TakenwithaGrainofSalt:ExperimentationandtheChemistryofArchaeologicalCeramicsfromXaltocan,MexicoWesleyD.Stoner& JohnK.Millhauser&EnriqueRodrguez-Alegra& LisaOverholtzer&MichaelD.Glascock#SpringerScience+BusinessMediaNewYork2013Abstract Neutronactivationanalysis (NAA) of ceramics fromXaltocan(n=651)displays highvalues for sodiumandpotassiumandlowconcentrations of manytransitionmetals andrareearthelements comparedtoother sites intheBasinofMexico. Given that Xaltocan was situated on an island in the middle of a saline lake,thepotential reasons for this chemical signaturearediverse. Ononehand, if thesodium and potassium were elevated due to some behavioral aspect of the potters, theXaltocan chemical groups provide a glimpse at the behaviors of Xaltocan potters thatpermitmoreprecisesourcedesignations.Ontheotherhand,ifthischemicalfinger-print arose due to contamination in a saline post-depositional environment, theXaltocanchemicalgroupswouldnotbevalidreferencesforprovenancestudies.Toevaluate these alternative hypotheses, we employ several lines of evidence: (1)comparison of the Xaltocan ceramics to over 5,000 NAAassays of clays andceramicsfromtheBasinofMexico,(2)experimentaldopingofclayswithwaterofJArchaeolMethodTheoryDOI10.1007/s10816-013-9179-2W.D.Stoner:M.D.GlascockArchaeometryLaboratory,UniversityofMissouriResearchReactorCenter,1513ResearchParkDrive,Columbia,MO65211,USAW.D.Stoner (*)Department of Anthropology, University of Missouri, 107 Swallow Hall, Columbia, MO 65211, USAe-mail:stoner.wesley@gmail.comJ.K.MillhauserDepartmentofSociology&Anthropology,NorthCarolinaStateUniversity,CampusBox8107,Raleigh,NC27695-8107,USAE.Rodrguez-AlegraDepartmentofAnthropology,UniversityofTexasatAustin,2201SpeedwayC3200,Austin,TX78712-1723,USAL.OverholtzerDepartmentofAnthropology,WichitaStateUniversity,114NeffHall,Wichita,KS67260,USAdifferent salinitiesandfiredtodifferent temperatures, (3) leachingexperimentsofarchaeological potterysherds(n=22) recoveredfromthesiteof Xaltocan, and(4)laser ablationinductively coupled plasmamass spectrometry of the clay and temperfractionof asmall sampleofXaltocanceramicstodeterminewhichcomponent isresponsible for the elevated sodium and potassium values. The results suggest that thehigh sodium and potassium values were present in the ceramic paste before firing. Wethenusethesenewlyestablishedreferencegroupstobetter understandtheroleofXaltocanintheregional economy. Thetypeof experimentationemployedinthisstudy has proven to be an important method for determining the behaviors of ancientpottersanddistinguishingthemfrompost-depositionalprocesses.Keywords Experimental archaeology.Ceramics .Exchange.Chemical analysis.Mesoamerica .Technological choiceIntroductionCompositional analysesofarchaeological ceramicshavebecomestaplesinmost ar-chaeologicalresearch.AsNeff(2000)andNeff et al.(2003,p.202)(Fig.2)observe,however, ceramic provenance studies often fall short of evaluating all potential sourcesofvariationthatcontributetoapotsbulkchemistry.Particularlytroublesomearethechemical effectsofuseanddiagenesisthat cansteerresearchersawayfromthetrueprovenance of pottery. Aminority of compositional studies actively use experimentationor supplemental chemical techniques to understand these potentially confounding vari-ables. In this paper, we establish an experimental method to differentiate cultural inputsfrompost-depositionalweatheringinexplainingthebulkchemistryofarchaeologicalceramics found at Xaltocan in the Basin of Mexico.Neutronactivationanalysis(NAA)ofceramicsfromXaltocanhasyieldedchem-ical compositions consistently high in sodium and potassium relative to other potteryintheBasinofMexico. Xaltocansat onahuman-madeislandinthemiddleofthesalineLakeXaltocan. Theenvironment aroundtheislandpossesses anumber ofmedia that could introduce these elements into ceramic pastes. Potters may have usedclays, temper, orwater highinthesealkali elements, or theymayhaveaddedsaltcrystals to the paste. Alternatively, diagenesis through physical saturation or chemicalweathering may have elevated their concentrations. We conduct experiments toobservethemobilityofsodiumandpotassiuminceramicmaterials(1)whendopedpriortofiringand(2)whensubjectedtopost-firingsalinesoakandleachtests.Theresults, coupledwithcomparisontothelargeNAAceramicdatabasefortheregionandmicroanalytical techniques, helpustodifferentiatethecontributingfactorsthatelevatedthelevelsofpotassiumandsodiumofXaltocanceramics. Ononeaxisofvariation, we distinguish between the natural composition of rawmaterials andcultural behaviorsthat includethepottersmodificationandmixingofthosemate-rials. Onanotheraxisofvariation, weconsiderthedistinctionbetweenproductionvariables,includingbothnaturalandculturalinputs,and post-productionalterationstoceramicchemistrythroughuseorpost-depositionaldiagenesis.Thisresearchhasseveral implicationsfor ceramicsourcingstudies. First, it de-velopsour abilitytoaccuratelyreconstruct patternsof exchangebyprovidingtheStoneretal.tools to distinguish the meaningful behaviors of potters from post-production chemi-cal changes. Thelattercanprevent accuratesourcing, or evenleadtofalsesourcedeterminations. Thefocusofthispaperoncommonsalt-formingelementsmakesitparticularlypertinent for chemical studiesincoastal andlacustrinesettings. Second,defining chemical reference groups for Xaltocan permits exploration of the sites role inproducingandexchangingpotterytothebroaderregion. Xaltocanheldpolitical andeconomicimportanceover muchof thePostclassicperiod(9501521CE) (Blanton1996; Brumfiel 2005b; Brumfiel and Hodge 1996; Overholtzer 2012). While re-searchers have examined Xaltocan as a consumer of pottery produced elsewhere (e.g.,Brumfiel andHodge1996; HodgeandNeff 2005; Nicholset al. 2002; Rodrguez-Alegraet al., under review), its roleas aproducer andexporter has beenpoorlyunderstood. Provenancestudies intheBasinof Mexicodependonbuildingrobustchemical referencegroups out of large ceramicsamples across theregion (e.g., Crider2011;Garraty2006;Hodge et al.1993;Minc1994; Nichols et al.2002;Neff, H.,&Glascock, M. D., 2000, Provenance analysis of Aztec period ceramics fromthe Basin ofMexico: final technical report, unpublishedpaper). Until recently, suchareferencegrouphas not existedfor Xaltocan(but see Neff, H., &Glascock, M. D., 2000,Provenance analysis of Aztec period ceramics from the Basin of Mexico: final technicalreport, unpublished paper). The current research allows us to identify Xaltocan productsin other Basin locations, thereby reconstructing exchange from this important site.TheoreticalandMethodologicalConsiderationsforCeramicCompositionalAnalysesThis paper highlights the fundamental distinctionamongnatural andcultural in-fluencesonpotterychemistryontheonehandandpost-production(useanddiagen-esis) chemical changes on the other. These issues have been debated since the first useof NAA(Sayreet al. 1957). Thebulkchemical datagarneredfromNAAdonotdiscriminate among these variables, so it is important to understand their influence onthechemistryofpotteryfoundinthearchaeologicalrecord.Natural determinantsofbulkceramicchemistryincludethechemical sumofallrawmaterials(clay, temper, aplasticinclusions, anythingelseaddedbythepotter)used to manufacture a paste (Neff et al. 2003). Natural chemical variation among rawmaterials boils down to a spatial problemregarding the geographic location ofsuitable andaccessible sources of clayandtemper andgeological processes thatmakethemchemicallysimilar or different (Arnold1985, pp. 2057; Bishopetal.1982,pp.275276).Culturalandnaturaldeterminantsofceramicchemistryco-varybecause potters userawmaterials foundintheir environment. Humans introducechemical variability, however, throughtheir selectionandprocessingof rawmate-rials;theproportionsinwhichtheymixclay,temper,andauxiliarymaterials;firingtechnology,fuel,temperature,andatmosphere;anddecorationwithpaintsandslips.Theseproductionvariablesenableprovenancestudies.The ways inwhicharchaeologists have approachedthese sources of chemicalvariationhavechangedconsiderablyoverthehistoryofNAAuseinthediscipline.Early NAA studies emphasized tracing pottery back to the geological source of clays.Kilikoglouet al. (1988, p. 37) suggest that themost secure approachis theTakenwithaGrainofSaltcomparison of the [ceramic] group in question with natural clays from the area wherethe site of interest is located. Sayre and Harbottle (1979) note in their oft-cited workthat thehumancomponent canoftenobscurethegeological provenanceof clays,mixingashedoesclayfromdifferentsources,levigatingit,addingtemperetc.(p.3).Theadditionoftempertorawclays,oftenreferredtoasthetemperproblem,haslongbeenseenasdistractingtheresearcher fromidentifyingthegeochemicalsourceofclays(Kilikoglouetal. 1988; Neffetal. 1988, 1989, 2003; Sayreetal.1957;SayreandHarbottle 1979).Manyresearchersadvocate theuseof petrographyand temper analysis to understand its influence on bulk chemistry (Bishop et al. 1982;Bishop 1980, pp. 4748; Day et al. 1999; Day and Kilikoglou 2001; Neff et al. 1988,1989;Stoner et al.2008;Tsolakidou et al.2002).In the 1990s, archaeologistslargely influenced by social theory of the prior decade(Bourdieu1977,1990;Giddens1979)andbehavioralstudiespotteryproductioninatechnological andecological frameworks(e.g., Arnold1985; Kolb1988; ReinaandHill 1974; Rice 2005 [original 1987]; Rye 1981; Schiffer and Skibo 1987,1997)treated choices made by potters as an aspect of their identity (Gosselain1992, 2000; Lemonnier 1992; Sillar andTite 2000; Stark 1998). This led totheframeworks oftechnological choice andchane opratoire that have been popularin archaeology over the past two decades. In brief, choices that potters make at differentstages of the production process, from selection of raw materials through their use anddiscard, leavesignaturetraitsonthepotterythat reflect thepotterschemical finger-print. Every potter has a unique paste recipe, but it is extremely rare that archaeologistsareable to isolateindividualsthroughmaterialsanalyses.Ideally,we strive to identifythetechnological fingerprintof communitiesof pottersat thehighestresolution possi-ble(e.g., Arnold et al.1991;Stark et al.2000).Thewaysinwhichpottersadd, subtract, andmixmaterialsinapastecanbenefitsourcingstudies.Ratherthan tracing potteryback tothegeologicalsource ofclay,thisperspectiveideallycomparesgroupsof ceramicsacrossthelandscape. Whilethisisperhaps the most common approach to pottery sourcing today, it is certainly not withoutits faults. In some cases, the potters add so much temper to the clays that natural chemicalvariation among them is erased. Using laser ablationinductively coupled plasmamassspectrometry (LAICPMS), several recent studies have successfully isolated the chem-istryoftheclayfractionwithinheavilytemperedceramics,effectivelyeliminatingthechemical effects of temper (e.g., Larson et al. 2005; Stoner and Glascock 2012). In othercases, thepreparation and mixing of materials provide characteristic information ofthepotters chemical fingerprint. Petrographers depend heavily upon this cultural variabilityamong ceramic fabrics, and their work permits a better understanding of the behavioralinputs to bulk chemical data (Bishop 1980; Day et al. 1999; Stoner et al. 2008).Today, the assertion of Bishop et al. (1982, pp. 275276) that sourcing studies areessentiallyspatialproblemsstillholdswater. Thebestmethodprovidesadetermina-tionofsourceatthefinestspatialresolutionpossible,regardlessofwhetherwerelyonnaturalorculturalvariables.Inallcases,however,post-productionuseandpost-depositionalchemicalweatheringpresentsanobstacletothisspatialreconstruction.Diagenesishasbeenanever-presentconcernofbulkchemicalanalysissincethebeginning(Sayre et al.1957),butattemptstoempiricallyevaluatepost-depositionalchemical alteration have been uncommon until recently (e.g., Buxeda i Garrigos et al.2002; Croissier 2007; Golitkoet al. 2012; Schwedt et al. 2004). As Neff (2012)Stoneretal.observes, resting ones arguments on presumptions about weathering with no supportingevidencecanleadtofalseconclusions.InresearchregionsliketheBasinofMexico,source identification relies on theformation of robust reference groups compiled fromintensive sampling of ceramics (Hodge and Neff 2005; Hodge et al. 1992, 1993; Minc1994; Nicholset al. 2002). Theregionthereforereliesmoreheavilyonidentifyingsources of pottery production rather than matching ceramics to raw clays. In the case ofXaltocan, referencegroupshavebeenformed, but thedistinctivechemistryoftheseceramics could be based on post-production use (as containers for salt or salted fish) orweathering after burial, neither of which reflects the paste recipes employed by potters.TheRoleofSaltandSalineWaterinPotteryStudiesCases of intentional use of salt, saline water, or naturally salty clays in pottery produc-tion are rare, but several examples exist. Traditional potters in Pakistan and Melanesiaadd salt to pottery pastes to increase vessel strength (Rye and Evans 1976, pp. 91, 128;see also Rye 1976). Pakistani potters also used salt in the slips of water vessels, statingthat thecoarseslipkeepsthewatercoolerbyincreasingsurfaceareaandpromotingevaporationthroughthevessel walls (RyeandEvans1976, p. 53). Matson(1971)documents a similar process for ancient Mesopotamian ceramics. Arnold (1971, pp. 2930)describeshowpottersatTicul, intheMexicanYucatn, tastethewet clayswithpreferencefor thosewithasaltytaste. Mitchem(1982) conductedexperimentsthatindicate the probable addition of seawater to prehistoric Pasco Series ceramic pastes inpeninsular Florida. Abbott (2008) conducted experiments that demonstrate the probableaddition of salt incombination with Ca-richclays and calcium carbonate to HohokamBuff wares in the Southwestern United States. Cuomo di Caprio (1991) suggested thathalite (NaCl found in common table salt) was added to medieval glazed pottery in Sicilytolightenitscolor. Bonifay(2004) describesasimilar whiteningeffect byuseofseawater to mix ceramic pastes of late Roman pottery in northern Africa. Neff (Neff, H.,2009, Plumbate technology revisited, unpublished paper) argues based on depth profil-ing using LAICPMS and scanning electron microscopy that Plumbate pottery repre-sentsthefirst applicationofsalt-glazingtechnologyintheNewWorld. Thesecasesrepresentpottersintimateknowledgeofmaterialsavailablewithintheirnaturalenvi-ronments and generations of experience in mixing them into paste recipes.A number of additional variables confound the conceptually simple task of matchingproduct toproducer. Thefiringstageof potteryproductionhas beentreatedas animportant sourceof cultural variation(e.g., BurtonandSimon1993; Carpenter andFeinman1999; Gosselain1992; Livingstone-Smith2000; Pool 2000; Sillar 2000).While we agree with the cultural significance of pottery firing choicesincluding typeof fuel, temperature, and firing technologyfiring has little immediate effect on the bulkchemistry of pottery. While volatile elements exit the ceramic matrixat differenttemperatures, theimmediatefiring-inducedchangestopotterycompositionaremorereadily observed in its mineral structure.Important tothediscussionofsodiumconcentration, diagenesis, andfiringtem-perature is a series of firing experiments (Bearat et al. 1989; Buxeda i Garrigos et al.2002; Picon 1991). Buxeda i Garrigos et al. (2002) demonstrate that in ceramics firedhigher than 9501,000 C, the Na-rich zeolite analcime forms either fromtheTakenwithaGrainofSaltdecompositionofgehleniteorthecrystallizationoffreematerialleftaftertheglassyphaseis reached(seealsoPicon1991). This process takes placeinapost-burialenvironment that introduces sodium to the ceramic. Because Xaltocan ceramics werenotconsistentlyfiredover9501,000Candtheyareelevatedin bothsodiumandpotassium,BuxedaiGarrigossfiringexperimentsdonotdirectlyexplainthechem-istryofXaltocanceramics.Theydo,however,highlighttheimportanceofconsider-ing post-depositional processes before uncritically accepting our high-sodiumchemicalgroupsasvalid. Theseexperimentsareparticularlyimportantwhenalkalior alkaline earthmetals exhibit chemical variationwithina sample, as these areamongthemost mobileelementsinpotteryperturbation(e.g., Golitkoetal. 2012;BuxedaiGarrigos1999;Buxeda et al.2002;SchwedtandMommsen2004).Amorecommonsource ofpost-productioncontaminationat Xaltocanmaysimplyinvolvethephysicalsaturationofpotterywithsalinewaterand/orsaltduringuseandburial. Salt was produced and used in intensive household production of salted fish andperhaps dyed cloth (Millhauser 2012), so naturally ceramics would be used to store it.Cooking would involve saline water, which may saturate cook wares with sodium. Morepervasive chemical alterations take place inthe ground. Potterywouldrepeatedlysaturateanddehydrate,leavingbehindsaltcrystalsasthelakeswaterlevelsroseandfell annually. The elevation of sodium and potassium due to this physical process mayeasily be reversed. As detailed in the preservation handbook published by theUnderwaterArchaeologyprogramatTexasA&MUniversity,thebasicprocedureforremoving soluble salts, such as those that form in and around Lake Xaltocan, is soakingin water(Hamilton1997,p. 26). This suggests that if post-depositionalcontaminationwerethereasonfor theelevatedsodiumandpotassiumconcentrations, it couldbereversed by soaking in a deionized (DI) water bath (tested below).This brief discussion establishes several alternative scenarios for the high levels ofsodium and potassium in Xaltocan ceramics. Variables useful for chemically tracing itback to the production source include selection of salty clays, addition of salt or salinewater tothematrix, or additionof temper highinthese elements. Variables thatconfound identification of production source are any variety of post-productionchemicalalteration.Thesealternativescenariosaretestedbelowfirstthroughexam-inationoftheextensivegeochemicaldatabaseon-handattheUniversityofMissouriResearch Reactor (MURR) and second through experimentation that permits a betterunderstandingofthemobilityofsodiumandpotassiuminceramics.XaltocanandItsEnvironsPrior to the arrival of Spanish in 1519, the lake systemin the Basin of Mexico hosted oneofthedensestPrehispanicsettlementsinMesoamerica(Sanders et al.1979)(Fig.1).Lake Xaltocan wasoneof thenorthernmost lakesin the region,just southandeast ofLake Zumpango. The lake was mostly drained by the turn of the twentieth century, andthesubsequentdiversionoftheCuauhtitlanRivertoMexicoCityhasteneditsdisap-pearance (see Archiga Crdoba 2004; Brumfiel and Hodge 1996, p. 420; Mathes 1970;Montoya Riviero 1999; Palerm 1973). During its Prehispanic occupation, Xaltocan wasan island within the lake with only a causeway connecting it to the western outer shore.The island was human-made beginning in the mid-ninth century A.D. (Brumfiel 2005b;Stoneretal.Brumfiel andHodge1996, p. 423; Fredericket al. 2005). It supportedsettlementsthrough almost the entire Postclassic period (9501521 CE). Isolated lakebed mounds,also hosting Prehispanic settlements, surrounded the island (Chimonas 2005). In 1521,Fig. 1 Map of the Basin of Mexico region showing sites mentioned in the text. The shoreline for the lakesisbasedoninspectionofa30 resolutiondigital elevationmodulefortheBasinofMexico. Thislikelyrepresentstheaveragewaterlevelsthroughout theyear.Dottedlinesdepict thelowwaterlevelsfortheindividuallakes.HighwaterlevelsinLakeXaltocanmayhaveextendedtothesiteofCuauhtitlanTakenwithaGrainofSalttheSpanish attackedandburned Xaltocan totheground(Corts1970,p. 118;Hassig2006, pp. 142143). The local population recovered and continued to inhabit the islandthrough the colonial period and today (Rodrguez-Alegra 2008a, b, 2010).Xaltocan was the capital of an autonomous Otom kingdom prior to its conquest byCuauhtitlan, partoftheTepanecstate,in1395A.D. (Carrasco1999, p. 104;Gibson1964, p. 10). According to documentary evidence, Xaltocan lay abandoned for a periodof about 30 years before incorporation into the Aztec Triple Alliance in 1428 (Brumfiel1991), though the archaeological evidence does not support any state-wide occupationalhiatus (Overholtzer 2012, pp. 133134). Xaltocans political systemhad alreadydisintegrated some decades prior to the formation of the Aztec empire. The Aztecs didnot reestablish a local dynastic line or kingly seat. Xaltocan was likely overseen insteadbytlacateuctli, or governingnobles (Hicks 1994). TheCodexMendozadescribesXaltocanas asubject that owedmilitaryobligationstotheTripleAlliance, but itspopulation also owed tribute to Texcoco and Tenochtitlan (Carrasco 1999, p. 99).Before the Aztec conquest, the island hosted a regional market and was a center foreconomicproductionandexchange(Brumfiel1991).Xaltocanalsoreceivedtributegifts frommanysettlements inthe northernBasinof Mexico(Brumfiel 1991,pp.181183).Blanton(1996,p.65)projectsamarketterritoryforXaltocanduringthe Early Aztec period that covers Lake Xaltocan and Lake Zumpango. Previous NAAindicatesthatplainorangeanddecoratedceramicsproducedatornearTenochtitlan,Chalco, Cuauhtitlan, andtheTeotihuacanValleywerefoundat Xaltocan(Garraty2006,p.136;HodgeandNeff2005;Peters2002).ThepatternsofBlack-on-Orangepotteryconsumptionat Xaltocanchangeover time. AztecI sherdswereproducedlocally at Xaltocan. Major production of Aztec II sherds shifted to Cuauhtitlan after itsconquest of Xaltocan. Aztec III ceramics found at Xaltocan were produced atTenochtitlan, theTenayucaregioninthewesternBasin, theTexcocoregionintheeasternBasin, andtheChalcoregioninthesoutheast Basin(Brumfiel andHodge1996,pp.431432;HodgeandNeff2005,Table13.1).Additionally,obsidianfromavarietyof sources has beenfoundat Xaltocan, includingobsidianfromPachuca,Otumba, Tulancingo, Ucareo, Zacualtipan, Oyameles-Zaragoza, andSanJuandelosArcos (Millhauser et al. 2011).Ethnohistoric documentation and economic models support the possible existenceof amarket at Xaltocanearlyinitshistoryof occupation, withdramaticshiftsinproduction, exchange, and consumption over time and relative to Xaltocans politicalhistory (Blanton 1996; Brumfiel 2005b). After its incorporation into the AztecEmpire,anymarketthatXaltocanmighthavehostedpreviouslylikelydecreasedinitsregional importance(Blanton1996, pp. 6869; HodgeandNeff2005; BrumfielandHodge1996). After theSpanishconquest, Xaltocamecas(peopleof Xaltocan)participatedactivelyinthecolonial market, anditseconomyflourishedforat leastsometimeintothecolonial period(Gibson1964, p. 366; Rodrguez-Alegraetal.,under review). Xaltocan and Mexico City were linked by one of the main roads in theBasin(Gibson1964,p.364).Duringthe Late Postclassic, Xaltocanproducedtextiles, reedmats, maize, andpossibly lime (Gibson 1964, p. 336; Morehart and Eisenberg 2010). After incorporationintotheAztecEmpire, textileproductionbecamemoreimportant, probablytomeettribute demands (Brumfiel 2005a, pp. 361363; Brumfiel and Hodge 1996, p. 429). Saltproduction also took place at Xaltocan, as evidenced by the presence of Texcoco FabricStoneretal.Marked salt basins (Brumfiel2005a,p. 363; Brumfiel andHodge 1996,pp.428429;Millhauser2012).Saltwasprobablynotboileddownfromthesalinewaters.Instead,tequesquite1a combination of NaCl, Na2CO3, Na2SO4, and KClwas extracted fromsalt impregnated soils along the edges and in the shallows of the lake. Salt minerals weremore concentrated in these soils, and they surfaced as crystalline deposits during the dryseason (Flores 1918; Gibson 1964, p. 338). Little direct evidence suggests the presenceofpotteryproductionatXaltocan,butthedistinctiveceramicchemistryfortheover-whelming majority of sherds sampled there presents the possibility of local production(see Hodge and Neff 2005; Millhauser 2012; Nichols et al. 2002; Overholtzer and Stoner2011; Rodrguez-Alegraet al., under review). Post-depositional contaminationmaycompromise this proposition.PreviousNAA WorkatandAroundXaltocanTodate, over 6,000neutronactivationassays have beenconductedonBasinofMexicoceramics at MURR. The studyarea is aninterior drainage basinwheresediments fromvolcanic parent materials draininwardandcomingle aroundthelakesattheheartoftheBasin.Anyvariationinhowtheyselectedandmixedthesematerialswillfacilitateprovenanceinvestigation.Research at the site of Xaltocan over the past decade has led to the examination of 651ceramic and clay specimens by NAA (Brumfiel and Hodge 1996; Crider 2011; De Lucia2011; Garraty 2006, 2007; Hodge and Neff 2005; Nichols et al. 2002; Overholtzer andStoner 2011; Rodrguez-Alegraet al., under review) (Table1). JohnMillhauser hasadditionally submitted 175 ceramics and clays from salt-producing sites in southern LakeXaltocan, including San Bartolom Salinas. These analyses have led to the identificationoftwochemical groups, Xaltocan1aandXaltocan1b, that differfromtheotherfivemajorBasinofMexicoreferencegroups(Figs. 2, 3, and4). TheprimarydifferencesbetweentheXaltocangroupsandotherBasinof Mexicoceramicsareelevatedsodiumandpotassiumvalues.Theyarealsodepletedinmanytransitionmetalsandlanthanideserieselements, whichcouldarisethroughdilution. Bothsubgroupsdisplaythesamechemical patterns, but Xaltocan 1b is more pronounced in its chemical divergence fromthe other Basin ceramics. All pottery that composes these reference groups was recoveredfrom the site of Xaltocan with a minority coming from San Bartolom Salinas, so we areconfident that they reflect the local paste recipe.TheenrichmentofXaltocanceramicsinsodiumandpotassiumclearlyhasvariablepotentialsourcesinthiscase.IfpottersatXaltocanchosesalineclay,temper,water,oraddedsalt tothepaste(all of whichareavailablelocally), theelevatedsodiumandpotassiumvaluesfor their ceramicsreflect their chemical fingerprint. Thisdistinctionmay not have been visible to the producer or consumer, but the pottery production processat Xaltocancanbe differentiatedfromother sites throughchemical characterizationtechniques. Whilethismaynot constituteanovert expressionof identity, thesesaltyceramics do nevertheless allow archaeologists to identify them through their chemistry.1From theNahuatlterm tequisquitlwhichreferstothesaltycrusts deposited along saline lakerimswhenthewaterlevellowersduetodessication(Parsons2001,p.146)TakenwithaGrainofSaltTable1 ElementalmeansandrelativestandarddeviationsforsevenmajorBasinofMexicoceramicchemicalreferencegroupsChalco(n=324) Cuauhtitlan(n=110) Texcoco(n=194) Otumba(n=305) Tenochtitlan(n=343) Xaltocan1a(n=80) Xaltocan1b(n=135)Mean RSD Mean RSD Mean RSD Mean RSD Mean RSD Mean RSD %DifferenceaMean RSD %DifferenceaNa(%) 1.51 11.4 1.39 19.4 1.39 9.2 1.43 15.5 1.43 15.3 2.12 11.4 48.8 2.11 9.9 47.4Al(%) 9.69 5.0 10.4 4.0 10.0 3.3 9.46 4.8 10.3 4.7 9.79 6.0 1.7 9.23 6.1 7.4K(%) 1.20 28.4 1.01 25.1 0.99 14.0 1.18 22.4 1.03 21.5 1.61 17.3 48.8 1.92 24.9 77.4Ca(%) 2.52 11.6 1.95 20.7 2.21 12.8 2.34 13.4 2.03 19.1 1.99 24.9 9.7 2.40 11.5 8.4Sc(ppm) 14.2 7.9 13.1 5.3 13.4 4.3 12.1 5.4 13.4 5.8 11.7 5.7 11.8 10.4 7.3 21.3Ti(%) 0.51 10.5 0.52 13.1 0.50 8.9 0.49 11.0 0.49 10.2 0.50 15.1 0.4 0.42 13.2 16.3V(ppm) 91.3 13.9 91.8 11.4 87.1 9.8 77.4 12.9 90.5 10.4 83.4 15.6 4.8 56.5 12.3 35.5Cr(ppm) 115 11.5 77.0 6.2 101 6.4 80.0 9.5 89.0 10.0 72.0 5.1 22.4 59.0 5.4 36.3Mn(ppm) 760 13.4 894 26.7 819 9.7 772 15.9 803 27.6 602 33.5 25.6 592 19.1 26.9Fe(%) 4.22 7.5 3.68 6.8 4.00 4.9 3.59 5.3 3.82 9.0 3.37 9.5 12.7 2.96 8.8 23.4Co(ppm) 17.0 9.6 16.8 13.4 16.7 5.5 14.5 9.6 16.4 13.8 12.9 16.6 20.6 10.7 12.4 34.6Zn(ppm) 75.2 10.4 76.8 16.1 71.8 14.1 69.4 15.9 76.4 14.1 70.9 61.6 4.1 60.3 9.9 18.4As(ppm) 2.30 29.2 2.11 30.7 2.39 33.7 2.06 29.4 2.51 41.0 2.79 48.1 22.8 1.84 45.5 19.2Rb(ppm) 50.0 10.0 49.7 11.9 60.9 7.8 52.0 12.0 52.1 11.9 52.1 13.0 1.6 61.6 10.4 16.4Sr(ppm) 433 14.7 347 23.0 374 14.3 426 17.8 372 22.2 469 35.9 20.2 466 17.3 19.3Zr(ppm) 150 13.1 170 11.6 154 12.0 133 14.2 146 12.1 147 15.3 2.6 119 14.3 20.8Sb(ppm) 0.26 14.7 0.36 15.3 0.33 15.7 0.29 17.5 0.32 18.9 0.28 35.0 9.5 0.25 63.6 21.0Cs(ppm) 2.77 13.3 5.35 18.7 3.66 11.1 2.77 18.4 3.82 15.7 3.64 21.6 1.0 2.85 14.9 22.5Ba(ppm) 590 32.0 612 43.7 641 32.6 923 44.7 585 33.2 532 58.4 20.6 426 52.0 36.4La(ppm) 24.2 8.1 29.7 12.7 25.8 6.5 22.3 10.3 25.7 9.2 21.6 15.9 15.5 20.4 15.4 20.1Ce(ppm) 50.9 7.2 59.6 12.2 54.7 5.2 46.7 10.5 53.2 8.6 42.1 17.3 20.5 41.3 17.7 22.1Nd(ppm) 25.9 12.0 30.4 14.8 27.1 9.9 22.8 13.1 27.5 11.5 21.9 19.5 18.2 20.6 19.0 22.9Sm(ppm) 5.39 8.5 6.44 11.8 5.63 6.8 4.98 9.7 5.72 9.0 4.64 15.2 17.5 4.43 15.7 21.4Eu(ppm) 1.49 6.8 1.69 8.6 1.51 4.4 1.33 6.5 1.55 6.9 1.31 11.7 13.8 1.22 11.9 19.4Stoneretal.Table1 (continued)Chalco(n=324) Cuauhtitlan(n=110) Texcoco(n=194) Otumba(n=305) Tenochtitlan(n=343) Xaltocan1a(n=80) Xaltocan1b(n=135)Mean RSD Mean RSD Mean RSD Mean RSD Mean RSD Mean RSD %DifferenceaMean RSD %DifferenceaDy(ppm) 3.63 13.2 4.63 13.4 4.00 9.4 3.63 12.2 3.96 11.4 3.00 20.8 24.5 3.17 18.2 20.0Yb(ppm) 2.03 11.2 2.55 10.1 2.27 8.6 2.00 12.2 2.17 9.8 1.69 18.8 23.5 1.70 18.7 22.9Lu(ppm) 0.28 15.8 0.36 10.2 0.32 9.0 0.28 10.7 0.31 10.6 0.25 18.8 20.8 0.24 18.2 21.8Hf(ppm) 5.66 6.2 6.49 7.1 5.80 4.4 5.23 7.2 5.75 5.5 5.74 10.0 0.7 4.64 8.4 19.7Ta(ppm) 0.74 9.1 0.75 7.9 0.79 6.0 0.72 8.8 0.68 7.7 0.66 10.1 10.7 0.65 9.0 11.3Th(ppm) 5.95 7.6 7.52 12.3 6.73 6.0 6.14 11.6 6.20 11.3 5.60 15.7 13.9 5.72 13.6 12.1U(ppm) 1.26 24.7 2.59 21.2 1.60 23.2 1.44 29.1 1.86 35.4 1.53 40.4 12.7 1.45 24.6 17.3ThepercentdifferenceforeachelementiscalculatedfromthevalueofXaltocangroupscomparedtotheaveragefortheBasinofMexicowithXaltocanremovedBoldtextdenoteselementsinwhichXaltocanceramicsaresignificantlyhighercomparedtootherBasinofMexicogroups,whereasitalicsdenoteelementsinwhichXaltocanceramicsaresignificantlyloweraPercentdifferencewascalculatedasthepercentdifferencecomparedtotheaveragefortheBasinofMexicoTakenwithaGrainofSaltThesenewdataprovidethemeanstobetter discerntheroleof Xaltocanintheregional productionand exchange system. To date, researchers have viewed thepositionofXaltocanintheregionalpotteryeconomyprimarilyfromtheperspectiveof consumer (Brumfiel and Hodge 1996; Hodge and Neff 2005; Nichols et al. 2002).IftheXaltocanchemicalgroupsaretheresultofchoicesmadebylocalpotters,andnot post-productionchemical alteration, researcherscanbetter figurethesitespo-tentialroleasproducerandexporterofpottery.LevelsofSodiumandPotassiumintheClayandTemperFractionoftheCeramicPasteIn an effort to determine which component of the paste contributes the high values of theelements of interest, we conducted a small study using LAICPMS to target the clayand temper portions of a sample of the Xaltocan 1a and 1b chemical groups (n=14) andonespecimenpreviouslyassignedtotheCuauhtitlanreferencegroup. LaserablationICPMS is useful for generating chemical data on different components of the ceramicpaste to determine which component contributes more sodium and potassium. Methodsfor LAICPMS are detailed elsewhere (Stoner and Glascock 2012; see alsoDussubieux et al. 2007; Gratuze et al. 2001; Speakman and Neff 2005).We determined that the aplastic fraction (temper and natural inclusions) ofXaltocanceramics contributedmoresodiumbut less potassiumtothebulkNAAchemical signaturethantheclayfraction(Fig. 5; Table2). AplasticsaregenerallyFig.2 ScatterplotofsevenmajorBasinofMexicoreference groupsprojectedondiscriminantfunction1anddiscriminantfunction2. Ellipsesrepresent90%confidenceintervalsofgroupmembershipStoneretal.lowerinpotassium, suggestingthatfelsicrocks(suchthatcontainK-feldspars)werenot present in the ceramic paste. Volcanic rocks found in the region, such as andesites,dacites,and basalts, are typically higherin sodium (and/or calcium) thanlocal clays.These rocks are widely available, and most ceramics in the Basin possess sodium-richaplastic materials. The higher concentration of sodiumin the aplastic fraction,therefore, doesnot necessarilyexplaintheuniquechemistryofXaltocanceramics.ComparisonoftheXaltocanclayfractiontorecent analysesoftheclayfractionofceramics fromTeotihuacan (conducted by the lead author), a non-saline post-depositionalenvironment,showsthattheformerisabout90%higherinsodiumonaverage (see Table 2). The clay fraction of Xaltocan ceramics could become elevatedinsodiumandpotassiumthroughanumberofprocesses.Themostobviousofthesearethat thenatural claysusedtoproducethepotterywerehighintheseelements.Alternatively,saltorsalinewatermayhavebeenaddedtothepaste.PatterningofSodiumandPotassiuminClaysAcrosstheBasinofMexicoThree clay samples were collected at Xaltocan by Liz Brumfiel (analysis undertaken in1999), and six were submitted by Mary Hodge froma satellite mound to the south of themain island (analysesundertaken in 1994). Surprisingly,only oneof theclay samplescollectedbyBrumfiel andnoneofthosecollectedbyHodgepossessedhigher-than-averagelevelsofsodiumorpotassium(Figs. 6, 7, and8). Theydid, however, yieldFig.3 ScatterplotofthesevenmajorBasinofMexicoreferencegroupsprojectedonaxesofpercentNaandK.DatapointsforXaltocan1aandXaltocan1bareaddedtodemonstratetheirrelativelyhighvaluesamongthesetwoelements. Ellipsesrepresent90%confidenceintervalsofgroupmembershipTakenwithaGrainofSaltvaluesfortransitionmetalsandrareearthelementsintherangewithceramicsfromXaltocan (see Fig. 8). Clays were also high in calcium, which is not a feature of Xaltocanceramics. In short, only one of the three clay specimens analyzed from Xaltocan couldhave produced the ceramics found on the site. These were opportunistic samples takenfromexcavation units, a sampling strategy that rarely identifies the location of clays usedby potters. These few data do suggest, though, that there is considerable variation in thesalinity of post-depositional contexts at the site.Fig. 4 Scatterplot of thefivemajor Basinof Mexicoreferencegroupsprojectedonaxes of partspermillionCrandLa.DatapointsforXaltocan1aandXaltocan1bareaddedtodemonstratetheirrelativelylowvaluesamong thesetwoelements. Ellipsesrepresent 90%confidence intervalsofgroupmembershipFig. 5 LAICPMS data for the clay and temper fractions of a sample (n=14) of Xaltocan 1a and Xaltocan1b ceramics that display the full range of Na and K values found in the reference groups. Clay and tempercompositions are expressed as ratios to the bulk NAA signature, which retains the constant value of 1 acrossthechart. Positiveornegativedeviationsfromthebaselineindicatethat theclayfractionofthepotterycontributedmoreorless(respectively)ofthatelementtothebulkcompositionStoneretal.Clay, adobe, andpossibleovenlinings sampledbyMillhauser at SanBartolomSalinas are calledout inthe same figures forcomparison.SanBartolom Salinas was asalt production site that was either a large island near the edge of the lake or a peninsula onthewesternshoreline(Millhauser2012). SedimentsfromSanBartolomSalinaswererelatively high in sodiumand potassium. In fact, the rawmaterials sampled there are goodchemical matches for the clay fraction of Xaltocan ceramics. San Bartolom Salinas maynot be the precise location that Xaltocan potters collected their clays, but many sedimentsalong outer shoreline of Lake Xaltocan may possess similar soil chemistry.Hundreds ofclay sampleshave been collectedwithinthebroaderBasin of Mexico(Fig.9).ClaysampleslocatedclosertoLakeXaltocangenerallypossesshighercon-centrations of alkali andalkalineearthelements (e.g., barium, calcium, potassium,sodium, strontium) and relatively low concentrations of transition metal and rare earthelements. In Fig. 9, darker colors on the chloropleth map represent high values for theTable 2 Comparison of the average composition of the clay and temper fractions of a sample of Xaltocanpottery,determinedbyLAICPMS,tothebulkchemistry,determinedbyNAA,ofthesamesamplesXaltocan(n=14) Teotihuacan(n=36)Averageclay 15,828.5 10,505.61Averagetemper 20,356.29 naAveragebulk 28,441.2 naThe clay fraction for a sample of Teotihuacan ceramics were also analyzed via LAICPMS as a referenceFig.6 LoglogscatterplotofclayssampledfromXaltocanandSanBartolomeSalinasandelsewhereintheBasinofMexicoprojectedonaxesofHf(inpartspermillion)andNa(inpercent)TakenwithaGrainofSaltcombined concentrations of sodium and potassium (in parts per million).2These mobileelementserodeoutoftheparentrockinthemountainsandwatereventuallydepositsthem into the lake.ThesumofalldatasuggeststhatXaltocanpottersgatheredclaysfromtheoutershoreline of the lake. Because these sediments naturally possess salts, the addition ofsalt tothepasterecipewouldhavebeenredundant andunlikely. It still remainsapossibilitythat thepastesweremixedwithsalinewater, but thewatersthemselveswouldhavepossessedlowersalinitythanthesoils.DistributionofHighSodiumandPotassiumCeramicsintheRegionNot all ceramics at Xaltocan are uniformly elevated in sodium and potassium. About21 % of all ceramics collected on the site of Xaltocanhave sodium valuesbelow themean for all Basin of Mexico ceramics. For potassium, 38 %of the sample is belowtheBasin of Mexico mean. No meaningful difference of sodium/potassium concentrationsoccurs between surface and excavation collections (Table 3). Low sodium/potassiumpotslikelyarrivedatXaltocanthroughtrade(seeBrumfielandHodge1996;Hodgeand Neff 2005; Overholtzer and Stoner 2011; Rodrguez-Alegra et al., under review).2ItmustbekeptinmindthatrawclaysamplingacrosstheBasinofMexicoishighlylocalizedandnotrepresentative of the whole region. Interpolated values are based on a Gaussian function built into ArcGIS10.Interpolationsclosetoactualdatapointsaremostreliable.Fig.7 LoglogscatterplotofclayssampledfromXaltocanandSanBartolomeSalinasandelsewhereintheBasinofMexicoprojectedonaxesofK(inpercent)andCa(inpercent)Stoneretal.Majolica (tin-enameled pottery) produced in Mexico City during the Colonial periodisperhapsthemost definitiveimport foundat Xaltocan. TheseColonial ceramicsdisplay below average sodium and potassium levels compared to the Basin of Mexicoandmatchpreciselyother ceramics producedinMexicoCity(Rodrguez-Alegraet al., under review).Non-saline post-depositional environments inother parts of the Basinpossessceramics that are chemical matches for production at Xaltocan. We filtered theBasin of Mexico sample according to the chemical ranges exhibited by the nine mostimportant elements (Ca, Co, Cr, Eu, Hf, K, La, Na, andRb) for inclusionintheXaltocan groups. Over 200 ceramics fromnon-saline depositional environmentsdisplaysimilar chemistrytoXaltocanceramics. At aminimum, thissuggests thatceramics of this composition exist in the Basin in contexts where diagenetic elevationofsodium/potassiumwouldbeunlikely.ThepossibilitythatXaltocan-likeceramicsarrivedatthesesitesthroughexchangeisexploredbelow.ExperimentingwithSaltTheabovediscussionseverelyweakensthepotential thatdiagenesisaloneexplainsthe high sodium and potassium values for Xaltocan ceramics. Since we are attemptingto establish Xaltocan reference groups that will be used to source additional ceramicsFig.8 LoglogscatterplotofclayssampledfromXaltocanandSanBartolomeSalinasandelsewhereintheBasinofMexicoprojectedonaxesofEu(inpartspermillion)andCr(inpartspermillion)TakenwithaGrainofSaltinthefuture,wecannotcomfortablyruleoutdiagenesiscompletely.Webelievethatexperimentationcanhelpus further evaluatethealternatives bydetermininghowsodiumbehaveswithinunfiredclaysandwithinfiredpottery.Fig. 9 ChloroplethmapofrawclaychemistryintheBasinofMexico. Valuesrepresenttheadditionofsodium and potassium concentrations in parts per million. Interpolation of chloropleth values is based on aGaussian function based on data points where raw clays have been collected. Interpolated values closest todatapointsarethemostreliableStoneretal.Theexperiment involveddopingtwodifferent clays withwaters of differingsalinity (Fig. 10). Chemical data for each category within the experiment aregeneratedbyshort-irradiationNAAtoyieldconcentrations for nine short-livedelements:aluminum(Al),barium(Ba),calcium(Ca),dysprosium(Dy),potassium(K), manganese(Mn), sodium(Na), titanium(Ti), andvanadium(V) (Glascock1992; Neff 2000). WealsoreanalyzedviaNAAaset of 21ceramics that wereassigned to the Xaltocan 1a and Xaltocan 1b chemical groups after soaking them inDIwaterfor48hwith4hinasonicationchamberat50C.ClaysTwoclays werepreparedfor theexperiment. OhioRed(RedArt 200mesh) clayminedinOhiopossessescharacteristicsofverylowwaterabsorptionrateandhighironcontent. ThelowwaterabsorptionresultsfromthekaoliniteclaymineralsthatcomposeOhioRed. Theotherclaywasa3:2mixtureofOhioRedandbentonite.Bentoniteweathersfromvolcanicrocks(particularlybasalt)andvolcanicash,anditis likely that the clays found in the Basin of Mexico comprise at least some bentoniteclayminerals.3Bentonitehashighwaterabsorptionratesandtendstoswellgreatly.Mixing Ohio Red and bentonite, therefore, increases the absorption rate of the former.Swellingisgenerallybadfor potteryproduction, but it isimportant tocontrol fordifferent water absorptionratesinthisexperiment aswater isthemediumfor theintroductionofsodiumandpotassiumions.TemperWe tempered the clays with play sand to enhance its workability. This is a multimineralsandavailableat most homesupplystores. Visual inspectionsuggests that thesandcontains primarily quartz, but feldspar, and an iron-rich mineral are also present.Aplastics drag through the clay as vessel walls are smoothed, leaving behind small voidsTable3 ComparisonofpotterysherdsrecoveredfromsurfaceandexcavationatXaltocanSamplesize Excavation Surfacen=609 n=38NaMean(ppm) 18,240 18,712SD(ppm) 4,178 4,366RSD 23 23Range 2,053.431,387.8 6,197.823,120.2KMean(ppm) 14,959 14,924SD(ppm) 5,346 3,666RSD 36 25Range(ppm) 3,183.841,558.9 5,818.022,299.43ClaysoilsintheBasinaremostlyvertisolandfeozem,bothofwhichconsistofhighlyabsorptiveclayminerals.TakenwithaGrainofSaltorpores.4Voidswillfacilitatetheformationofsodiumorpotassiumcrystalswithinthepottery (e.g., Na2CO3, NaCl, K2CO3, KCl). The sand temper used in this experiment wassievedsothatonlygrainsbetween0.50and1.00mmwereaddedtotheclay,thoughaminor percentage of both larger and smaller grains were included. Ohio Red and bentoniteclays were tempered with 1015 % play sand (measured by dry weight of both materials).Average elemental concentrations for two replicates of the sand are shown in Table 4. Theoverwhelming majority of the temper is composed of Si as the major mineral componentis quartz.WaterWe prepared both clays with deionized water and then desiccated them in an oven at100C.Sampleswerethenrehydratedwithfivecategoriesofwater:(1)DIwater,(2) 1.5 % saline solution, (3) 3.0 % saline solution, (4) 9.0 % saline solution, and (5)Fig.10 Schematicofthesaltexperiment4Voidsare alsoformed bythebubbling ofgases (suchas CO2)thatget caughtwithin theglassy phaseoftheceramic.Stoneretal.27.0 % saline solution. The density of the experimental water was measured using ahydrometer.ThesalinityofLakeXaltocanvariedwiththerisingandfallingwaterlevelsof thelake(GalvnMorenocitedinParsons 2001, pp. 144146). Duringthe dry months, the water levels would be lower and the salinity higher. Toreproduce the seasonably variable saline environment of Lake Xaltocan, salinewater was madebyaddingamixtureof 50%Na2CO3, 40%NaCl, and10%K2CO3(bydryweight) todeionizedwater indifferent proportions. Theratioofdifferent mineral salt additions derives fromdata for the average tequesquitecomposition published for Lake Texcoco (Galvn Moreno 1945, p. 12, summarized inParsons 2001, p. 146).5Rehydrationofclayswasconductedbyaddingastandardamount ofwatertoasample ofthe desiccatedclays. Ingeneral, weaddedthe waterslowlyuntil thepointthat theclaysbecameplasticenoughtoformpottery. Claysmixedwithbentoniteabsorbedmorewater thantheOhioRed. Moresodiumandpotassiumions were,therefore,introducedintothepaste.FiringOncerehydratedwithwatersofdifferentsalinity,wecreatedasinglecoilforeachclayandsmootheditoutasifformingavesselwall.Thisproducedslabsbetween8and10mmthickness.Theslabswerethenfiredtothreetemperatures:700,900,and 1,050C. Openfiring methods generallyachieve sustainedtemperaturesmuchlower than1,000C(Rice2005, p. 82). Theuseof kilns couldachievehighertemperatures,andthepotterhasmorecontroloveratmosphere, thedistributionofheat acrossspace, andtherateoftemperatureincreaseanddecrease(e.g., Arnold1985, p. 213; Arnold1991; Pool1990;Rice2005, p. 153; Rye1981, p. 98). Rye(1981, p. 98)notesthat simpleupdraft kilnscanreachmaximumtemperaturesof1,000 C. In practice, however, temperatures between open and kiln firings overlapconsiderably (see P. Arnold 1991; Gosselain 1992, p. 224; Livingstone-Smith 2000;Pool2000).5ItshouldbenotedthatK2CO3wassubstitutedforKClduetocostandavailability.Table4 Elementalmeans(inpartspermillion)andstandardde-viationsfortworeplicationsofplaysandtemperElement Mean(n=2) SD(n=2) RSDAl 3,834 684 17.9Ba 102 0.11 0.1Ca 1,801 55.0 3.1Dy 0.68 0.03 4.2K 530 36.2 6.8Mn 125 3.99 3.2Na 136 9.08 6.7Ti 1,344 62.2 4.6V 31.4 1.43 4.6TakenwithaGrainofSaltPost-firingTreatmentsTheslabswerebrokenintofragments(referredtohereafterastiles)andseveral ofthem underwent post-firing treatments. Control tiles fired without saline solution (i.e.,usingonlydeionizedwater)weresoakedinthefourdifferent salinesolutionsafterfiring. The tiles were soaked for 48 h and sonicated (placed in a water-filled chamberandbombardedwithsoundwaves)for4hat 50Ctoensurepermeationofsalinesolution. Longacre et al. (2000) have demonstrated that ceramics absorb nearly all thewater theycanholdwithinthe first 2minof soaking, sowe are confident thatthoroughsaturationoccurred.Thetilesweredriedinanovenat100Cthreetimesduringthiscycletoencouragecrystalformation.Afterthethirddrying, halfofthesetileswerepreparedforNAA. Theotherhalfwere subjected to a rinse in deionized water to determine how readily the sodium andpotassiumionsaddedafterfiringcouldbeliberated. Another set oftilesthat wereprepared and fired with saline water were soaked in deionized water to see if sodiumand potassium could leach out of the fired ceramic. The leaching experiment followedthe same soaking and sonication parameters mentioned above. Once dried, these tileswerealsopreparedforNAA.A sample of 21 pottery sherds collected from Xaltocan (belonging to the Xaltocan1a and 1b chemical groups) was reanalyzed after attempting to leach them in DI waterusingthesamesonicationprocesses. If thesesherdswereelevatedinsodiumandpotassiumdue to soaking in a saline post-depositional environment, the rinsingprocedureshouldsignificantlylowertheconcentrationoftheseelementsasseeninthepost-rinsingexperimental analysis describedbelow. Mineral crystals that mayforminXaltocanceramicswouldbethosefoundintequesquite6(GalvnMoreno1945, p. 12, summarized in Parsons 2001, p. 146). All of these minerals are soluble inwaterasfollows:35.9g/100mlforNaCl,21.6g/100mlforNa2CO3,112g/100mlforK2CO3,and34.4g/100mlforKClat20C.Ifpost-depositionalcontaminationcaused the formation of any of these minerals, leaching in DI water at 50 C with theadded use of sonication should significantly reduce concentrations of the sodium andpotassium.ExperimentResultsBeforediscussingthechemical data, aninterestingrelationshipwas observedbe-tween the addition of salt and the firing of the pottery. The 27 % saline tiles emergedfromthefurnacewithaglazedsurfaceafter firingto1,050C, but not at 900or700 C. Of course, salt glazing was a common practice for the production of historicceramics,andsomepottersstillusethistechniquetoday.Thesaltlowersthevitrifi-cationtemperatureof silicabydisruptingitscrystal network(Rice2005[original1987], p. 99). These are the onlytiles that displayevidence of vitrificationandoverfiring(aglassytexturewithlargeporeswheregasbubblesformedwithinthepaste).Itthereforeappearsthataddingsaltinhighpercentagesreducedthemelting6Though,itshouldbenotedthat tequesquitemineralsformprimarilyonthesurface.Stoneretal.point ofmineralsintheclayandtemper(seeRyeandEvans1976, pp. 4243andArnold1985,pp.2528forsimilarconclusions).Several significant patternsoccurwithintheexperimental dataset (Fig. 11). Thehigher the salinity of water used to prepare the clays, the higher the resulting sodiumconcentrations inthefiredtiles (Fig. 11ad). Surprisingly, this patternis strongeramongtheOhioRedclays,whichhavelowerabsorptionratesthanthemixedclays.Amongtheceramics subjectedtothepost-firingsalinesoak, bothclays absorbedabout equivalent amountsof sodium(Fig. 11e, f). OhioRedclaymust havemorepotential sodium bonding sites than bentonite. After firing, most bonding sites wouldhavebeenclosedtonewbonds.Fig.11 Scatterplotsshowingtheresultsofthesalinewaterexperiments.The x-axesarealinearexpres-sions of the percent salinity used to mix/soak the clays. The y-axes are linear expressions of Na (in parts permillion).Pleasenotethelargerscaleofthe y-axisinaandcTakenwithaGrainofSaltThe effects of firing temperature are inconsistent. Among the pre-firing-doped tiles(Fig. 11a, b), theOhioRedappears todisplayadefinitetrendof higher sodiumretentionwithlower firingtemperatures. However, theglazeonthesurfaceof theOhio Red ceramics fired to 1,050 C was removed prior to analysis. We analyzed onetile with the glaze intact, and it yielded sodium concentrations about 50 % higher thanthosewiththeglazeremoved. It thereforeappears that muchof thesodiumwasexpelled to the surface of the ceramic during drying and as firing took place, more sowiththehigherfiringtemperatures.Amongthepost-firing-saline-soakassays (Fig. 11e, f), firingtemperaturedoeshave a clear effect on sodium absorption. The lower-fired tiles accepted more sodium,whichprobablyresultsfromtheirlowerdegreeofvitrificationandhigherporosity.Higherfiredtileswerelessporousduetothehigherlevelofvitrification.The leaching experiments also produce clear patterns. Among the pre-firing-dopedtiles, leachingcausedonlyaslightdecreaseinsodiumconcentrations(Fig. 11c, d).Thehigherthesalinityoftheoriginaldoping,themoresodiumthatwaslostduringleaching.Again,thisreflectsthefactthatmoresodiumionswereintroducedbythehigher salinitysolutionsthancouldbeabsorbedbythemolecular structureof theclays. The extra sodiumions were likelytrappedwithinthe glassyphase of theceramic. WeconductedICPMSonthewatersusedforthefirst leachingcyclefortilesfiredto700C(Fig.12).Theresultwasadefiniteincreaseinbothsodiumandpotassium within the leaching water of ceramics with higher levels of doping. Averysubtlepatternappearsof lowfiredtileslosingmoresodiumduetoleaching. Lowfiringtemperaturesarethereforelessefficientatcementingsodiumintotheceramicstructure.Perhaps the clearest and most important result of this experiment derivesfromtherinsethat followedthepost-firingsalinesoak. Rinsingremovedalmostall of the sodiumadded through repeated saline soakings of fired tiles (thosenot dopedprior tofiring). This reinforces the argument made above that post-Fig. 12 Loglogscatterplot showingtheresults of theleachingexperiments. Thesevaluesreflect theamount of Na and K leached from experimental sherds mixed with saline solution before firing. The x-axisis the percentage saline solution used to fire the ceramics scaled to log base 10 expressions. The y-axis is theconcentrationsofNaandKinpartsperbillionbutscaledtologbase10expressionsStoneretal.depositional saline saturationof ceramics caneasilybe reversedbysoakinginDI water.Based on these experiments, we formulate a general hypothesis regarding the roleofpottingbehaviorversuspost-depositional contaminationinrelationtothehighersodium (and potassium by inference) levels in Xaltocan 1a and 1b ceramics. Sodiumcontainedwithintheceramicpastepriortofiringbecomesfixedwithintheresultingpottery, while non-doped pottery deposited in a saline post-depositional environmentmoreeasilygivesupsodiumwhenexposedtoarinse.Afinalstepintheexperimentinvolvedreanalyzingasample(n=21)ofceramicsassignedtoboththeXaltocan1aandXaltocan1bgroups.Thesamplewastakentorepresenttherangeofchemicalvariabilityinthesecompositionalgroups(Table5).The average change in sodium after leaching was a 4.1 % decrease. The mean changefor potassiumisa2.7%increase. Themedianchange, however, isonly 2.0forsodiumand+2.2for potassium.7The divergence betweenthe meanandmedianvalues is affected by outliers. Looking at individual samples, two in particular greatlyaffecttheaveragelossinsodium. AZX114andAZX139areexamplesthatshowadefinite loss of sodium possibly due to leaching. These two were among the smallestsamplesbyvolume,though,raisingtheissuethattheymaynotberepresentativeofthewholepot.Onlyonespecimendisplayssignificantlyleachedpotassiumconcen-trations (AZX055). All other specimens show a relatively smaller decrease in sodiumand potassium and some actually increased concentrations in both elements. To arriveat concentrations similar to the average for the Basin of Mexico, the leachingexperiment wouldhavetoremoveat least 25%ofthesodiumandcloserto35%of the potassium. Even with instrumental and sampling errors considered, the leachedceramics (with two possible exceptions) do not come close to this percentage of loss.SummaryWehavedemonstratedthattheelevatedlevelsofsodiumandpotassiuminchemicalgroupsXaltocan1aandXaltocan1blikelyresultedfrombehaviorsspecifictothepottersresidingat Xaltocanandpossiblyother nearbysites. Multiplelinesofdatasupport this conclusion. First, since the experimentally leached samples fromXaltocan did not experience greatly reduced levels of sodium/potassium, theseelementalconcentrationsmusthavebeenlockedintotheceramicstructurebyfiring.Thisconclusionisbasedontheobservedmobilityofsodiuminpre-andpost-firingexperiments detailed above. Second, imported wares that display low-to-averagesodiumandpotassiumlevelshavebeenrecoveredatXaltocaninthesamecontextsasceramicswithhighlevelsof sodiumandpotassium. Third, ceramicsfrombothexcavationandsurfacesurveyat Xaltocandisplaynopatternedchemical variation.Fourth, pottery sherds with chemistries identical to those found at Xaltocan have beenidentified in non-saline depositional environments elsewhere in the Basin of Mexico.Fifth, theLAICPMSdatademonstratethat theclays usedtoproduceXaltocan7The rise in sodiumor potassiumspecimens after leachingcould result fromthe leachingof othercomponentstherebyconcentratingtheseelements. Alternatively, samplebiasandinstrumentalerrormaycontributetotheelevatednumbers.TakenwithaGrainofSaltceramics are higher in sodium than other clays in the Basin. Finally, the small sampleof raw clays analyzed from Xaltocan indicates that not all depositional contexts on thesite are of high salinity. These data disprove the hypothesis that the elevatedsodium/potassiumlevels amongXaltocanceramics resultedfrompost-productionuseordiagenesisafterburial.Mostlikely,theelevatedsodiumandpotassiumresultedfromthesaltymaterialsthat thepotters selectedand/or their behavior of mixingpastes withsalinewater.Although the aplasticfractionof the ceramicsis high in sodium (butnot potassium),Table 5 Comparison of two NAA assays on the same pottery sample collected from Xaltocan, displayingthepre-leachedandpost-leachedcompositionsANID Pre-leachedNa(%)Post-leachedNa(%)%ChangeNa/AlANID Pre-leachedK(%)Post-leachedK(%)%ChangeK/AlAZX114 2.8 2.0 30.9 AZX055 1.7 1.4 21.1AZX139 2.9 2.4 20.1 JKM079 1.3 1.1 12.7AZX103 2.1 1.9 9.8 XAL089 2.2 1.9 8.2AZP219 2.1 2.0 7.7 AZX077 1.6 1.4 6.8AZX019 2.0 2.0 7.3 AZX141 2.1 2.1 5.0JKM046 2.2 2.1 6.7 AZX019 2.1 1.9 3.4AZX141 2.3 2.3 4.5 AZX139 2.2 2.2 1.7AZX003 2.2 2.2 4.1 AZX073 2.0 2.1 0.3AZX023 1.9 1.8 3.7 AZX095 1.5 1.5 1.7AZX007 2.3 2.1 2.7 AZX033 1.7 1.8 2.0AZX135 2.1 2.1 2.0 AZX103 1.4 1.5 2.2AZX063 1.9 1.9 1.7 AZP219 2.0 2.2 3.8AZX073 2.2 2.2 1.4 AZX007 1.5 1.6 4.1XAL069 1.9 1.8 1.2 AZX114 1.5 1.6 4.4AZX089 2.0 2.1 0.6 AZX135 1.7 1.9 4.7JKM079 2.7 2.7 0.3 AZX023 2.0 2.1 4.8AZX095 1.9 1.9 0.9 AZX063 1.6 1.7 5.0AZX055 2.2 2.3 1.4 AZX089 1.6 1.8 8.5AZX077 2.4 2.3 4.1 JKM046 1.2 1.4 12.6AZX033 2.0 2.1 5.7 XAL069 1.4 1.6 24.9XAL089 2.2 2.2 6.7 AZX003 1.3 1.7 36.4Mean 2.2 2.1 4.1 Mean 1.7 1.7 2.7Median 2.2 2.1 2.0 Median 1.6 1.7 2.2SD 0.3 0.2 8.5 SD 0.3 0.3 12.1RSD 13.3 10.2 NA RSD 19.5 17.6 NAUncertaintyestimatesformeasuresinNaare1.5%andinKare2.5%.Sothemaximuminstrumentaluncertaintyover two assaysis 3.0% forNaand 5.0 %forK. These uncertainty rangesassume thatthesampleirradiatedinbothassaysisofidentical composition, whichisnot thecaseheresinceweusedadifferent piece of the same sherd originally submitted. This might have added and unknown sampling errorto the instrumental uncertainty. Numbers in bold indicate deviations that are greater than what instrumentalerroralonecanexplain,butgreaterdeviancesmaybeexplainedbyslightlydifferentcompositionsofthepieceofthesherdexaminedStoneretal.this is the case for ceramics throughout the Basin. The compositionof the clayfraction of Xaltocan ceramics is very similar to the rawsediments sampled byMillhauserat SanBartolomSalinas(seeLevelsofSodiumandPotassiumintheClayandTemper Fractionof theCeramicPaste). Manysediments ontheoutershorelinedisplaythesamecomposition, andXaltocanpotterslikelyusedthesaltyclayson the outershoresof the lake.Peoplemight have combined clayprocurementwith salt production, as this was the preferred location to wash salt brine from soils. Inadditiontousingsalt-ladenclaypotters might have mixedthe paste withsalinewaters.PotteryTradefromXaltocantoOtherSettlementsintheBasinofMexicoConfident that theXaltocanreferencegroupsreflect thechemical fingerprint ofitspottersratherthanpost-productionchemicalalterations, wenowexamineexchangeof the Xaltocanpottery.Duringthe EarlyAztec,LateAztec,and Colonial periods, itis likely that the population of Xaltocan exported ceramics to the destinations listed inTable6.Potters atothersitesinthe northernBasinsubregionprobably producedtheFormative, Classic, Epiclassic, and Early Postclassic ceramics. The island ofXaltocanwas not constructeduntil theEpiclassic, andthesitehadlittleregionalinfluence until the Early Aztec, which overlaps with the end of the Early Postclassic.Whileweincludepre-AztecdatainTable6,wedonotsummarizethembelow.Toidentifypossible tradewares,we calculatedMahalanobisdistance-based prob-abilitiesofmembershipinthesevenmajorBasingroups. Ceramicswithmorethan1 % probabilityof belonging to eitherXaltocanchemicalgroup (1aor 1b) and morethantwicetheprobabilityof belongingtoanyother referencegroupwereinitiallyincludedas potential trade wares. These specimens were thenexaminedthroughcanonical discriminant analysis and inspection of elemental concentrations relative tothe Xaltocan reference groups to eliminate members that were not a good fit. The endresult is160potential exportsfromtheNorthernBasintoother subregionsof theBasinof Mexico(seeTable6). About 65%of thesehadbeenassignedtoothersources previously: including Cuauhtitlan, Zumpango, Tenochtitlan, and severalothers in minor proportions. The Cuauhtitlan reference group resembles the chemistryof Xaltocan1a, but theyseparatebasedoncertainelemental axes (seeTable1).Garraty (2006) and Crider (2011) defined the Zumpango reference group. The site iswithinproximitytosedimentsthat must beverysimilartothoseusedat Xaltocan.While Xaltocan likely supported a market that serviced the northern Basin, we cannotruleout thepossibilitythat Xaltocan-likeceramicsfoundintheZumpangoregionwerelocallyproduced.8ExportsfromtheLakeXaltocanregioncovera3,000-yeartimeframefromtheFormative through Colonial periods. Only 3 %of these potential exports wereassignedtotheXaltocan1bsubgroup. Thesefivespecimens areamongtheonlyceramics recoveredoutsidethesiteof Xaltocanthat fit theXaltocan1bchemicalsignature. The remainder was placedintoXaltocan1a, whichwas distributedinminorproportionsovermuchoftheBasinofMexico. Forthediscussionbelow, it8ThesewerealmostallEarlyPostclassicwaresthatarerareorabsentatthesiteofXaltocanitself.TakenwithaGrainofSaltTable 6 Ceramics identified as potential exports from Xaltocan sorted according to time period and regionPeriod/siteofarchaeologicalrecoveryBasintotalBasinsubregionofarchaeologicalrecoveryEast North Northeast Northwest South West TotalFormative(BCE1500100CE) 203 2 1 3Cuanalan 2 2ElTerremote 1 1Classic(100650CE) 436 1 3 1 5Azcapotzalco 1 1CerroPortezuelo 1 1SanMarcos 1(1) 2Teotihuacan (1) 1Epiclassic(650900CE) 661 1 1 3 1 2 8Azcapotzalco 1 1Ixtapalapa 1 1LosReyes 1 1Teotihuacan 2(1) 3Xico 1 1ZumpangoRegion 1 1EarlyPostclassic(General) 755 5 2 7CerroPortezuelo 5 5Teotihuacan 2 2MazapanEarlyPostclassic(8501000CE)7 1 8CerroPortezuelo 7 7ZumpangoRegion 1 1TollanEarlyPostclassic(10001100CE)6 25 7 2 3 43CerroPortezuelo 4 4ChalcoRegion 3 3Chiconautla 4 4Chimalhuacan 1 1Coacalco 2 2SanPedroChicoloapan 1 1Teotihuacan 2 2Xometla (1) 1ZumpangoRegion 24 (1) 25EarlyAztec(11001350CE) 723 2 10 6 1 19CerroPortezuelo 2 2Chiconautla 10 10Cuauhtitlan 6 6Xochimilco 1 1LateAztec(13501521CE) 1,372 1 5 11 4 3 31 55CerroPortezuelo 1 1Stoneretal.is important to remember that settlements surrounding the eastern and northern shoresof Lake Xaltocan (e.g., Zumpango, Citlaltepec, Tultepec) may have had access to thesamematerialsasXaltocan,butnonefigureasprominentlyasXaltocaninethnohis-toricaldiscussionsoftheregionseconomy.TheEarlyAztecperiod(9001350CE)isrepresentedbyAztecIandAztecIIBlack-on-Orange ceramics, which are defined based on decorative styles. AtXaltocan, AztecI ceramicsappear by900CE, whichcoversthedurationof theEarlyPostclassic. By1240CE, AztecII ceramicsappear alongsideAztecI, buthouseholdparticipationineachstylewasnotuniformacrossthesite. EarlyAztecceramics disappear by1350CEandarereplacedbyAztecIII Black-on-Orange(Overholtzer 2012, pp. 120123). Brumfiel andHodge (1996, p. 431; see alsoHodgeandNeff2005, p. 333)arguethat pottersat XaltocanmanufacturedAztecIservingwares,buttheynotedthatlargersamplesizes wereneededtoconfirmtheargument. Wecanconfirmthat bothAztecBlack-on-OrangeandReddecoratedservingwaresfit securelywithintheXaltocanchemical groups. Infact, manyofthesedecoratedtypesresembletheXaltocan1bpasterecipethat isfoundalmostexclusivelyatXaltocan.Table6 (continued)Period/siteofarchaeologicalrecoveryBasintotalBasinsubregionofarchaeologicalrecoveryEast North Northeast Northwest South West TotalChalco 3 3Chiconautla 9 9Cuauhtitlan 4 4Culhuacan 1 1Ixtapalapa 5 5MaquixcoArriba 2 2Nonoalco 1 1SanMiguel 2 2Tenochtitlan 17 17Ticoman 1 1Tola 0 3 3Zacatenco 1 1ZumpangoRegion 5 5Colonial(1521?CE) 398 3 1 2 6 12Acolman 1 1Cuauhtitlan 2 2Temascalapa 1 1MexicoCity(Tenochtitlan) 6 6ZumpangoRegion 2 2Total 23 34 35 14 9 40 160All totals consist of those ceramics found outside Xaltocan that possess either Xaltocan 1a or Xaltocan 1bpaste recipes. All data are for membership in Xaltocan 1a, except those in parentheses denote membershipinXaltocan1bTakenwithaGrainofSaltIn addition to producing decorated Aztec serving wares locally, Xaltocamecas alsoconsumed pottery produced from other locations. Aztec I ceramics came to Xaltocanfromavarietyofother places, mostlyfromthesouthernBasin. However, tradeofAztecIIceramicscontractstoincludeonlytheCuauhtitlan-controlledregiontothewest ofLakeXaltocan. Thesetwositeswereengagedinalongwarthat ledtotheeventualconquestofXaltocanbyCuauhtitlanin1395CE.Itwouldbecounterintu-itivetosuggest thatthewarintensifiedeconomic interactions,butasimilarsituationhas recently been noted for the Middle Postclassic by Nichols et al. (2009). BrumfielandHodge (1996) andHodge andNeff (2005) arguethat Xaltocamecas didnotproduceAztec II Black-on-Orangebecausethe small sample analyzed previouslybyNAAdidnot yieldthedistinctivechemical signatureof locallyproducedAztecIpottery. It is likely that Aztec II ceramics were imported at the same time that Aztec IceramicswereproducedatXaltocan.Becausethepotential exportsareeither PlainOrangeundecoratedwaresornon-diagnostic Aztec Black-on-Red (and three figurines) and we lack detailed chronologicalcontext for the export samples, we cannot determine a more precise chronology for theexportactivityfromtheXaltocanregion(Table6andFig.13).Wegenerallyrefertotrade during the Early Aztec, combining Aztec I and Aztec II ceramics. Xaltocan potteryexports comprise 2.6 % of the Early Aztec database. They reached only two sites amongthe currently available sample. The first is Chiconautla, which likely served as a hub ofFig. 13 Maps depictingthe distributionof Xaltocan-producedceramics duringtheEarlyAztec, LateAztec, and Colonial periods. Site symbols are size graded for quantity of Xaltocan imports and hypotheticalpatternsofinteractionaredemarcatedwith solid arrow linesStoneretal.tradefocusedonthelakes,anditsawincreasedtradeactivityduringtheEarlyAztec(Nicholsetal. 2009, p. 23). TheotherisCuauhtitlan. Between1250and1395CE,endemicwarfarebetweenCuauhtitlanandXaltocanplaguedtheregion(AnalesdeCuauhtitlan1992, pp. 5861;Morehart2010, p.138;Overholtzer2012, pp.8283),but thesetensionsdidnot negativelyaffect trade(seeabove). EarlyAztecceramicsrepresent preconquest times at Xaltocan. The data presentedhere, combinedwithpreviousdeterminationsofimport fromtheCuauhtitlanregion, suggest symmetricalexchange relationships between Xaltocan and the Cuauhtitlan region.TheLateAztecperiod(13501520CE)ismarkedbyAztecIIIBlack-on-Orangeceramics, but alsosometransitional AztecIIIIVwares. AztecIII replacesearlierstylesby1350CE.AtXaltocan,AztecIIIBlack-on-Orangeceramicsmostlyresem-blepasterecipesemployedat Tenochtitlan, themost powerful capital intheAztecTriple Alliance. Because Aztec III ceramics appear at Xaltocan almost 80 years priortotheirintegrationintotheAztecEmpire,itisunknownwhethertheseimportspre-dateor post-datetheformationof theTripleAlliance. Blanton(1996, pp. 7172)arguesthat Xaltocanlost itsmarketplaceduetopolitical subjugationbythistime.Noneof theearlier analysesfromXaltocanrecognizedAztecIII Black-on-Orangewareasbeingproducedlocally,butRedwaresandPlainOrangewareswereamongitsproducts(Garraty2006).Export activity from Xaltocan is 4 % of the Basin sample for the Late Aztec phase.ThedirectionofexportshiftstowardthesouthernhalfoftheBasin(seeTable6andFig. 13). The types of ceramics exportedinclude PlainOrange (n=17), figurines(n=13), andPolychromes (n=9). AztecIII Black-on-Orange(n=4) andAztecRed(n=3) wares were also exported in minor quantities. The export of Aztec III Black-on-Orange contradicts the previous assertions that this ware was not produced inXaltocan(Hodge andNeff 2005). Alsoamongthe Xaltocanexports are incenseburners (n=3), onepipe, andonewhistle. Thehighest proportionof thesegoodswasdestinedfortheAzteccapitalofTenochtitlan.XaltocanappearsonthetributelistforbothTexcocoandTenochtitlan.Wedonotfavor tribute as the predominant method of Xaltocan pottery export to the capital for theseveral reasons. First, Hodgeetal. (1993, p. 132)notethat ceramicsweregivenastribute, but not specificallytheAztecBlack-on-Orangeware. Second, not asinglepotentialXaltocanpotteryexportappearsatTexcoco(thesecondmostpowerfulcityinthe Triple Alliance and a documentedreceiver of Xaltocantribute). Onlyonespecimen was potentially traded to the entire eastern Basin (Texcocos territory).Hodge and Neff (2005) also highlight the conspicuous absence of Texcoco-made potteryat Xaltocan(p. 339). Nicholset al. (2009, pp. 2425)findthesameat Chiconautla,though some plain pottery found there did come fromTexcoco (Garraty 2006). Xaltocanpotteryexportsandimports,therefore,followedanexclusivenorthsouthtraderoute,connectingthesiteeither directlytoTenochtitlanand/or throughChiconautla. Thisimplicates a symmetrical pattern of market interaction instead of, or in addition to, theone-sided flow of tribute payments.The decreased importance of Xaltocans own marketplace during this period(Blanton 1996, pp. 7172) would have forcedpotters to findanother market to tradetheir wares. The Tlatelolcomarketplace situatedadjacent tothe Aztec capital atTenochtitlanwasthelargest intheregion. Xaltocamecasmaynot havephysicallyattended the market in the capital, but their products followed a southern course mostTakenwithaGrainofSaltlikelyviacanoetransport onthelakes. Lendingsupport tothis hypothesis is thesubstantial number of Xaltocan 1a pots at Chiconautla. During the Late Aztec phase,ChiconautlasawagreatincreaseinAztecIIIBlack-on-OrangepotteryimportsfromTenochtitlan (Nichols et al. 2009, pp. 2425), much like at Xaltocan (Hodge and Neff2005, p. 337). XaltocanandTenochtitlanmayhaveinteractedthroughChiconautlamerchants. Whether trade between the Aztec capital and Xaltocan was direct or withan added break-of-bulk point at Chiconautla, the geographic movement of interactionwasessentiallythesame.DuringtheColonial period, Xaltocanproducedasignificant amount of itsownpottery, but it also imported Majolica ware from Mexico City (formerlyTenochtitlan) (Rodrguez-Alegra et al., under review). The proportionof poten-tial Xaltocan-regionexports is 3%of the perioddatabase. Amongits principalexports arePlainOrangeandRedwares (seeTable6andFig. 13). MexicoCityis the primary recipient of exported ceramics fromXaltocan (n=6). All of theRed wares (n=4) traded fromXaltocan went there. All other potential exportswent tothenorthernhalf of theBasin. Thispatternsuggeststwodiscreet spheresof interaction. Xaltocan likely participated in a restricted market systemthatcovered only the northern Basin subregion. The products of Xaltocan pottersalso made their way to Mexico City. It is very unlikely that indigenous RedWares were sent tothe capital as tribute.Thisdiscussionoftradeandtributeisnot acomprehensiveconsiderationoftheeconomicroleofXaltocanacrosstime. Suchataskisreservedformorethoroughtreatmentelsewhere.ItdoesexemplifytheadvantageofdefiningXaltocanreferencegroupsforceramiccompositionalstudies.ConclusionsAll artifactual, chemical, andexperimental datadiscussedinthis paper point tothe validity of using Xaltocan 1a and Xaltocan 1b as reference groups forchemical provenance studies. We determined that Xaltocan ceramics wereelevated in sodium/potassiumprior to firing. This pattern could be explainedeither bygeological variabilityinresourceavailabilityor thebehavior of pottersin the selection or mixing of materials. In either case, the newly createdXaltocan 1a and Xaltocan 1b reference groups reflect meaningful cultural andnatural variation representative of the potters living in the Xaltocan region.Further differentiationof cultural andnatural contributions of potterychemistryshouldbecome clearer withfuture work.This studyhasgreatimportance fordeterminingtherole of Xaltocan in thepotteryeconomyof theBasin. Colonial documents selectivelymentionasmall number ofcommunities that were known as producers of ceramics in the Colonial period, includingHuitzilopochco, Texcoco, Xochimilco, Azcapotzalco, and Cuauhtitlan (Gibson 1964, p.350). Potters from these communities distributed their wares to places all over the Basin(Hodgeet al. 1993). Chemical characterizationstudies have helpedidentifyotherceramic production centers, including the Texcoco region, Ixtapalapa, Chalco (Garraty2006; Hodge et al. 1993), Tenochtitlan, and Teotihuacan (Hodge and Neff 2005; Hodgeet al.1992;Nichols et al.2002).Pottery-producingtownsmentionedinthehistoricalStoneretal.sourcesdonot correspondcloselytoproductioncentersidentifiedarchaeologically,underscoring the value of combining historical, archaeological, and chemical evidenceto build a more complex picture of pottery production and exchange in central Mexico.WebuilduponthesepreviousstudiesbychemicallyidentifyingXaltocanasapotteryproduction center.UsingtheXaltocanchemical referencegroups, weidentifyprobablepatterns ofpottery export fromthe region. Patterns of Xaltocan export generally formthe reciprocalpattern with regards to the geographic relationships previously identified for the importof ceramics to the site (Brumfiel and Hodge 1996; Hodge and Neff 2005; Nichols et al.2002).Whilethemajorpatternsofinteractioncloselyfollowpoliticalrelationships,itnowappearsasthoughpotteryexchangebetweenXaltocananditspoliticalsuperiorswas symmetrical rather than one-sided. We suggest that the movement of pottery to andfromXaltocanintheEarlyAztec,LateAztec,andColonialperiodsinvolvedmarketsthat shifted geographically over time. The disappearance of Xaltocans ownmarket by the end of the Early Aztec may have forced its potters to market their waresin Tenochtitlan and Mexico City during the Late Aztec and Colonial periods(respectively). These data also potentially support the reconstruction of Chiconautla asa trade intermediary between Xaltocan and Tenochtitlan (see Nichols et al. 2009).The studyalsoelucidates important behaviors that affect potterychemistry. Wedemonstratethat pottersin theXaltocanarea selected salt-laden clays, saline water,orboth to manufacture their pottery. Potters at Xaltocan may not have consciously selectedsaline water over freshwater, or salty clays over non-salty clays such as in the Yucatn.Choice of these materials may simply reflect use of resources close at hand. However, ifsaltwaterorsaltyclayswereusedinthepotteryproductionprocessanywhereintheBasinofMexico, Xaltocanwastheonlyplacetodosoconsistentlyovertime. Thisunique practice forms an important part of the chemical fingerprints of Xaltocan potters.This study also makes a significant methodological contribution. We add to a diverseset of experimental tools designed to explain chemical variation in ceramicprovenance studies(seeBlackmanandBishop2007;Boulanger et al.2012;BuxedaiGarrigos1999;BuxedaiGarrigos et al. 2002;Golitko et al. 2012;Neff et al. 1988,1989, 2003; Schwedt and Mommsen 2007; Schwedt et al. 2004; Stoner et al. 2008). AsNeff (2000) and Neff et al. (2003) have repeatedly argued, such evaluations should bestandard in all compositional research. It is not enough, however, to rely on generaliza-tions about pottery perturbation. Issues of diagenesis have figured prominently in somerecent debates, and the approach presented here provides a method to evaluate betweenpotting behaviors and post-depositional chemical alteration.Acknowledgments This research was made possible, in part, through NSF grants #1110793 and#0922374awardedtotheUniversityofMissouri ResearchReactor. Additionally, NSFgrant #1035319,awardedtoJohnK. Millhauser. Millhauser, fundedtheinitial analysisthat ledtotheredefinitionoftheXaltocanceramicsampleintonewreferencegroups.AssistancewithpreparingsampleswasprovidedbyTimothy Ferguson, Cody Roush, and Erin Gillespie. We also wish to thank researchers who have submittedsamples from Xaltocan over the years, including the late Elizabeth Brumfiel, the late Mary Hodge, DeborahNichols, Destiny Crider, Christopher Garraty, and Kristen De Lucia. Their initiative has made the Basin ofMexico one of the most thoroughly researched regions through NAA. Discussions with a number of othercolleagues, includingJeffreyFerguson, MatthewBoulanger, andJaumeBuxeda, alsohelpedtofacilitatethedesignofthisresearchortopoint out examplesintheliteraturethat haveconductedsimilarexperi-ments.JamesGuthriehelpedtorunasubsampleofsalinesolutionandleachedliquidsthroughtheICPMS.BarryHigginswasconsultedseveraltimesduringtheoperationoftheLAICPMS.TakenwithaGrainofSaltReferencesAbbott, D. R. (2008). 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