Aimers et al (2011) Handheld XRF analysis of Maya ceramics

Chapter 13 Handheld XRF analysis of Maya ceramics: a pilot study presenting issues related to quantification and calibration Jim J Aimers, Dori J Farthing and Aaron N Shugar Introd uction The investigation of archaeological ceramics has a long and varied history with regard to the analytical instrumentation used (for gcneral examples, see Peacock 1970; Bishop er al 1982; Rice 1987; Pollard er al 2007) In recent years newer applications have been used for the analysis of ceram ic materials as wcll, including rCP-MS (Fenno et al 2008; Man nino and Orecchio 20 I) and INAA (G lascock 1992; Neff 2000) In most cases the motivation to obtain chemical concentrations from archaeological ceramics has been to establish the source of the clay matrix This has proven possible using instrumentation with low detection limits (i c trace element analysis techniques such as NAA, ICP, AAS, and WD-XRF) Handheld X-ray fluorescence spectrometry was developed in the carly 1960's (Piorek 1997) but did not enter the world of archaeology outside of isolated research , until the early to mid 2000's when the instrumentation became more affordable (e.g Uda et al 2000; Cesareo et al 2004; Ida and Kawai 2005; Newman and Loendorf 2005) With the flourishing use of handheld XRF by non-trained scientists and other researchers who may not be trained in the basic (and advanced) theories of X-ray fluorescence, its misuse and the misinterpretation of results is prevalent (see chapter I of this volume for more detai l) Several papers have recent ly been published dealing with the provenancing of ceramics using handheld XRF with varied success (e.g., Morgenstein and Redmount 2005; Tagle and Gross 2010; Barone et al 20 I I; Goren el al 20 I I; Speakman el al 20 I I) Unfortunately, the 'boxed' calibrations that come with these instruments are not designed to deal with the complex nature of archaeological ceramics Ceramics are by nature heterogeneous with numerous components (such as temper) all having variable particle size They can have surface alterations and coatings and over time, the chemistry of the surface can alter as well In addition, archaeological ceramics often have altered chemical surfaces related to their burial environment Manufacturer calibrations are more geared to modern applications and modern materials that are uniform in makeup , and expectjng calibrations 424 Jim J Aimers, Don J Fa rthing and Aaron N Shugar designed for these purposes to be effective with archaeological ceramics is unreasonable The desire for quantification, whether it be for provenancing studies or characterization studies , requires the user to create material specific calibrations (see Hein el al 2(02) This paper discusses current investigations of Maya ceramics from Belize The focus of this study is not to determine the source/provenance of the clay bodies, but to investigate the potential for establishing handheld XRF as an on-site analytical tool for the characterization and potential classification of ceramics based on their chemical signatures The development of an empirical calibration is presented including the process involved in creating reference materials for that calibration Overview of Maya chronology and pottery The date of the arrival of people in the Maya lowlands is currently a matter of debate (see Lohse 2010) but lies somewhere in the Archaic period (8000-2000 B.C.) with maize pollen indicating farming by about 3000 B.C (Pohl er al 1996) The Preelassic period dates from roughly 2000 B.C to A.D 250 with the earliest well-documented Maya pottery about IIOO-goo B.C in the Cunil ceramic complex of the Belize Valley (Sullivan and Awe 20 12) By the Late Preelassic period (often dated 250 B.C to AD 250) Maya pottery was very weU made and styles were widely spread across the entire Maya lowlands Although most of the significant cultural aspects of Maya civilization were in place by the Late Preelassic, the subsequent Classic period (AD 250-8(0) is generally considered the height of Maya development The Classic Maya lived in a literate, highly stratified society which produced monumental all and architecture and elaborate polychrome pictorial pottery [n tbe Terminal Classic period many sites were abandoned and profound changes swept across the Maya world (Aimers 2007b) Dates vary because different sites were abandoned or transformed at different times from about A D 750-1050 but the Terminal Classic has traditionaUy been dated to about AD 800-goo Pottery of the Terminal Classic varies across the lowlands but it was still well-made with an emphasis on elaborate modeling and incising over polychromy The Postclassic period follows the Terminal Classic and ends at the arrival of the Spanish in the Maya area at about AD 1540 Postelassic pottery also emphasizes incising and modeling and is typically well-made The pottery of the Postelassic period is the focus here Handheld XRF analysis of Maya ceramics 425 The Postclassic period and its pottery , N • YUCATAN Pt;.N I'SLLA , , ,., GU ATEMAL A ,, I , : IJI.IrpfqI.Ie PACIFIC OCEAN ".HOND U RA S • ; - t , EL SAlVAOQR \, Figure 13.1: Map of the Maya region showing key Postdassic and trading si tes relevant to this study (After McKillop 2(05) In the early years of Maya archaeology the Postelassic period was neglected as a period of cultural deeline following the Classic "collapse" , but more recent research at Postclassic sites has revealed population movement, innovative political strategies increased exchange and commcrciali.mtion , iconographic innovation, and intense Mylistic interaction (Smith and Berdan 2000, 2(03) A key characteristic of the Maya Postelassic period is evidence of extensive trade , especially among sites along the Caribbean coasts and on rivers, and involving sites in the northern half of the peninsula such as Mayapan and Chichen Itza (Figure 13.1 ) A beller understanding of the economic and political milieu of the Postelassic would be greatly aided by more detailed documentation of the nature 426 Jim J Almers, Don J Farthing and Aaron N Shugar and degree of interacti on among Pos tclassic si tes, and one of our most informative rtifact classes is pottery b (((('//// /!)// J) ~ Lflf./c( J!{IP_ ' /~ )"11 "- l~ j )~~~"'j , Figure 13.2: Red Payil Group Ceramics Payil Red is the plain type, Palmul Incised is the incised verSion (aher Sanders 1960: Fig 4, 5) 7 Handheld XRF analysis of Maya ceramics 427 Aimers has been investigating the Postclassic period since his dissertation research on the Maya collapse and its aftermath (Aimers 2003, 2004c, 2004a, 2007b) and particularly with research on the pottery of two sites that were not abandoned in the Terminal Classic: Tipu (Aimers 2004a: Aimers and Graham 2012a) and Lamanai (Aimers 2008 ,2009 ,20 10; Aimers and Graham 2012b) In the summer of 2011 Aimers began a pilot study to investigate the chemical variability of a plain red type (Payil Red) and a re lated incised type (Palmul Incised) (see Figures 13.2 and 13.3) using XRF with samples from the inland site of Tipu, and the site of San Pedro on the island of Ambergris Caye These types are not particularly common but they are widely distributed in the Late Postela sic period, and they are much more common at coastal sites and are thought to have been produced along the coast of the Mexican state of Quintana Roo (e.g , the sites of Ichpaatun , Tancah and Tulum, see Figure 13.1; (Sanders 1960: Aimers 2(09) As we discuss later, the original goal of the research was to identify compositional groupings within these types which might help in addressing trade and exchange in the Paste lassie period We not expect to link the pots to their production location except in rare cases (sec comments below), but we hope that chemical characterization can help us map the distribution of pottery types better than surface style and form alone These distributions can help in the construction of inferences about Maya pottery production and trade A larger study is planned to follow the pilot stud y with more stylistic types and samples from more sites Figure 13.3: Palmul Incised sherd from San Pedro, Belize, showing the surface inCISing and the red slip This example also has blue pigment which is thought 10 have been distributed from the site of Mayapan (Mexico) 428 Jim J Ai mers, Dori J Farthing and Aaron N Shugar Major analytical techniques used in maya pottery studies In th e Maya area, a stylistic classification system known as type-variety has dominated the study of pottery since its introduction from American Southwestern archaeology in the 1950's and 1960's (S mith , Willey, and Gifford 1960) Typevariety organizes pottery hierarchically into wares (based on broad characteristics of paste andlor surface) grollps , which are clusters of types (defi ned by a set of anributes such as color and decorative treatment) and varieties which are often based on single attributes So, Payi l Red and Palmul Incised are types within the Red Payil Group ofTulum Red Ware Each of these types only has a si ngle variety because these types arc macroscopically quite homogenous in paste and surface treatment - this is one reason they were chosen for the XRF study (sce Cecil 20 I2 for more detail on the pastes, AA data and petrography) Type-variety has been used widely because it is a rapid and inexpensive " Iow-tech" way to organize the thou sands (sometimes millions) of sherds that are produced by excavations at sites in the Maya area (Aimcrs and Graham 20 12b) Aimers' research to date has involved assessing interaction among sites and regions using type-va riety analysis of pouery from hands-on examination of collections from across the Maya lowlands (see e.g Aimers 2004a, 2004b 2007a 2008,2009,2010) Type-variety provides a common language for archaeologists and has facilitated the compari son of pottery across sites and regions in addressing issues as varied as chronology function trade/exchange and cultural meaning Nevertheless, type-variety has been subject to a number of important criti cisms One of the most important issues is the characterization of fabrics (which Mayanists generall y call pastes) at the ware le ve l (Rice 1976) Paste variation has been used by some archaeologists as a key discriminating attribute and thus uscd to make distinction at the highest (ware) level (e.g in Rice's work cited in this chapter), but it has been considered by others to be a minor factor and occurs randomly in , for example, type or variety description s or to create varieti es (Gifford 1976) Attention paid 10 paste has tended to vary with research questions Those interested in manufacture and production have tended to pri vilege paste variation for the insight it can provide into these issues Those interested in consumer choice, stylistic analysis and comparison, or meaning, have often considered pa~ l c variation irrelevant Thus type-variety classification is problematized by inconsistencies in the treatment of paste variation that are n01 weaknesses in the system itself but result from the fact that like all classifications, type-varicty methods vary according 10 the research questions addressed (A imers 2012a) Still , it is reasonable for Mayani'" to seek greater accuracy consistency and comparability in the characterization Handheld XRF analysis of Maya ceramics 429 of paste variation and the oldest established technique for the close examination of paste variation is petrography (Jones 1986 199 I) Maya petrographic studies have been surpri;ingly rare in comparison to work in the Old World and to the amount of research on pottery in the Maya area This is probably because Maya pottery is stylistically varied , exceptionally elaborate and often well-preserved, so macroscopic characterizations have been adequate for chronology and broad comparative studies Petrography is of course time-consuming and destructive which poses a problem with large or complex sample sets Petrographic studies of pottery have tended to focus on issues related to manufacture production and distribution (e.g Rice 1977, 1991 , 1996; Cecil 200lb 200la; Cecil and Pugh 2004; Howie 2005; Cecil 2(09) One of the challenges for the petrographic study of Maya ceramics is that the geology of the Maya area is relatively poorly mapped (see extensive comments about these issues in Howie 2005: 120- 16 I for Belize) so until more sampling of geology and clays is done it can be very difficult to tie pottery to its clay sources Successful petrographic studies have tended to be focused on a fairly local level (e.g Cecil and Rices work in the Pet6n Lakes; Howie 2005) where the geology is well known or distinctive, andlor where clay sampling has been undertaken Materials science approaches to the study of archaeological ceramics are advancing rapidly Petrography is of course well established, and recent studies of archaeological pottery have used XRF (Bakraji el al 2010; Bakraji el al 2006; Hall 200 I ; Thomas el al 1992), portable XRF (PXRF) (Papadopoulou el 01 2007; Papadopoulou el 01 2006; Papageorgiou and Lizritis 2(07), mineralogical analysis using XRD (McCaffery el al 2007; Mitchell and Hart 1989; Rasmussen el 01 2009: Stanjck and Hausler 2004; Zhu el al 2004) , trace chemistry determination by NAA (many, e.g , Glascock 1992; Glascock el 01 2004; Hancock el al 1989; Lopez-del-Rioelal 2009; Olin and Blackman 1989),structural and microstructural characterization techniques such as SEM (Ow nby el al 2004: Palanivel and Meyvel 2010) or combinations of various techniques (Marghussian el al 2009; Padilla el al 2005; Speakman el al 20 II) The best overview on the use of all of these techniques in the analysis of archaeological pottery was done by Rice (1987) Of the elemental analysis techniques, Mayanists have considered NAA to be the most valuable because of its sensitivity, accuracy, few matrix effects, and the range of trace elements that can be identified, including rarc earth elements (Neff 1992:2) The disadvantage of NAA is its cost and the fact that it can only be conducted in facilities with research reactors In addition, the sample size required for NAA is quite small, typically a small drilling is all that is required For thi s reason sample heterogeneity could have an adverse effect on the resulting I I 430 Jim J Aimers, Don J Farthing and Aaron N Shugar chcmislry obtained This issue is recognized by reseruchers and now larger samples are taken and powdered for analysis (see Speakman el 01 20 II for example) Many of the other elemental analysis techniques (e.g., PIXE) are also expensive and require equipment that is not easy to acquire This has led to continued interest in petrography and the use of XRF and XRD because many universities and museums have access to these instrumental methods Although XRD analysis does not provide elemental analysis data insights it compliments other techniques because it provides information on the mineral makeup of analyzed samples For example, Tenorio el 01 (2010) used NAA, XRD , and SEM in a study of pottery from Laganero, Chiapas, Mexico Current trends in Maya ceramic analysis The introduction of a new and readily accessible analytical technique typically results in optimism about its utility for the investigation of archaeological problems, For example, Culben and Schwalbe (1987) published an early study of the application of standard XRF to Maya ceramics from Tikal (see al;o Schwalbe and Culben 1988) This study and others were criticized concerning issues of precision and especially comparability of results to other studies (Bishop el 01 1990:543; Neff 1992:4) Recently, ponable and handheld XRF technology created a similar wave of optimism but critical evaluations did not lag far behind Shackley (20 I0) provides the most straightforward critique of handheld XR F on issues of reliability and vaUdity as well as the ·'near religious fervor" with which the technology been embraced by people who are not adequately familiar with the methodological and interpretive issues involved (Shugar 2009: also addresses these issues) This is cenainly the case in Maya studies The Mayanist here (Aimers) learned of handheld XRF relatively recently and was excited by what appeared to be a fast way to acquire ·'hard" compositional data in the field Like many others he had no background in XRF methodologies and no awareness of the challenges of sensitivity, precision, accuracy, and comparability of results using this new technology This chapter brings together the differing experience of the three authors in an investigation of these issues in relation to archaeological pottery In the study of Maya pottery new analytical techniques after a period of enthusiastic experimentation, are typically absorbed into research projects which combine them with more established techniques, In panicular, petrography combined with quantitative chemical analysis broadens the scope of all investigation to include both the paste makeup (e.g, the characteristics of the clay Handheld XRF analySis of Maya ceramics 431 body, natural inclusions, and temper which are Chaine operaroire or specific manufacturing process), and its particular chemistry (as stated above - potentially to source clays) Type-variety despite its problems, is also sti ll a very useful organi zational and descriptive structure for Maya pottery There is broad agreement that results from multiple techniques of ana lysis are always more useful than anyone alone (Cu lbert and Rands 2(07) In a discussion of the challenges of characterizing Aegean pottery Day el al (1999: 1034) reached mOTe indicative of the a sim ilar conclusion : different sources of chemical variation emphasize the need for the integration of olher information; mineralogical, technological and stylistic; which enables the researcher to attribute differences to provenance or aspects of clay paste technology The complex interplay of these natural and human SOUIees of variation means that such analyses cannot take place in isolation in a " black-box , approach On the contrary, it is imperative for mineralogical and elemental ana lyses, at least in the Aegean , to be conducted in an integrated programme which exploits complementary types of archaeological and analytical information Pottery variability and the potential of XRF and handheld XRF Inter-observer inconsistency is always an issue in type-variety, especially for rare types (Aimers 20 12b) but many types, including the ones discussed in this chapter, are recognized by experienced specialists with little if no debate So , why is there a need for XRF and other means of compositional characterization? In the case of Red Payil Group sherds , the problem is their macroscopic consistency We know that these types are widely distributed and we assume that like most widespread types, they were made by multiple producers and probably at multiple locations Pool and Bey (2007:36) note that the "vast majority of [Maya] pottery was made and consumed locally" (see also Arnold el al 1991) This has been found repeatedly for Maya ceramics, most famously with the Preclassic Sierra Group types which are very stylistically consistent across the entire Maya lowlands This pilot project was designed to see if XRF could detect intra-type compositional groups that could be investigated and hopefully confirmed by other techniques such as petrography SEM and XRD The longer-term thinking was that if standard XRF would reveal compositional groups in this otherwise homogenous pottery, handheld XRF would have the potential to the same The ability to use handheld XRF on large numbers of samples in the field could allow 432 Jim J Almers, Don J Farthing and Aaron N Shugar the establishment of what are essentially technological varieties of Payil Red and Palmul Incised In so me ways, this new portable and handheld technology could solve what Aimers (2007a) has called ''The Curse of the Ware" - the inconsistent treatment of paste in type-variety (see Ri ce 1979 for an extended disc ussion of this iss ue) Another reason for the interest in handheld XRF is that the ability to export large number of sherds from Belize, or any co untry is difficult at best, making traditional analysis difficult , and analysis of large sample groups even morc problematic Being able to transport the XRF to the field would allow for on-site analysis of the sherds and could help archaeologist direct the ir research questions ill siw Benchtop XRF sample preparation and analysis To investigate the viability of the XRF as a "discriminating" tool, a selection of samples representing the Payil Red and Palmul Incised types were analyzed for their major and trace clement composi tions in the Department of Geological Sciences ar SUNY Geneseo All samples were analyzed with a PANalytical AXIOS Sequential WD-XRF Spectrometer The XRF uses a kW Rh-anode X-ray source and both a flow and a scintillation detector The now detector is ideally suited for the analysis of transi tion clements and the scintillation detector is ideal for the analysis of heavy elements A set of internal curved crystals (including the following options: LiF 200, LiF 220, PE 002, and GE III ) are also used in every analysis to disperse the X-rays emitted from the sample according to their different wavelengths using diffraction The crystal s are connec ted to a turret that rotates to insert one crystal at a time into the beam path The crystal that is selected depends upon the element that is being analyzed (Table 13 1) The XRF is operated at vo llages that range from 10 kV to 60 kV and currents that range from 10 rnA to 125 rnA Typically flow detectors and scintillation detectors have resolutions below 1000 eV, but when used in unison with a crystal spectrometer, that resolution j", greatly improved to range from approximately 12 eV (LiF 220) to 31 eV (LiF 2(0) (Jenkins 1999: 1(0) All pottery samples were cleaned with water and an ordinary nylon-bristle toothbrush to remove soil and particulate matter that was loosely adhered to the pottery and not considered original to lhe pottery body The samples were then crushed using a SPEX SamplePrep Mi xerlMili and a hardened steel grinding canister equipped with gri nding balls The grinding process produced a powder that passed through an 80-mesh sieve size This was achieved by milling Handheld XRF analysis of Maya ceramics 433 -10 grams of sherd material for minutes If after milling large pieces of sample remained the sample wa, milled for an additional I to minu tes depending on the abundance of large fragments Between each sample the bal I mill was cleaned by milling quanz sandbox sand for minutes The cleaning san d was disposed of and the canister was then blown out with compressed air to femo ve any additional sand Small sized particles are easier to fuse into glass beads/d isks because they have a greater amount of surface area and dissolve easier in th e fusing process I Small and even particle sizes are also essential for preparing compacted pellets I , because the small and even-sized particles are easier to hom ogenize and will compact into a morc coherent Hat-faced pellet with no major nicks and divots in the sample surface Both fused beads and well-made pressed pe lIets arc essential for obtaining the best possible XRF analysis because they mininu.ze matrix effects, which can skew the data and not accurately represent the overa II chemistry of the sherd The powdered materials are also in the ideal form for X-ray diffraction (XRD) analysis and samples can be analyzed first with XRD an d then the powder can be re-used in the preparation of XRF samples Cry~lal name PANal)'lical's o;;ugge~tions , , , for use LiF 200 crystal Used for routine analysis for elements ranging from LiF 220 crystal Used for routine analysis for elements bel wcen V and U It is not as reflective as the LiF 200 but has a highcrdi " "0 X ~ 15 30 21 6,7 Z, Y Nb Mo 10 10 53 5.4 456 45 13 15 148 130 5,9 5.9 1,7 1,8 23 3.8 9.5 10 23 1.8 7,1 6.5 450 446 19 20 136 124 6.9 6.7 2.5 1.9 6.2 3.6 9.1 10 2.4 1.5 2.0 10 10 12 12 4.8 6.1 449 435 18 18 166 180 8,6 8.9 3.0 0.6 6.6 7, 10 1.7 32 28 17 83 6,8 12 11 8.1 8.0 592 544 18 18 185 11 72 73 2,6 5.4 8.7 10 7.4 2.0 7.4 13 28 29 6.4 6,1 10 10 8,2 7.0 453 426 20 17 137 112 5,9 6,1 1,7 1,7 6,7 3.4 9,2 10 3,9 3.0 11 21 41 40 7.7 7.4 10 10 7,2 7.7 330 358 20 23 156 158 7.1 7.7 3.0 2.0 4.7 3.5 7.6 9.5 0,6 13 ~ i3 SP8 17 18 27 12 7.8 7.8 9.4 10 6.1 6,4 362 337 15 15 148 155 6.8 6.8 1.8 2.0 4.9 2.4 10 0.0 1,4 ;Z SP9 14 21 31 31 7.0 7.5 10 11 7.9 83 394 375 16 16 184 182 7,4 7,4 1.0 2.0 4.2 3,7 10 9.4 33 2.5 SP1 39 38 '" SP13 13 24 SP14 30 29 41 10 10 10 10 33 19 9.5 10 13 45 46 10 10 11 37 88 10 10 11 8.0 8.1 255 255 21 20 184 184 8.9 7,7 2,8 1,9 4,8 4,7 11 10 4.0 -0.1 12 11 387 386 21 20 236 222 9.2 23 4.2 93 10 2,4 2.5 10 5.6 5.5 363 257 17 15 21 207 9.5 7.9 0.98 2.0 4,7 2.1 10 10 4.8 2.4 12 10 10 348 348 17 17 212 93 10 22 2.0 6.6 71J 93 10 1,6 1.6 8,4 2,6 21J 10 12 9.0 2.1 '"a:x SP15 2.7 40 40 10 10 11 11 7.5 7, 479 44 19 19 165 154 8.8 ~ SP1 11 18 150 150 10 10 11 11 8.5 9.1 504 407 14 14 182 182 8,8 8,7 14 14 18 17 11 11 400 449 36 35 33 339 10 63 5.5 469 369 16 17 17 14 13 499 500 32 32 304 304 13 28 335 335 14 23 124 183 8,9 8.0 0.5 1.5 SP17 57 58 50 49 :l;' SP18 12 18 29 29 8.2 8.0 11 o" SP1 80 80 46 46 12 12 17 ::> " "0 X SP20 74 74 51 51 14 14 16 16 14 14 329 328 28 -~ T1 7B 63 63 52 11 13 10 9.4 57 42 16 co I >Ql' 41 100 :; " '" ~ i" 10 36 @ Sb 7.6 co Sn Cd 83 " 7.0 S, Rb 10 SPIO '"I Th Ga 26 '"::> "- >- >- ::r o::w x'" 62 93 141 1.6 2.0 5.2 4.0 10 10 1.9 1,7 6.9 6,8 12 9,4 3.4 4.2 3.0 15 14 3.1 31J 5,6 5,6 10 10 3.0 140 6.9 6.8 3.1 2.0 1.1 21J 8.4 10 3.0 3,6 13 2,6 2,8 5.5 5.5 8.9 91J 13 3.1 2.8 5.7 10 5.6 2.0 11 2.5 5.2 10 35 33 10 42 2.5 5- ~ '""-::> ::> V> " C