- present study and in another previous study [9], the same phia, PA 19104, USA. Microscopy Consulting Technologist, P.O. Box 6124, Thomasville, GA 31758, USA. Department of Agricultural Sciences, West Texas sherd sample was used in the present paper to analyze A&M University, Box 60998, Canyon, Texas, TX 79016-0001, USA. archeological pottery pigments. Both of these previous studies employed a JEOL JSM 6400 high vacuum SEM Acknowledgements The authors acknowledge Kenny Witherspoon and Mandi Hellested of IXRF without variable pressure capability. The Tescan Vega 3 Systems for the Micro-XRF-SEM analysis of the sherd and for sharing their LVSTD detector used in the present study better revealed scientific expertise. Pendleton et al. Herit Sci (2016) 4:11 Page 7 of 7 Competing interests 4. Nelson FW. A simple method for distinguishing between organic The authors declare they have no competing interests. and inorganic paints on black-on-white Anasazi pottery. Amer Antiq. 1975;40:348–9. Received: 26 June 2015 Accepted: 19 April 2016 5. Roney JR. The prehistoric Pueblo world A.D. 1150–1350. Tucson: Univer- sity of Arizona Press; 1996. 6. Ellis EA, Pendleton MW. Vapor coating: a simple, economical procedure for preparing difficult specimens for scanning electron microscopy. Micros Today. 2007;15:44. 7. Witherspoon KC, Cross BJ, Hellested MD. Combined electron and X-ray References excitation for spectrometry in the SEM. Micros Today. 2013;21:24–8. 1. Pendleton MW, Washburn DK, Ellis EA, Pendleton BB. Distinguishing 8. Pendleton MW, Ellis EA, Washburn DK, Pendleton BB. An analysis of pre- between mineral paint and carbon paint on ancestral Puebloan pottery. historic pottery paint composition utilizing energy dispersive spectros- Micros Today. 2012;20:32–6. copy. Micros Microanal. 2009;15:1518–9. 2. Stewart JD, Adams KR. Evaluating visual criteria for identifying carbon- 9. Pendleton MW, Washburn DK, Ellis EA, Pendleton BB. Detecting iron- and iron-based pottery paints from the Four Corners region using SEM- based pigments on ruthenium-coated archaeological pottery by SEM- EDS. Amer Antiq. 1999;64:675–96. EDS and by micro-XRF-SEM. Micros Microanal. 2014;20:2030–1. 3. Shepard AO. Ceramics for the archaeologist. Washington: Carnegie Insti- 10. Goodhew PJ, Humphreys J, Beanland R. Electron microscopy and analysis. tution of Washington; 1956. London: Taylor and Francis; 2001. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Heritage Science Springer Journals

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

Loading next page...
 
/lp/springer-journals/detecting-iron-based-pigments-on-ruthenium-coated-ancestral-pueblo-oQJiTi7Y0X
Publisher
Springer Journals
Copyright
Copyright © 2016 by Pendleton et al.
Subject
Material Science; Materials Science, general
eISSN
2050-7445
DOI
10.1186/s40494-016-0082-5
Publisher site
See Article on Publisher Site

Abstract

Background: Ancestral Puebloan black-on-white ceramics of the American Southwest can be classified as contain- ing 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 concentra- tions 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 previ- ously 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 charg- ing 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 spec- trometry maps for iron was obtained by the micro-X-ray fluorescence detector on the Hitachi S-3400 N scanning electron microscope. *Correspondence: mikep@tamu.edu 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 © 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. Pendleton et al. Herit Sci (2016) 4:11 Page 2 of 7 Keywords: Iron, Pigment, Archeological pottery, Scanning electron microscopy Background used scanning electron microscopy (SEM) coupled with The classification of the type of pigments applied to the energy dispersive spectroscopy (SEM–EDS). To prepare pottery sherds or vessels recovered at an archeological the sherds for SEM, they applied a coating of carbon site is vital to the determination of the cultural and tem- on the surface of their sherds to control SEM charging poral context of the site. The traditional Pueblo pigment effects. These effects can reduce the clarity of the image, types are carbon-based (containing organic compounds, cause a smearing of the image, and produce bright areas primarily potassium [1], mineral-based (containing pri- with corresponding loss of detail. While the carbon coat- marily iron [2] compounds), or in some cases a mixture ing allowed precise mapping of the boundaries of the of these types. Throughout the Pueblo sequence from iron (in the mineral based pigment) in backscatter SEM the advent of pottery making (c. AD 600 to the contact mode, the dark black carbon coating visually obscured period, AD 1600), these pigment types appear to corre- the paint pigment boundaries. To avoid the use of carbon spond to regional cultural groups and temporal changes coating to reduce charging, this paper describes alterna- in these groups. For example, in southern Utah and tive methods to control charging such as (1) the addi- southwestern Colorado, carbon-based paints character- tion carbon and copper tape (Fig.  1) to the outer edges ize the type sequences of western Pueblo black-on-white of the sherd, (2) the use of ruthenium vapor coating [6] pottery types in Arizona and in the Mesa Verde/Colorado to reduce charging without the visual color change asso- River/San Juan River area. In contrast, in northwestern ciated with sputtered metal coatings and (3) the use of and southwestern New Mexico, mineral painted wares a lowered SEM vacuum pressure in secondary imaging dominate the Chaco and Mimbres sequences. These pig - mode. To demonstrate the effect of ruthenium coating ment types are typically distinguished in the field and in most lab analyses by visual inspection. The carbon- based 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 vali - dated the accuracy of these observer-based identifica - tions 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 min- eral (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 inade - quate 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 vis- ual 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 Fig. 1 Proscope digital camera image of sherd with area of interest than merely visual observation was stipulated by [5] to outlined by black carbon tape and copper tape. Tabs of carbon tape classify the pottery of the eastern San Juan Basin and the (arrows) added to locate area of sample. The green dotted lines denote the area of the sherd shown in Figs. 2 and 6. The yellow dashed lines Acoma-Laguna regions of the American Southwest. denote the area of the sherd shown in Figs. 3, 4 and 5. The blue solid In order to distinguish mineral pigments from organic lines denote the area of the sherd shown in Fig. 7. Scale 5 mm pigments in painted Ancestral Puebloan pottery, [2] Pendleton et al. Herit Sci (2016) 4:11 Page 3 of 7 as an aid to reduce charging, a secondary electron image charging was present in the unpainted areas of the sherd (Fig.  2) of the sherd before coating was applied taken (Fig.  2) but the same sherd (following ruthenium coat- (using 30 keV) utilizing a JEOL 6400 SEM at a chamber ing) had far less charging present in the unpainted areas vacuum of 5.0e−3 Pa. Another secondary electron image of the sherd (Fig. 3). The painted areas were grounded by of the same sherd (Fig.  3) following ruthenium coating the iron present in the pigment which provided a path was obtained (30  keV) at a similar vacuum (2.1e−2  Pa) to ground. At a reduced vacuum (60  Pa), but the same with the Tescan Vega SEM. For the uncoated sherd, beam energy (30  keV), the charging effects were greatly reduced on the unpainted areas of the same sherd sur- face so that the darker painted areas could be easily iden- tified (Fig.  4). In this image, the caliche deposit on the sherd surface was easily resolved with the Tescan Vega 3 XMU SEM using the low vacuum Tescan secondary detector (LVTSD) with a built-in turbo molecular pump. An Oxford Energy Dispersive Spectrometry detector produced a map of the location of iron as an overlay in green (Fig.  5) using a vacuum of 2.3e−2  Pa over a sec- ondary image (30 keV) of the same sherd by the Tescan Vega 3 XMU SEM secondary detector. Another image of the same sherd (Fig. 6) was obtained (using 30 keV at 2.2e−2  Pa vacuum) on the Tescan Vega 3 XMU using the conductive annular mono-crystal scintillator-type (retractable) backscatter electron detector which dem- onstrated compositional contrast which allowed the identification of iron present in the brighter portions Fig. 2 JEOL 6400 SEM image of un-coated (no ruthenium) sherd at 30 keV and a chamber vacuum of 5.0e−3 Pa. A JEOL Everhart–Thorn- of the painted pigment area. Using a Hitachi S-3400  N ley secondary detector is used for this image. White arrow carbon SEM (at 5.0e−3  Pa vacuum), micro-X-ray fluorescence tape, C area of charging, P area of paint pigment. Scale 1 mm was used to map (Fig. 7) several elements including iron in the same sherd. Fig. 3 Tescan Vega 3 XMU SEM secondary electron image of ruthe- Fig. 4 Tescan Vege 3 XMU SEM secondary image of ruthenium nium coated sherd at a vacuum of 2.1e−2 Pa. A Tescan Everhart– coated sherd at 60 Pa vacuum. Tabs of carbon tape (arrows) added Thornley secondary detector (without a turbo-molecular pump) is to locate area of sample. Low vacuum secondary Tescan detector used for this image. Tabs of carbon tape (arrows) added to locate area (LVSTD) is used which has a turbo-molecular vacuum pump inside of sample. Bright areas on image are caused by charging. Scale 2 mm the detector. Almost all charging effect is eliminated. Scale 2 mm Pendleton et al. Herit Sci (2016) 4:11 Page 4 of 7 Results and discussion The SEM images used in this paper demonstrate differ - ent techniques used to reduce or eliminate the effects of charging without applying a carbon coating on the pot- tery sample which visually obscures the painted pottery designs. The image shown in Fig.  4 demonstrate that high resolution details of iron-based pigment with lit- tle or no charging can be best produced using the low pressure secondary detector at 60 Pa vacuum. The other images (Figs.  2, 3, 4, 5 and 6) were compared using the same primary beam energy as used in Fig. 4 (30 keV) but with a different vacuum setting, a different detector, or an absence of coating for the same Ancestral Pueblo black- on-white pottery sherd. SEM secondary image production begins with a pri- mary beam of negatively charged electrons that penetrate the surface of the sample to induce the production of secondary electrons which are attracted to the positively charged grid in front of the secondary detector. These Fig. 5 Tescan Vega 3 XMU SEM secondary image (30 kV ) of ruthe- secondary electrons are used by the secondary detector nium coated sherd at 2.3e−2 Pa vacuum with an overlay using an to produce an image. However, if too many negatively Oxford Energy Dispersive Spectrometry detector to demonstrate the charged electrons do not penetrate the sample surface location of iron as a green color with Aztec software. Tabs of carbon tape (arrows) added to locate area of sample. Scale 2 mm and cannot find a pathway to a positive ground (along the ruthenium coating to the conducting carbon and cop- per tape attached to ground), then charging can occur with the accumulation of electrons on the sample. These accumulated electrons can be attracted to the secondary detector by the positively charged grid mounted over the scintillator producing bright areas on the image (charg- ing) which do not reflect the topography of the sample surface. The backscatter detector is typically mounted around the objective lens of the SEM and is divided into four quadrants. No positively charged grid is needed for attracting the electrons for this detector. The backscat - tered electrons were originally part of the incident beam but some may have lost energy. The backscatter detector will only detect electrons which have interacted elasti- cally or quasi-elastically. Depending on how the signals from the four quadrants of the detector are combined, areas of the sample’s atomic number, surface topography, and surface crystallography can be analyzed. The Ancestral Pueblo pottery sherd is shown in Fig.  1 with carbon and copper tape surrounding the periph- ery to reduce charging. The pigment is visible as a black vertical band and triangular sections of black conduc- Fig. 6 Tescan Vega 3 XMU SEM backscatter image of ruthenium tive tape (white arrows) were added to the unpainted coated sherd at 2.3e−2 Pa vacuum. Tab of carbon tape (arrow) added area to facilitate locating the pigment areas in different to locate area of sample. Bright areas of compositional contrast are imaging modes. The green dotted lines denote the area due to iron in paint pigment which is correlated to painted pattern of the sherd shown in Figs.  2, 6. The yellow dashed lines shown in Fig. 1. Scale 1 mm denote the area of the sherd shown in Figs. 3, 4. The blue Pendleton et al. Herit Sci (2016) 4:11 Page 5 of 7 Fig. 7 Micro-XRF-SEM map showing areas of the sherd containing iron, aluminum, potassium, calcium, sulfur, and silicon. Tabs of carbon tape (arrows) added to locate areas of sample. A Hitachi S-3400 N SEM equipped with an X-ray source (IXRF Systems 10 μm X-beam Micro-XRF using an X-ray source monocapillary optic Rh tube operating at 50 kV and 1000 μA) and an IXRF Systems (30 mm) EDS detector was used to produce the Micro-XRF-SEM elemental maps Scale 5 mm solid lines denote the area of the sherd shown in Fig.  7. and of aluminum, potassium, calcium, sulfur, and sili- Figures  2, 3, 4, 5 and 6 are all images of the ruthenium- con (Fig.  7). The greater number of elements detected coated sherd obtained with the Vega 3 SEM at 30  kV. by Micro-XRF-SEM is due to the better signal to back- Figure  2 image (produced with a JEOL 6400 SEM at ground ratio of XRF compared to SEM–EDS. The Mini - 30  keV and a chamber vacuum of 5.0e−3  Pa) was not mum Detection Limits (MDL) of an electron beam are ruthenium-coated in order to demonstrate the increase about 1000 ppm (or 0.1 weight percent) but the MDL of in charging produced with no ruthenium coating present. an XRF beam is less than 100 ppm [7]. The painted area was grounded by the iron pigment so that only the unpainted area of the sherd demonstrated Experimental the bright areas of charging. Another secondary elec- An Ancestral Pueblo pottery sherd from the American tron image of the ruthenium coated sherd (Fig.  3) was Southwest (Fig. 1) was used for this study. The dark black obtained at a similar vacuum (2.1e−2  Pa) with the Tes- painted designs on the sherd matched the visual criteria can Vega 3 XMU to demonstrate the reduction of charg- used [2] to define mineral (or iron-based) paint pigment. ing due to the ruthenium coating. The image taken with a The pottery piece was vapor coated with a thin coating vacuum of 2.1e−2 Pa in secondary mode (Fig. 3) using an of ruthenium as described in [6] to help reduce charg- Everhart–Thornley detector did not show clear pigment ing effects (blurred images from the SEM). Along with boundaries on the sherd. However, the image of the same the ruthenium coating, carbon and copper tape were sherd taken at a lower vacuum of 60  Pa (Fig.  4) showed applied to the outside edges of the sherd (except for the very well resolved pigment boundaries using the other section to be observed) and grounded to the specimen Everhart–Thornley secondary Tescan detector (with a holder to further reduce the effects of SEM charging. The turbo-molecular vacuum pump inside the detector). image of the sherd prepared for SEM in Fig. 1 was taken An Oxford Energy Dispersive Spectrometry detec- with a Proscope camera mounted to a dissection light tor (using Aztec software) produced a map of the loca- microscope. tion of iron as an overlay in green (Fig. 5) using a vacuum A Tescan VEGA 3 XMU variable pressure scanning of 2.3e−2  Pa over another image of the sherd made by electron microscope (VP-SEM) was used for this study the Tescan Everhart–Thornley secondary detector at equipped with two Everhart–Thornley type second - 2.1e−2  Pa with the Tescan Vega 3 XMU SEM. Figure  6 ary detectors: a low vacuum secondary Tescan detec- demonstrates the compositional contrast (brighter areas tor (LVSTD) and another Everhart–Thornley secondary of iron) obtained with the Tescan Vega 3 XMU SEM at a detector for use with vacuum settings near 2.3e−2  Pa. vacuum of 2.3e−2 Pa. The LVSTD produces a secondary image by using a turbo While the SEM–EDS detector (Oxford Inca X-act) on molecular pump to keep the detector components vac- the Tescan Vega 3 produced an elemental map of iron uum (as low as 1000 Pa) while an electron porous barrier (Fig.  6), the Micro-XRF-SEM detector on the Hitachi isolates the detector from the higher vacuum (2.3e−2 Pa) S-3400  N was able to produce elemental maps of iron SEM chamber. The low vacuum mode of the Tescan Vega Pendleton et al. Herit Sci (2016) 4:11 Page 6 of 7 3 serves to reduce the tendency of the sherd sample to the pigment boundaries of the pottery paint due to the charge in the SEM chamber (Fig. 4). A conductive annu- lower SEM chamber vacuum and vapor coating which lar mono-crystal scintillator-type (retractable) backscat- reduced charging effects (Fig.  4). The same Micro-XRF- ter detector is also used in the Tescan Vega 3 VP-SEM EDS data presented in [9] was included in the present to analyze the pigment present on the sherd (Fig.  6). paper for comparison with the SEM–EDS data obtained An Oxford X-act energy dispersive spectrometer (EDS) by VP-SEM. A more detailed explanation of the different (using Aztec software) is used on the Tescan Vega 3 to detector designs and function of the SEM can be found in produce SEM–EDS (high vacuum mode) the elemental introductory sources such as [10]. map of the location of iron in green on the sherd overlain by a secondary electron image (Fig. 5). Methods A Hitachi S-3400  N SEM equipped with an X-ray In order to prepare samples for vapor coating, the sherd source (IXRF Systems 10  μm X-beam Micro-XRF using was attached to an SEM mounting stub with carbon an X-ray source monocapillary optic Rh tube operating tape and copper tape applied to the edges of the sample at 50  kV and 1000  μA) and an IXRF Systems (30  mm) (Fig. 1) and placed in the vapor chamber. This ruthenium EDS detector was used to produce the Micro-XRF-SEM vapor protocol must be conducted in a properly func- elemental maps (Fig.  7) of the same Ancestral Pueblo tioning fume hood with a minimum flow rate of 100  ft/ pottery sherd analyzed with the Tescan Vega 3. The same min. Although osmium vapor coating has been used in ruthenium coating along with conductive carbon and some cases, only ruthenium vapors should be used for copper tape were utilized during Micro-XRF-EDS analy- this type of SEM–EDS study since the osmium X-ray sig- sis in order to provide similar experimental sample con- nal can interfere with the iron X-ray signal in the spectra. ditions for a comparison with SEM–EDS. A plastic bottle cap was used to hold the vapor solution of 1  ml of 10  % (wt/vol.) sodium hypochlorite to 0.02  g Conclusions ruthenium chloride (RuCl ·3H O). Ruthenium chloride 3 2 The SEM images used in this paper demonstrate differ - and 10  % sodium hypochlorite were purchased from ent techniques used to reduce or eliminate the effects of Sigma Chemical Co., St. Louis, MO. Once these chemi- charging without applying a carbon coating on the pot- cals are combined, the reaction is rapid. A beaker of hot tery sample which visually obscures the painted pottery water on the vapor chamber expedites the vapor coating designs. For this study, ruthenium vapor coating was of the sample [6]. After the reaction, the sample (attached applied to the sherd to reduce charging because the coat- to the stub) can be placed in the SEM for observation ing did not visually obscure the painted designs of the and analysis. Unlike the appearance of a sherd following sherd. Figure  2 shows a greater degree of charging prior carbon coating, the ruthenium-coated sherd does not to vapor coating while Fig. 3 shows the sherd with vapor appear darker after the application of vapor coating. coating exhibiting a lesser degree of charging than Fig. 2. The image shown in Fig.  4 demonstrate that high resolu- Abbreviations tion details of iron-based pigment with little or no charg- SEM: scanning electron microscope; EDS: energy dispersive spectrometer; ing can be best produced (with ruthenium vapor coating) SEM–EDS: energy dispersive spectrometry produced by electron beam in SEM; VP-SEM: variable pressure-scanning electron microscope; Micro-XRF-SEM: using the low pressure secondary detector at 60  Pa vac- micro-X-ray fluorescence produced by X-rays detected by EDS in SEM; LVSTD: uum. The images (Figs  2, 3, 4, 5, 6) were produced using low vacuum secondary Tescan detector; MDL: minimum detection limits; XRF: the same primary beam energy (30  keV) but with a dif- X-ray fluorescence. ferent vacuum setting, a different detector, or an absence Authors’ contributions of coating for the same Ancestral Pueblo black-on-white MWP completed technical operation of VP-SEM and EDS, DKW provided pottery sherd. Micro-XRF-SEM is more effective than the archeological context, EAE assisted in determination of optimal sample preparation, and BBP contributed to project conception and presentation of SEM–EDS in locating trace elements (aluminum, potas- scientific data. All authors read and approved the final manuscript. sium, calcium, sulfur, and silicon in addition to iron) (Fig. 7) in the paint and matrix of Ancestral Pueblo black- Author details Microscopy and Imaging Center, Interdisciplinary Life Sciences Building, on-white pottery for the sherd sample tested. A previ- Texas A&M University, Mail Stop 2257, College Station, TX 77843-2257, USA. ous study [8] examined a piece of the sherd used in the 2 American Section, University Museum, University of Pennsylvania, Philadel- present study and in another previous study [9], the same phia, PA 19104, USA. Microscopy Consulting Technologist, P.O. Box 6124, Thomasville, GA 31758, USA. Department of Agricultural Sciences, West Texas sherd sample was used in the present paper to analyze A&M University, Box 60998, Canyon, Texas, TX 79016-0001, USA. archeological pottery pigments. Both of these previous studies employed a JEOL JSM 6400 high vacuum SEM Acknowledgements The authors acknowledge Kenny Witherspoon and Mandi Hellested of IXRF without variable pressure capability. The Tescan Vega 3 Systems for the Micro-XRF-SEM analysis of the sherd and for sharing their LVSTD detector used in the present study better revealed scientific expertise. Pendleton et al. Herit Sci (2016) 4:11 Page 7 of 7 Competing interests 4. Nelson FW. A simple method for distinguishing between organic The authors declare they have no competing interests. and inorganic paints on black-on-white Anasazi pottery. Amer Antiq. 1975;40:348–9. Received: 26 June 2015 Accepted: 19 April 2016 5. Roney JR. The prehistoric Pueblo world A.D. 1150–1350. Tucson: Univer- sity of Arizona Press; 1996. 6. Ellis EA, Pendleton MW. Vapor coating: a simple, economical procedure for preparing difficult specimens for scanning electron microscopy. Micros Today. 2007;15:44. 7. Witherspoon KC, Cross BJ, Hellested MD. Combined electron and X-ray References excitation for spectrometry in the SEM. Micros Today. 2013;21:24–8. 1. Pendleton MW, Washburn DK, Ellis EA, Pendleton BB. Distinguishing 8. Pendleton MW, Ellis EA, Washburn DK, Pendleton BB. An analysis of pre- between mineral paint and carbon paint on ancestral Puebloan pottery. historic pottery paint composition utilizing energy dispersive spectros- Micros Today. 2012;20:32–6. copy. Micros Microanal. 2009;15:1518–9. 2. Stewart JD, Adams KR. Evaluating visual criteria for identifying carbon- 9. Pendleton MW, Washburn DK, Ellis EA, Pendleton BB. Detecting iron- and iron-based pottery paints from the Four Corners region using SEM- based pigments on ruthenium-coated archaeological pottery by SEM- EDS. Amer Antiq. 1999;64:675–96. EDS and by micro-XRF-SEM. Micros Microanal. 2014;20:2030–1. 3. Shepard AO. Ceramics for the archaeologist. Washington: Carnegie Insti- 10. Goodhew PJ, Humphreys J, Beanland R. Electron microscopy and analysis. tution of Washington; 1956. London: Taylor and Francis; 2001.

Journal

Heritage ScienceSpringer Journals

Published: May 19, 2016

References