Dating of ancient kilns: A combined archaeomagnetic and thermoluminescence analysis applied to a brick workshop at Kato Achaia, Greece

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Original

article

Dating

of

ancient

kilns:

A

combined

archaeomagnetic

and

thermoluminescence

analysis

applied

to

a

brick

workshop

at

Kato

Achaia,

Greece

Evdokia

Tema

_,

_Georgios

_Polymeris

_,

_Juan

_Morales

_,

_Avto

_{Goguitchaichvili}

_,

Vassiliki

Tsaknaki

a_{DipartimentodiScienzedellaTerra,UniversitàdegliStudidiTorino,viaValperga35,10125Torino,Italy} b_{ALP-AlpineLaboratoryofPalaeomagnetism,viaG.U.LuigiMassa6,12016Peveragno,Italy}

c_{InstituteofNuclearSciences,AnkaraUniversity,TandoganCampus,06100Ankara,Turkey}

d_{LaboratorioInterinstitucionaldeMagnetismoNatural,InstitutodeGeofisica,UNAM,CampusMorelia,Michoacan,Mexico} e_{6thEphorateofPrehistoricandClassicalAntiquities,197,Alex.YpsilantouStreet,26110Patra,Greece}

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Articlehistory: Received6May2014 Accepted29September2014 Availableonline7November2014

Keywords: Dating Archaeomagnetism Thermoluminescence Bricks Secularvariation

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Wepresentherethe resultsofadetailed archaeomagneticand thermoluminescenceinvestigation performedonbricksfromtwoancientkilnsexcavatedatKatoAchaia,Greece.Magneticmineralogy mea-surementshavebeencarriedouttodeterminethemainmagneticcarrierofthesamples.Thedirectionsof thecharacteristicremanentmagnetizationofeachstructurehavebeenobtainedfromstandardthermal demagnetisationproceduresandtheabsolutearchaeointensityhasbeendeterminedwiththeThellier modifiedbyCoemethod,accompaniedbyregularpartialthermoremanentmagnetization(pTRM)checks. Thefullgeomagneticfieldvectorwasusedforthearchaeomagneticdatingofthetwokilns,after compar-isonwiththereferencesecularvariationcurvescalculateddirectlyatthesiteofKatoAchaia.Independent datinghasalsobeenobtainedfromthermoluminescence(TL)analysisonfourbricksamplesfromeach kiln.Thedatingresultsobtainedfromthetwomethodshavebeencomparedandthelastfiringofeach kilnhasbeenestimatedfromthecombinationofthetwotechniques.Usingtheindependentdateoffered byTLdating,thenewarchaeomagneticdatahavebeencomparedwithotherdatafromthesametime periodandtheycanfurtherbeusedasreferencepointstoenrichourknowledgeaboutthepastsecular variationoftheEarth’smagneticfieldinGreece.

©2014ElsevierMassonSAS.Allrightsreserved.

1. Introduction

Datingof archaeological remains is essential in archaeologi-calresearch,inordertoplaceinchronologicalorderfindingsand civilizations.Inscribedobjects sometimesbear an explicitdate, orpreserve thename of a knownindividual (e.g.a kingor an emperor). However, this is not always the case and often the contributionof a scientific dating technique is necessary. Dur-ingthelastdecades,severaldatingmethodssuchasradiocarbon dating,obsidianhydration, dendrochronology, potassium-argon, archaeomagneticandluminescencedatinghavebeenincreasingly usedinarchaeology.Eachoneofthesedatingtechniqueshowever

∗ Correspondingauthor.Tel.:+00390116708395;fax:+00390116708398. E-mailaddresses:evdokia.tema@unito.it(E.Tema),polymers@auth.gr

(G.Polymeris),jmorales@geofisica.unam.mx(J.Morales),avto@geofisica.unam.mx

(A.Goguitchaichvili),vtsaknaki@gmail.com(V.Tsaknaki).

has its own advantages and limitations, mostly related to the availabilityof appropriatematerial,thetype andcharacteristics of the studied samples, their preservation conditions and the chronologicalperiod. For this reason, when possible, the com-binationof more datingtechniques togetherwiththeavailable archaeologicalevidencemayofferthebestapproachfor obtain-ingamoreprecisechronologicalframeworkforanarchaeological site.

Archaeomagnetic dating is based on the principle that the magneticmineralscontainedin manybakedclayarchaeological artefacts(e.g.,kilns,hearths,bricks,pottery),whenheatedathigh temperaturesandcooledinthepresenceoftheEarth’smagnetic field,mayacquireathermalremanentmagnetization(TRM)with directionparallelandmagnitudeproportionaltotheambient mag-netic field. For the regions where a detailed reference secular variation(SV)curveisavailable,archaeomagneticdatingis possi-bleafterthecomparisonoftheremanentmagnetizationmeasured ontheundisturbedarchaeologicalartefactswiththereferenceSV

http://dx.doi.org/10.1016/j.culher.2014.09.013

Fig.1. a:Generalviewofthestudiedkilns;b,c:kilnsKL3andKL5;d,e:locationofthecollectedbricksamplesrespectively.

curve.Duringlastdecades,importantprogressonarchaeomagnetic datinghasbeendoneandithasbeensuccessfullyappliedinseveral casestudiesintheMediterraneanarea,mainlyinvolvingthestudy ofthedirectionofthegeomagneticfieldvector(e.g.[1–7]).

Inananalogousway,luminescencedatingisbasedonthefact thatnaturally-occurringmineralslikequartzandfeldsparsactas naturaldosimetersandpreservearecordofirradiationdose,i.e. energy perunit mass,receivedthrough time. Thisdose results mainlyfromthedecayofnaturalradionuclides,i.e.,232_Th,40_Kand naturalU,alongwithcosmicrays,whichprovideaconstantsource oflow-levelionizingradiation.Theaccumulateddoseisstoredby meansoftrappedchargeincrystaldefects,whichisstableoverlong periodsoftimebutcanbereleasedeitherbyheatingorexposingthe crystaltolight.Thisreleasecantakeplaceaccidentallyinanatural wayoralternativelyartificially,atthelaboratory,givingthusrise tothermoluminescence(TL)andopticallystimulatedluminescence (OSL)respectively[8–10].Thebrightnessoftheluminescence sig-nalreflectstheamountoftrappedcharge.Consequently,itisalso proportionaltothetotalirradiationdoseaccumulatedandthusto thetotalage.Thenumberoftrappedelectronsisincreasingaslong asthematerialisirradiated.However,everytimethatthematerial issubjectedtoprolongedheating(asinthecaseoffiringpottery) orintenselightexposure(asinthecaseofsunlight),electronsare evictedand trapsareemptied.Inthat case,thesignalistotally zeroed.Then,energystartstoaccumulateintheformoftrapped electronsinordertorefilltheemptytrapsonceagain.Thetotal numberoftrappedelectronsformsa luminescent“clock”which startsmeasuringfromthebeginning(t=0)everytimethatthese trapsarezeroed.Therefore,light-exposedmaterialscouldbedated totheirlastexposuretolight,whileburntmaterialstotheirlast heating.KilnsbelongtothelattercaseandTLcaneffectivelydate theirlastuse.

Archaeomagnetic and TL dating techniques share the same rationale,datingexactlythesameeventthatisthelastheatingof bakedclayartefacts.Therefore,simultaneousapplicationofboth techniquestothesamearchaeological materials,suchasbricks fromkilns,yieldstheimportantadvantageofcrosscheckingages. Eventhoughtheircombinationcanofferapowerfultoolfordating ofarchaeologicalartefactsduringHolocene,uptonowsuch com-binedstudiesareextremely limited[6,11–13].Wepresent here theresultsofadetailedarchaeomagneticandTLinvestigation per-formedonbrickscollectedfromthestructureoftwoancientkilns excavatedatKatoAchaia,Greece.Thedatingresultsobtainedfrom thetwomethodshavebeencomparedandthelastfiringofeach kilnhasbeenestimatedfromthecombinationof thetwo tech-niques.UsingtheindependentdateofferedbyTLdating,thenew archaeomagneticdatacanbefurtherusedasreferencepointsatthe constructionofthesecularvariationoftheEarth’smagneticfield inthepast.

2. Archaeologicalsiteandsampling

The studiedkilns werediscovered during theworks for the establishmentofthefundamentalsofanewbuildinginthecorner ofParodosAg.IoannouandPapaflessa,KatoAchaia,andarepart oftheextensiveceramicworkshopfoundinthewestpartofthe ancientcityofDyme,situatedatthesamewideplateauofthe mod-erncityofKatoAchaia(38.15o_N,21.55o_{E),Peloponnese,Southern} Greece.Thearchaeologicalresearchinthisplotrevealedacluster ofceramickilnsofvariousdimensionswithadditionsand modi-ficationswhichdenotethetimeframeoffunctionandactivityof theworkshopduringthewholeHellenisticperiod.Forthepresent study,twocircularkilnsweresampled,namedKL3andKL5(Fig.1).

Fig.2.a:IRMacquisitioncurvesforrepresentativesamplesfromKL3(left)andKL5(right)kiln;b:StepwisethermaldemagnetizationofthreeIRMcomponentsfor representativesamplesfrombothstudiedkilns.Symbols:diamond=Soft-(0.1T);triangle=Medium-(0.5T);square=Hard-(1.6T)coercivitycomponent.

KL3issituatedontheN-Saxisandduringitsoriginal construc-tionit wascircular,and internallycovered bya claylayer.The centralheatingchamberis4minexternaldiameter,witha surviv-ingheightofaround1.35–1.50m.Theentranceofthekilnislocated atthenorthandbearstheformofapointedarch,1.48mheight and0.27–0.72mwide(Fig.1b).Duringthesecondphaseofitsuse, theorientationoftheKL3kilnwaschanged.Thecentral mouth-openinginthenorthwasblocked,andanewonewasopenedon thewestside,1.54mheightand0.56mwide.Furthermore,a0.50m widewallmadeof bricks, tilesand soilwasconstructedinside theareaoftheheatingchamberthuslimiting itsdimensionsto 2.30–2.50m.Consequently,anewchamberwithdifferentcapacity wascreated.Theinternaloftheheatingchamberbearssuccessive claylayers,acommonfeatureaimedtoretainasteady tempera-tureandavoidheatdispersal.Cutsontheupperpartofthecircular wallgiveevidenceoftheexistenceofthesupportsofthebaking floor(eschara).KL5issituatedatasmalldistanceandatthe east-ernsideoftheKL3kiln(Fig.1).ItsconstructionissimilartoKL3 witha2mexternaldiameteranda0.70mhighcentral cylindri-calsupportgraduallywideningontheupperpart(Fig.1c).Both KL3andKL5kilnswereprobablyconstructedatthesameperiod andwerecontemporaneouslyusedforalongperiodoftime,afact whichbecomesevidentnotonlybythepotteryfoundinsidebut alsobythefunctionalitydenotedbythecommonorientationofthe mouth-fireentrancesofthetwokilns.Afirstoverviewofthe pot-teryproductsofthetwokilnssuchasfragmentsoftiles,bricks,clay masses,toxylia,andthepotterydepositwestofthefireentrance, datetheabandonmentofthekilnsnotearlierthantheendofthe firstcenturyBC.Morespecifically,andwithreferencetothepottery recoveredfromthekilnsandfromthevicinityareathefollowing

maybededuced.Theexistenceofseveralshapesofcoarseware inpercentage90%comparedtofinewarewithred–blackslipor blackslip,denotedthattheproductionwasconcentratedmostlyin everydayusepots,ratherthaninmoreexpensivefineones.InKL3 twopieceswithcountingmarkswerealsoidentified,acommon practiceforpotters,enablingthemtokeeptrackoftheorders.The majorityofthematerialfounddatesthesitetothemiddleandlate Hellenisticperiod.

Systematic archaeomagnetic sampling was carried out col-lecting 9 brick samples from the first kiln (KL3) and 12 brick samplesfromthesecond kiln(KL5).Allbricksamplescollected werepartofthemainstructureofthekilnsandwereorientedinsitu usinga magneticanda solarcompass. Mostofthebrickswere positionedhorizontallyin thekilns’walls andthecentralpillar (Fig.1d,e).Fromeachindependentlyorientedsample,onetothree cylindricalspecimensofstandarddimensions(diameter=25.4mm, height=22mm)weredrilledinthelaboratory.Fourbricksamples fromeachkilnhavealsobeencollectedforthermoluminescence analysis.

3. Archaeomagneticanalysis

3.1. Magneticmineralogy

Rock-magnetic measurements were carried out on several representativesamplesfrombothkilnsattheALP-Alpine Palaeo-magnetic Laboratory (Peveragno, Italy). Isothermal remanent magnetization(IRM)acquisitioncurveswereobtainedby apply-ingstepwiseincreasingmagneticfieldsupto1.2T,withanASC pulsemagnetizerandthemagneticremanencewasmeasuredwith

Fig.3.ResultsofstepwisethermaldemagnetizationofrepresentativesamplesrepresentedasZijdervelddiagrams:kilnKL3(upperpart)andKL5(lowerpart).Symbols:full dots=declination;opendots=apparentinclination.ResultshavebeenillustratedandelaboratedusingtheRemasoftsoftware[17].

aJR6spinnermagnetometer(AGICO).Stepwisethermal demag-netisationofacompositethreeaxesIRMwasalsoperformedafter applyingfirstamaximumfield(1.6T)alongthecylinder–axis(Z), thenan intermediatefield (0.5T)along theY-axis andfinally a minimumfield(0.1T)alongtheX-axis.

The IRM curves obtainedfrom different samples from both kilnsindicatethatthesaturationofthemagnetizationis gener-allyreachedatlowfieldsvaryingfrom0.2to0.4T,indicatingthe presenceofalow-coercivitymineralsuchasmagnetiteand/or Ti-magnetite(Fig.2a).Insomesamples(mainlyfromKL5 kilne.g., KL5-4andKL5-6), saturationisnot completelyreachedat1.2T showingthatasmallamountofa high-coercivitymineral,most probablyhematite,mayalsobepresent.Theseresultsarealso con-firmed bythe thermal demagnetisationexperiments of a three componentIRM[14].Theobtaineddemagnetisationcurves(Fig.2b) showthedominanceofthemagneticallysoftfraction(<0.1T)while themediumandhigh-coercivitycomponentsaregenerallyvery small.TheseresultspointtomagnetiteorTi-magnetiteasthemain magneticcarrierinthestudiedsamples,withpossiblysomesmall contentofhematiteinsomecases.

3.2. Archaeomagneticdirection

Thenaturalremanentmagnetization(NRM)ofallspecimens wasmeasuredattheALPlaboratorywithaJR-6spinner magne-tometer.Onetothreespecimensfromeachsample,accordingtothe materialavailability,havebeenstepwisethermallydemagnetized upto560◦C usinga TSD-2Schonstedt furnace.The demagneti-zationresultsareillustratedasorthogonalvectorprojectionsof theremanentmagnetization(Zijderveldplots)(Fig.3).Zijderveld

diagramsshowthatthemagneticremanenceisverystableandit consistsofonewell-definedcharacteristicremanent magnetiza-tion(ChRM).Insomesamples(mainlyfromKL3kiln)asecondary viscouscomponentisalsovisiblebutitiseasilyremovedduring thermaldemagnetization.

The direction of the ChRM has been obtained from princi-palcomponentanalysis[15,16]usingtheRemasoftsoftware[17]. Directionscalculatedatspecimenlevelarewelldefinedwith max-imumangulardeviation(MAD)anglesgenerallylessthat3o_(with onlyexceptionsspecimensKL3-3a,KL5-1bandKL5-8a).Allresults atspecimenlevelfromkilnKL3andKL5arereportedinTable1. Mean directions for each sample were calculated according to Fisherstatistics[18]andarereportedinTable1togetherwiththe meanarchaeomagneticdirectioncalculatedforeachkiln. Equal-areaprojectionsof theChRM directionsatsample level(Fig.4) showaverygoodconcentrationaroundthemeanvalue.The cal-culatedmeandirectionforkilnKL3is:D=353.0o_,I=56.6o_,k=245, ␣95=3.6oandforkilnKL5is:D=350.4o,I=57.7o,k=219,␣95=3.5o. Theverysimilardirectionsobtainedforthetwokilns(statistically indistinguishable)suggestthatthetwokilnswereinuse contem-poraneouslyandwereabandonedatthesametimeperiod.These resultshavebeenpreviouslypresentedby[19]andarehere com-plementedbythearchaeointensitydetermination.

3.3. Archaeointensitydetermination

Archaeointensitydeterminationshavebeencarriedoutatthe LIMNA palaeomagnetic laboratory of UNAM (Campus Morelia, Mexico) withtheclassical Thelliermethod[20] asmodified by Coe[21,22].Onetofourcubicspecimens ofsimilardimensions

Table1

Archaeomagneticdirectionalresults.

Specimen Temperaturerange(◦C) D(o_{) I(}o_{) MAD Samplemean}

D(o_{) I(}o₎ KilnKL3 KL3-1a 400–560 348.8 58.6 1.8 KL3-1b 400–560 350.2 56.5 1.8 KL3-1c 280–560 351.9 58.0 1.9 350.3 57.7 KL3-2a 220–480 1.6 52.3 1.1 KL3-2b 220–480 347.7 55.6 2.0 KL3-2c 220–480 343.7 55.9 1.0 351.3 54.9 KL3-3a 320–480 5.8 51.7 6.5 KL3-3b 160–520 337.2 55.3 1.4 352.1 54.4 KL3-4a 280–480 347.7 56.7 1.2 KL3-4b 280–520 351.7 54.0 1.5 349.8 55.4 KL3-6a 400–560 343.3 57.8 0.7 KL3-6b 220–560 340.6 55.2 1.2 341.9 56.5 KL3-7a 220–560 355.9 52.8 1.3 KL3-7b 160–520 350.4 53.4 1.8 KL3-7c 220–560 349.9 50.6 1.2 352.0 52.3 KL3-8a 100–520 340.0 60.8 1.1 KL3-8b 280–520 5.4 56.6 1.6 KL3-8c 280–520 11.4 55.5 1.3 359.9 58.3 KL3-9a 400–520 3.3 61.5 0.6 KL3-9c 220–520 16.1 61.6 1.4 9.69 61.7 Meanvalue N=8 n=20 Dm=353.0o Im=56.6o K=245 ␣95=3.6o KilnKL5 KL5-1a 200–500 351.7 56.0 2.0 KL5-1b 200–500 343.8 56.2 3.6 347.8 56.2 KL5-2a 200–500 347.4 54.8 1.3 KL5-2b 140–500 354.0 52.8 1.1 350.8 53.8 KL5-3a 140–500 358.9 53.8 2.1 KL5-3b 140–500 0.5 54.7 2.9 359.7 54.3 KL5-4a 140–500 356.9 64.9 3.0 356.9 64.9 KL5-5a 140–500 342.3 61.5 1.6 KL5-5b 80–500 348.1 59.0 2.5 345.3 60.3 KL5-7b 200–500 341.1 52.9 1.8 341.1 52.9 KL5-8a 300–500 348.1 59.0 4.3 KL5-8b 200–500 356.1 56.0 3.0 352.3 57.6 KL5-10a 140–500 0.1 60.2 1.8 0.1 60.2 KL5-11a 140–500 342.6 57.3 1.9 KL5-11b 140–500 340.4 57.1 2.9 341.4 57.2 Meanvalue N=9 n=15 Dm=350.4o Im=57.7o k=219 ␣95=3.5o

Columns:specimen;TemperatureintervalusedforthecalculationofthedirectionoftheChRMatspecimenlevel;Declination(o_{);Inclination(}o_{);MAD:MaximumAngular}

Deviation;MeanD(o_)andI(o_{)calculatedatsamplelevel;Meanvalueforeachkiln:N=numberofindependentlyorientedsamples;n=numberofspecimens;D}

m=mean

declination;Im=meaninclination;k=precisionparameter;␣95=95%semi-angleofconfidence.

(∼10mmlength)werecut inthelaboratoryfromeach sample, usingtheremainingmaterialofthedrilledcylindricalsamples, pre-viouslyusedfordirectionalanalysis.Atotalof16specimensfrom kilnKL3 and26fromkilnKL5 havebeenpreparedandstudied. AllspecimenswereheatedandcooledinaASCScientificTD48-SC furnaceandtheremanencewasmeasuredwithaJR6spinner mag-netometer.Heating/coolingcycleswereperformedinair.Fourteen temperaturestepsweredistributedfrom25◦Cto490◦C.Adirect laboratoryfieldof65.0±0.05␮Twasappliedduringheatingand

coolingofthedifferentspecimens.TwopTRMcheckswere per-formedinordertodetectpossiblechangesinthepTRMacquisition capacity.Additionally,apTRMtailcheck[23]wasperformedata temperatureof350◦C.CoolingratedependenceofTRMwas inves-tigatedfollowingaproceduresimilartothatdescribedby[24].At theendofthearchaeointensityexperiments,allspecimenswere heatedthreemoretimesat490◦Cinthepresenceofthesame lab-oratoryfieldusedduringthearchaeointensitydetermination.The firsttime,anewTRM(TRM1)wasgiveninthesameconditionsas

N 90 180 270 N 90 180 270 Down Up

Kiln KL3

Kiln KL5

Kiln KL3

Kiln KL5

Fig.4.EqualareaprojectionoftheChRMdirectionsatsamplelevelforkilnKL3(up) andKL5(down).

thatgainedduringthelaststepoftheThellierexperiment,using ashortcoolingtimeofaround45min.ThenasecondTRM(TRM2) wasgivenwithalongercoolingtime(∼6h)andfinallyathirdTRM (TRM3)wascreatedusingthesameshortcoolingtimeasthatused duringtheTRM1(approximately45min).Thecoolingrate correc-tionfactorswerecalculatedasthevariationbetweentheintensity acquiredduringashortandalongcoolingtime[24].Thecooling ratecorrectionwasappliedonlywhenthecorrespondingchange inTRMacquisitioncapacitywasbelow15%.

Theobtainedresults,interpretedusingNRM–TRMplots(Fig.5), arereportedinTable2togetherwiththestatisticalparameters calculated according to [22]. To be considered as trustworthy estimationsoftheancientfield,archaeointensitydeterminations obtainedinthisstudyhadtofulfillthefollowingacceptancecriteria

[25]:

• directionsofnaturalremanentmagnetization(NRM)end-points ateachstepobtainedfromarchaeointensityexperimentshaveto fallalongastraightline,trendingtowardtheoriginintheinterval chosenforarchaeointensitydetermination;

• nosignificantdeviationofNRMdirectionstowardstheapplied field direction should be observed, as revealed in vector (Zijderveld)plots;

• anumberofalignedpoints(N)ontheNRM-pTRMdiagram≥9; • NRMfractionfactor(f,[22])≥0.5.Thismeansthatatleast50per

centoftheinitialNRMwasusedforarchaeointensity determina-tion;

• aqualityfactorq=(f×g)/␤[22]≥5(generallyabove10,Table2); gisthegapfactor[22]and␤therelativestandarddeviationof theslope;

• archaeointensity results obtained from NRM-pTRM diagrams mustnotshowanevidentconcaveupshape,sinceinsuchcases remanenceisprobablyassociatedwiththepresenceofMDgrains

[26,27];

• positivepTRMchecks,i.e.,thedeviationof“pTRM”checksshould belessthan15%.

Average cooling rate correction factors close to 0.98 were applied to raw archaeointensity data. Evaluation of pTRM-tail checkswasinmostcaseslowerthan2%.Althoughindividual inten-sitydeterminationsobtainedrangefrom53.2to74.5␮T forthe kilnKL3andfrom54.9to73.4␮TforkilnKL5,correspondingmean valueperkilnaresimilar:61.3±6.0␮Tand62.4±5.2␮T, respec-tively.

3.4. Archaeomagneticdating

Thefullgeomagneticfieldvector(declination,inclinationand intensity)obtainedforeach kilnhasbeenusedforthe archaeo-magneticdatingofthetwostructuresaftercomparisonwiththe referencesecularvariationcurvescalculatedfromtheSCHA.DIF.3K model[28].TheSCHA.DIF.3Kisaregionalarchaeomagneticmodel thatrepresentsthegeomagneticfieldvariationsinEuropeforthe last3000yearsmodelingtogetherthethreegeomagneticfield ele-ments.It is based onreferencedata comingfrominstrumental measurementsforthelast400yearsandondatafrom archaeo-logicalmaterialforoldertimes.For the400BC-500ADperiods, thedirectionalcurveobtainedfromtheSCHA.DIF.3Kmodelis sta-tisticallythesamewiththeGreekSVcurvecalculatedusingthe Bayesianstatistics[7].Forthisreason,bothcurvesshouldgivethe samedatingresults.Inthisstudy,theSCHA.DIF.3Kmodel refer-encecurvewasusedbecause,inrespecttothelocalSVcurves, itpresentstheadvantagethatpredictsthegeomagneticfieldat the site of interest, avoiding this way any eventual relocation error.

ArchaeomagneticdatingoftheKL3andKL5kilnshasbeen car-riedoutusingtheMatlabarchaeodatingtool[29].ReferenceSV curveshavebeendirectlycalculatedatthegeographiccoordinates ofKatoAchaiaandhavebeenusedforthecalculationof proba-bilitydensityfunctionsseparatelyfordeclination,inclinationand intensity.Thefinaldating ofthetwokilns isobtainedafterthe combination oftheseparatedensityfunctions(Fig.6).For each kilnseveralpossibledatingintervalsoccur.However,takinginto accountthearchaeologicalcontextofthesiteandthe archaeolog-icalfindingsthatproposeaHellenisticage,itissuggestedthatthe lastuseofthekilnsKL3andKL5occurredinthetimeintervals97 BC-133ADand85BC-37ADrespectively,calculatedat95%of probability.

4. Thermoluminescenceanalysis

4.1. Experimentalprocedure

ForTL dating,twodifferentphysicalquantitiesarerequired; thetotalaccumulateddoseduringthepast,termedaspalaeodose orequivalentdose(expressedinunitsofGy),aswellastherate at which this energy-doseis accumulated, termedas doserate (expressedinunitsofGy/yr).Theratioofthesetwoquantities,i.e. thepalaeodose(DE)overthedose-rate(DR),representstheageof thesample.Atotalofeightbricksamples,fourfromeachkiln,were subjectedtoTLdating. Treatmentand preparationwere under-takeninsubduedredfilteredlightconditions.Analmost0.5cm

Fig.5.RepresentativeexamplesofNRM–TRMdiagramsandassociatedZijderveldplotsfromsuccessfularchaeointensityexperiments.

thick,outerlayerwasremovedfromeachsampleinthelaboratory toeliminatethelight-subjectedportions.Thechemicalprocedure described by [30] was applied for sample preparation. Finally, grainswithdimensionsintherange4–12␮mwereextracted, sus-pendedin acetone and finally precipitatedonto 1cm diameter aluminiumdiscs[31].

For the equivalent dose estimation, the multiple aliquot, additivedoseprocedure (MAAD) in TL wasapplied; a detailed descriptionoftheprocedurecanbefoundin[8,9,32](foranoutline, readerscouldalsoreferto[33]).AllTLmeasurementswerecarried outusingaRisøTL/OSLreader(modelTL/OSLDA-15),equipped witha90_Sr/90_{Ybetaparticlesource,deliveringanominaldoserate} of0.071Gy/s.A9635QAphotomultipliertubewasusedforlight detection.The detectionoptics consistedof a combination of a PilkingtonHA-3heatabsorbingandaCorning7–59(320–440nm) bluefilter. AllTL measurements wereperformed in a nitrogen atmospherewithalowconstantheatingrateof1 оC/s,inorder toavoid significanttemperature lag,up tothemaximum tem-peratureof 500 оC.The additivedoses appliedwere7, 15 and 22Gy.

Thedoserateiscalculatedbasedonthedecayofnaturally occur-ringradionuclidesinsidetheclaymatrix,i.e.,40_K,232_Thandnatural U,alongwithcosmicrays,whichprovideaconstantsourceof low-levelionizing radiation.The lattertwo weremeasuredin units ofpartpermillion(ppm)usingthicksourcealphacounting[34], while40_{Kcouldbeestimated byScanning ElectronMicroscopy} (SEM,[35]).Dose-ratecalculationsweremadeusingtheconversion factorsof[36].

4.2. DEestimation

NaturalTLglowcurvesforallsamplesexhibitthesamemain characteristics, namely a glow curve that has the form of a

continuumwith two prominentoverlapping TLpeaks centered around275and350◦C(Fig.7).Eachglowcurveisthemeanvalue ofthreeindependentlymeasuredglowcurves.Equivalentdoses werecalculatedwith1␴errorvalues;atypicalplotofDEagainst glowcurvetemperatureispresentedinFig.8.Errorsderivedmainly fromtheuncertaintiesincurvefitting,are±1␴andwere calcu-latedbystandarderrorpropagationanalysis[37].Inallcases,DE plateausarewideenough,over90◦Cwide.Theequivalentdoses wereobtainedasthemeanvalues ofthebestplateaus foreach sample.Onlylinearfittingswereperformedtothedoseresponse curves.Thislinearitywasstronglyestablished.IntheinsetAof

Fig.8representative exampleofadditivebuild-upcurveisalso presentedasfilledsquaresforthetemperaturecorrespondingto thetemperatureinthemiddleoftheplateaurange,alongwiththe correspondinglinearfit.Thisinsetfigurestronglysupportsthe lin-earitymonitoredforthecaseofthedoseresponse.In thesame figure,insetBpresentsthecorrespondingsecondglowTL dose responsecurveafterapplyinglowdoses,indicatingthepresence ofsupra-linearityinthelow-doseregion[31].Thecorresponding supra-linearitycorrectionwasestimatedastheinterceptofthe lin-earpartofthesecondglowTLwiththedoseaxis;supra-linearity correction,I,equalstozeroifthisinterceptionpassesfromthe ori-ginoftheaxis.ThevaluesofIarethenplottedversustemperature andameanvalueisyieldedinthesametemperatureregionwhere theplateauismonitored[38].AsummaryoftheTLdatingdatais providedinTable3.InthecaseofkilnKL5,whichyielded supra-linearity,thecorrespondingindexIwasaddedtotheequivalent dosevalue,applyingthiswayaformoftheslidemethod.

Finally,since samples areexpected tocontainfeldspars,the anomalousfadingwasalsoestimated.For thepresentstudythe procedurepreviouslyappliedby [6]wasadopted; howeverthe doseappliedherewassimilartotheequivalentdoseofeachkiln whilethestoragetimewasthreemonths.Noanomalousfadingwas

Table2

Archaeointensityresults.

Specimen N Tmin-Tmax(◦C) m sigma f g q H(␮T) ␴(␮T) Hcr(␮T)

KilnKL3 KL3-1a 10 20–490 −1.036 0.080 0.529 0.797 5.3 67.3 5.2 66.5 KL3-1b 11 20–490 −0.965 0.026 0.596 0.857 19.6 62.7 1.7 62.8 KL3-1c 9 20–490 −1.014 0.049 0.630 0.782 10.1 65.9 3.2 61.0 KL3-2a 11 20–490 −0.891 0.028 0.932 0.766 25.5 57.9 1.8 56.4 KL3-2b 12 20–490 −0.885 0.038 0.896 0.875 20.6 57.5 2.5 55.9 KL3-3a 9 20–490 −0.936 0.022 0.888 0.492 19.9 60.8 1.4 60.5 KL3-3b 11 20–490 −0.925 0.031 0.885 0.694 19.8 60.1 2.0 59.9 KL3-3c 11 20–490 −0.866 0.036 0.837 0.758 17.6 56.3 2.3 55.5 KL3-4a 10 20–490 −0.828 0.034 0.703 0.863 17.8 53.8 2.2 53.2 KL3-4b 10 20–490 −0.861 0.066 0.657 0.851 8.5 56.0 4.3 – KL3-4c 10 20–490 −0.823 0.038 0.674 0.831 14.7 53.5 2.5 – KL3-6a Rejected KL3-6b Rejected KL3-8a 9 20–490 −0.980 0.044 0.704 0.771 12.3 63.7 2.9 62.7 KL3-8b 10 20–490 −1.061 0.062 0.690 0.788 8.8 69.0 4.0 67.0 KL3-8c 9 20–490 −1.155 0.028 0.699 0.762 19.0 75.1 1.8 74.5 Meanvalue Hm=61.4±6.3 Hm CR=61.3±6.0 KilnKL5 KL5-1a Rejected KL5-1b Rejected KL5-2a 5 20–425 −0.720 0.024 0.738 0.656 20.2 46.8 1.6 – KL5-2b Rejected KL5-3 8 20–490 −0.961 0.063 0.651 0.554 5.7 62.5 4.1 61.8 KL5-4a 10 20–490 −1.006 0.130 0.727 0.789 4.4 65.4 8.5 – KL5-4b 9 20–490 −0.800 0.065 0.908 0.582 8.1 52.0 4.2 – KL5-6a 11 20–490 −0.851 0.055 0.962 0.790 13.8 55.3 3.6 54.9 KL5-6b 11 20–490 −0.991 0.058 0.683 0.855 10.1 64.4 3.8 62.0 KL5-6c 10 20–490 −1.156 0.043 0.736 0.852 14.6 75.1 2.8 73.4 KL5-6d 5 20–400 −0.853 0.017 0.690 0.591 24.0 55.4 1.1 – KL5-7 10 20–490 −0.926 0.027 0.802 0.864 25.7 60.2 1.8 59.3 KL5-8a 10 20–490 −1.016 0.027 0.675 0.855 21.4 66.0 1.8 – KL5-8b 8 20–490 −1.050 0.023 0.669 0.816 23.7 68.3 1.5 – KL5-9a 9 20–490 −0.890 0.052 0.557 0.863 9.2 57.9 3.4 57.8 KL5-9b 9 20–490 −1.060 0.013 0.841 0.855 55.3 68.9 0.8 67.6 KL5-11a 9 20–490 −1.014 0.016 0.897 0.835 46.8 65.9 1.0 65.1 KL5-11b 8 20–490 −1.004 0.027 0.772 0.846 24.2 65.3 1.8 63.9 KL5-12a 6 20–425 −0.711 0.030 0.865 0.722 20.8 46.2 2.0 – KL5-12b 6 20–425 −0.703 0.017 0.853 0.720 36.1 45.7 1.1 – KL5-13a 5 20–425 −0.730 0.043 0.804 0.698 13.1 47.5 2.8 – KL5-13b 5 20–400 −0.766 0.031 0.671 0.747 16.2 49.8 2.0 – KL5-14a 9 20–490 −0.890 0.115 0.682 0.360 2.1 57.9 7.5 57.5 KL5-14b 9 20–490 −0.974 0.028 0.758 0.780 21.1 63.3 1.8 63.0 KL5-15a Rejected KL5-15b Rejected Meanvalue Hm=63.6±5.9 Hm CR=62.4±5.2

Columns:Specimen;N:thenumberofheatingstepsusedfortheintensitydetermination;Tmin–Tmax:minimumandmaximumtemperaturesusedfortheintensity

determination;m:slopeofthebestfit;sigma:standarddeviationofm;f:thefractionofNRMusedforintensitydetermination;g:thegapfactor;q:thequalityfactoras

definedbyCoeetal.(1978)[22];H:Archaeointensitybeforeanycorrection;␴:standarddeviationofH;Hcr:Archaeointensityaftercoolingratecorrection.Themeanvalues

foreachkilnhavebeencalculatedonlyfromspecimensthathavepassedallselectioncriteria(seetextformoreexplanation).

detected.Similarsignalswithoutanomalousfadingformaterials includingfeldsparswerealsoreportedintheliteratureby[39]and

[40].Thislackofanomalousfadinginconjunctionwiththeglow curveprominentpeaks,typicalofquartz,suggeststhedominant presenceofquartzinthestudiedmaterial.

4.3. Doserateassessment

Thedoseratewasassumed tobemainlyderived from natu-ralradioactivityinthekiln.Theannualdosecanbecalculatedas thesumofcontributionstothedosefromalpha,betaandgamma

Table3

AsummaryoftheTLdatingdata,thecontentofnaturalradio-nuclidesaswellastheexperimentallyobtainedagesforeachindividualkilnfragment.Eachvalueisaccompanied bythecorrespondingerrorinsideparenthesis.

Sample DE(Gy) PlateauT(◦C) I(Gy) DE+I(Gy) U(ppm) Th(ppm) K(%) DR(Gy/ka) AgeBP(years)

KilnKL3 KL3-2 8.11(0.53) 95 – 8.11(0.53) 4.12(0.15) 6.48(0.25) 1.15(0.02) 4.102 1977(±175) KL3-4 8.93(0.62) 95 – 8.93(0.62) 4.62(0.18) 6.94(0.27) 1.21(0.03) 4.472 1997(±183) KL3-6 8.52(0.64) 90 – 8.52(0.64) 4.56(0.25) 7.12(0.22) 1.15(0.03) 4.406 1934(±199) KL3-9 8.42(0.63) 90 – 8.42(0.63) 4.53(0.19) 6.82(0.25) 1.13(0.02) 4.329 1944(±195) KilnKL5 KL5-1 5.84(0.36) 110 0.77(0.11) 6.61(0.47) 2.89(0.10) 5.68(0.19) 1.07(0.01) 3.305 1999(±179) KL5-7 6.05(0.35) 95 0.82(0.17) 6.87(0.52) 3.02(0.13) 5.58(0.18) 1.11(0.02) 3.396 2023(±201) KL5-9 5.78(0.29) 100 0.75(0.12) 6.53(0.41) 2.62(0.14) 5.46(0.22) 1.09(0.02) 3.163 2058(±187) KL5-12 5.61(0.33) 110 0.75(0.13) 6.36(0.46) 2.56(0.09) 5.72(0.17) 1.06(0.02) 3.139 2026(±189)

Fig.6.Archaeomagneticdatingresultsfor(a)KL3and(b)KL5kilns.Datingintervalshavebeencalculatedat95%ofprobabilityusingthematlabarchaeodatingtool[29].

particlesgeneratedduringradioactivedecays.Inthestudiedkilns, thecontributionofthegammaraysmainlyarisesfromthemain bodyofkiln(seesamplespositioninFig.1).Aboutonegramof untreatedclayfromeachsamplewasemployedtoperformthick

sourcealphacountingwithaZnSdetector.Themeasurementswere performedbothintheintegralandinthepaircountingmode,for thediscriminationbetweenThandU[34].It wasassumedthat Uand 232_{Thconcentrations wereuniformlydistributed allover}

0 100 200 300 400 500 1000 10000

_(c)

(d)

(b)

TL

(a.u.)

KL5 - 12

(a)

Fig.7. a:Naturaland(b-d)natural-plus-betadoseglowcurvesforsampleKL5-12. Theadditivedosesdeliveredwere7,15and22Gy(curvesb,candd,respectively). Reheatshavebeensubtracted.Eachglowcurveplottedistheaverageofthree indi-viduallymeasuredglowcurves.

thesample.Measurementswithdurationlongerthan5dayseach werecarriedout,accordingtothemethodologyproposedby[8]. Thesamplegavesealedoverunsealedratioof1.054,whichis con-sideredastorepresentinsignificantRnescapeunderlaboratory conditions[8].Thek-factor,i.e.theefficiencyofthealpha parti-clescomparedtobetaparticleswasadoptedtobe0.1[41].Table3

presentsalsotheoutlineofthedoserateassessmentprocedure.

4.4. TLdating

TLdatingwascalculatedseparatelyforeachoneofthefour sam-plesperkilnandtheobtainedresultsatsamplelevelareanalytically presentedinTable3.Basedontheseresults,thefinaldatinginterval foreachkilnwascalculatedasthemeanvalueofthefourstudied samples:thisis1963(±29)(±96)BPforKL3kilnand2027(±25) (±98)forKL5kiln.Eachmeanvalueisaccompaniedbytwoerrors: a)thestandarddeviationonthemeanvalue;lowvaluesindicate therepeatabilityoftheindividualagesyieldedfromeachsample ofthesamekilnandb)theerrorestimatedaccordingtoeach indi-vidualerrorvalue(around200years)accordingtostandarderror propagationanalysis[37].Takingintoconsiderationbothofthese errors,theageofthelastfiringofKL3kilnis46BC-146ADandfor KL5kiln112BC-84AD.

5. Discussion

ThebricksamplescollectedfromthetwokilnsatKatoAchaia, provedtobeverygoodrecordersof boththepastgeomagnetic fieldandtheTLsignal,suggestingthatthesetwodatingtechniques canbesuccessfullycombinedinthecaseoffiredarchaeological structures, suchas kilns. Directional and archaeointensity data havebeensuccessfullyobtainedforallstudiedsampleswithonly 7rejectedarchaeointensitydeterminations.Thefullgeomagnetic fieldvectorhasbeenusedforthearchaeomagneticdatingofthe twokilnsandtheobtainedageshavebeencomparedwiththeTL dating(Fig.9).ArchaeomagneticandTLdatingresultsareinvery goodagreementsuggestingthatbothkilnswereinuse contempo-raneously,andabandonedmostprobablyattheendofthe1stBC andthebeginningofthe1stADcentury,eventhoughtheKL3kiln couldhavebeenabandonedslightlylater.Theseresultsarealsoin verygoodagreementwiththearchaeologicalfindingsinthesite. Thisstudyshowsthatarchaeomagneticdatingbasedonthefull geomagneticfieldcangivemorepreciseresultscomparedtothe datingbasedonlyondirections[19]andcanofferaverypromising datingtoolforarchaeology,mainlyforthetimeperiodsforwhich

200

300

400

0

5

10

15

20

Additive Dose (Gy)

D_E (A) 0 1 2 3 4 5 6 7 0 1000 2000 3000 4000 5000 6000

Second Glow TL (a.u.)

Regenerative Dose (Gy)

I (B)

KL5 - 12

D

(Gy)

T (

C)

Fig.8. Equivalentdoseplateauplottedversustemperaturewherethewide tem-peraturerangeoftheplateaucanbenoticed.Insets:ArepresentativeNTLplusbeta (filledsquares,insetA)andregenerated(secondglow,filleddots,insetB)plotforthe temperatureof300◦_{C.Thearrowsindicatetheequivalentdoseandsupra-linearity}

correctionfrominsetsAandBrespectively.

adetailedreferencecurveisavailable,asforexampleHellenistic andRomanperiodsinEurope.

UsingtheindependentdatingprovidedbytheTLresults,the newarchaeomagneticdatapresentedherearecomparedwith pre-viousliteraturedatafromtheBalkanPeninsula[42].Allavailable dataforthe200BCto200ADperiodhavebeenrelocatedto Thessa-loniki(40.60o_N,23.00o_{E)andplottedinFig.10,togetherwiththe} BalkanSVcurvesandtheSCHA.DIF.3KEuropeangeomagneticfield model.Suchcomparisonshowsthatthenewdatafitverywellto theperviousliteraturedataaswellastotheavailableregionaland EuropeanSVcurves.Theyarehighqualitydataandcanbeusedas referencepointstotheGreekSVcurvescontributingtothe enrich-mentoftheGreekdataforthe1stADcentury,forwhichonlyvery fewdirectionaldataareavailable[7].

This study shows that the combination of archaeomagnetic andTLstudiescanbeaverypromisingtoolforbotharchaeology andgeomagnetism.ArchaeomagnetismandTLpresentthegreat advantagetodateexactly thesameeventthat isthelast firing ofabakedclayarchaeologicalartefactandcanthusofferprecise crosscheckeddating,particularlyimportantinthecaseofrescue excavationswherethearchaeologicalsiteusuallygetdestroyed, preventinganypossibilitytofurtherinsituinformation acquisi-tion.Atthesametime,datinginformationofferedbyTLcombined toarchaeomagneticinvestigationofthesamematerialcanbeused forreconstructingthepastgeomagneticfieldvariations.Thisisvery importantmainlyforthetimeperiodsforwhichonlyfewwelldated archaeological findingsareavailable,andthus welldated refer-encearchaeomagneticdataaremissing,e.g.themedievalperiod in Greece. We hope that this study would encourage a closer

Fig.10.Thenewdeclination(up),inclination(middle)andintensity(down)data obtainedinthisstudyplottedtogetherwithliteraturedatafromtheBalkanareafor the200BC-200ADperiodandtheBalkan(redline)andSCHA.DIF.3k(blueline)SV curves.

collaborationbetweenarchaeologists,archaeomagnetistsandTL physicianscontributingtotherescueofourculturalheritageand improvingourknowledge aboutthepastEarth’smagnetic field variationsduringHolocene.

Acknowledgements

Dr. Christina Rathosi is highly acknowledged for assistance duringthefield sampling.Threeanonymousreviewersarealso acknowledgedfortheircommentsonourmanuscript.

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