Revista Mexicana de Física 23 (974) FA87-FAI04 FAS7

THERMOLUMINESCENCE MEASUREMENTS IN MESOAMERICAN•POTTERY WITII A COMMERCIAL TL READER

A. Moreno y Moreno*. and Klaus Bccker

I/ea/lh Physics Division. Oak Ridge Naliona/ Laboralory

Oak Ridge. Tennessee, USA 37830

(Recibido: enero 10. 1974)

(Versión revisada: ocrubre 29. 1974)

ABSTRAer: The feasibiljty of an unmodified commercial TI~D reader(lIarshaw 2000) for authenticity tests of ancient ceramics was

demonstTated by TI.•measurements in seven diHerent typcs ofprecolombian potsherds reom various paMS of Mexico as well as

modeen pottery, including faked artifacts. The techniques for

carrying out such tests are described in detail, and the possi.

bilities of dating, detection of irradiat,"d fakes, and long .•term

integration of low environmental gamma radiation exposures by

reading the TI.. stored in the interior of large quartz grains Insamples of known age, are discussed.

Research sponsored by eh," U.S. Atomic Energy Commision under contraet withUn ion Carbide Corporarion••Visiting scientisr £rom Institute of Physics, Universidad Nacional Autónoma deMéxico.

FA88

INTRODUCTION

Moreno and Beeke1

Ancient ceramics, such as pottery, ccrracottas, bricks, and tiles aceoot onIy of scientific imponance for Que underscanding of rhe cultural andsocioeconomic history of roan, bul a150 frequently of great artistic, andcons~quendy monetary valuc. A Greek vase, rhe Metropolitan Museum' scontroversial erarer, recentIy changed hands for une million dollars. Notsurprisingly, faking of such relics became rachee widespread. Collectors,art dealers, and museum directors estima te thar currently 50 to 75% of rhesupposedly oId precolumbiao pots and s(atue s 00 rhe market are probablyrecent fakes. Families, and perhaps whole villages are in the business ofproducjng such fakes, employi~g aEtco rachee sophisticated techniques. Forexample, use of the same ciay, che same molds, and the sarne firing techniqueswhich were used in antiquity, and careful "aging" by burying, etc., makes italmost impossible to detect forgeries by visual or microscopic inspection,or chemical and mineralogical anaIysis.

ObviousIy, a reIativeIy simple method for testing the authenticity ofan art object is of great interest to archeologists, art' historians, museums,and art dealers and collectors, Measuring of the radiation-induced thermo.Iuminescence (TI.) signaI which has been accumulated in quartz and otherconstituen[s of ceramics since it was manufactured by firing (resuIting in ananneaIing ol all previous TI... signals) provides such a method. Althoughflrst suggested twenty year ago1 and studied for more than twelve years byvarious groups, up to now only about ten laboratories in the world were e-quipped to carry out such tests. The main reason was that the pioneers in[his fieId repeatedIy pointed out in their publications (for sorne good modernreviews, see re£. 2-5) that highIy sophisticated and correspondingly expensiveTI... equipment with a vacuum system, pIJoton counter, etc., would be requiredfor this type ol studies, thus discouraging smaller laboratories and less ex-perienced investigators. Commercially available Thermoluminescence Dosi-metey (TLD) instruments have, however, been considerably improved in recentyears (for a compilation, see rel. 6). It was therefore,one of the primarygoals of this study to determine whether one ol [he better and more widelyused commercial TLD readers, the I1arshaw 2000 instrument (several ol whichare presently in use in Mexico City) could be employed for such investigations.

The next steps in this investigation about which sorne preliminary re-sults also are included in this report will inelude:

l. To check the feasibility ol unrnodified or modified comrnercial TIJDreaders for actual age determination within reasonable limits oíaccuracy (t 5-10%). TL dating is gaining rapidly in importance

Tbermoluminescence measurements ••• FA89

because of the well ...known difficulties in obtaining suitable samples,and disagreemcnt in the results of other absolute dating techniques,such as l"e, dendrochronology, obsidian hydration, and fissiontrack dating.

2. Assuming that fakers already have, or may begin soon to irradiatetheir products in order to simulate a greater TL age, we intend toestablish simple and reliable methods to distinguish between re ...cently irradiated and truly ancient samples.

3. It should be determined if there is a stable Thetmally StimulatedExoelectron Emission (TSEE) signal in those samples, whetherit can be used for dating, and how it compares with the TL signals.

4. For the first time, we will al so attempt to use the TL signal whichis produced by penetrating environmeotal radiatioo to determineeovironmental radiation levels averaged over hundreds or thousaodsof years, in order to compare them with current levels and annualor seasonal fluctuations.

The principIe of this lalter method IS simple .. The age of a samplecan be determined 00 the basis of the equation

age = TL signal/(av. dose-rate x sensitivity)

With a sample of known age, this equation can be reversed to result 10

av. dose-rate = TL signal/(age x sensitivity)

We intend to use large (> 100,um) quartz grains which are koown to containalmost no radioactivity and'can, therefore, be used as a reliable detectorfor the long-range gamma radiation, after the alpha radiadon effect at theirsurface has be en eliminated by etcllÍng away the alpha. exposed surface layerwith hydrofluoric acid, and the "internar" beta radiation dose has been sub ...tracted.

EXPERIMENT

In aH experiments, a standard lIarshaw 2000 rcader with a factoryprovidcd slightly extended temperaturc range (500 instead of 400oC)* and a

•T-he heater as supplied with the standard instrument would actually be capable ofreaching temperatures of almost IOOO"C; the high-temperature modifications conslstof a few simple measures, such as a change in the galn of the thermocouple ampli-fier, repJacement of the temperature meter, and re-ealibration.

FA90 Moreno and Becker

plarinum-iridium "permanent'" planchct was employed. Unless mentionedothcrwisc, a red illumination (reJ-painted light bulbs) was used during samplepreparar ion, hecausc sorne of (he samples sccrncd to respond [O the normalfluorescenl Iaboratory light. Becausc small traces of oxygen are known (Oproduce a ¡arge spurious signal, lhe rcadout c1uunhcrwas Oushed fOf no lessthan 30 seconds with pure n¡trogen prior to rcadoul. Longer f1ushing did ootimprove the results; neither did additional washing of the n¡trogen in aconcentrated aqueous solution of KOII and pyrogallic aeid (v.'hich is known toremovc traces of oxygen Croro gases) fol1owedby desiccation of me gas streamwith calcium chloridc. Ir has beco suggested7 ro mix the materials with aheat.resistant silicone oil to minimize environmental effects. We found,however, that the oil used for lhis purpose, begins to evaporate when heatedfor an extended period ro 450-500° C, thus possibly contaminating dle interiorof the TL reader.

The heating rate was ~lffiost linear and about 20°C/sec, the heatingtime 30 sec., the photomultiplier vohage 750 V, the range for recording theglow curve usually 10.8 or 10..9A and ((or the heater set at 9.0) the maximumtemperature"" 450°C. In sorne instances, samples h.ave also been read at alower maxlffium temperature of about 400°C (heater set at 7.5). This stillreleases most of the ancient and modern TL, but reduces dramatically thebackground signal, and is, therefore, morc suitable for dctcrmining the arcaunder ,he glnw peak.

As the temperature indicadon of the reader is not considered very ac-curate, no ternperatures are given in the graphs ((or orientation purposes: theleh end of the graphs corresponds ro "" 25 ° C; the main stable peak of quartzis in the 300-350oC range'; background bccomes substantial a' 350'0 400°C;and the curves break off al "" 450°C). The recording rate in a plotmaticX -y plouer was 0.5 cm/ sec. Switching to the operation mode of the readerto photon counting would ha ve be en easy by adding a preamplifier and a scaler,but it was not considered neccssary because none oC the disturbiog "bursts"which have been reponed by others2 have been observcd

Pottery samples wcre taken by eithcr breaking oc sawiog a picce a fewhundced mg in weight off (rom a larger potshcrd oc figure, cemoving the upper-most"" 1 mm which have been exposed to intense light, and crushing thesample gently bctween aluminum foil with pliers. This was followed by

•Although only a broad single pea~ is seen at the high heating rate used in these ex-periments. it should be noted that it actually consists of tWQ peaks at 325 aud375°C, with half-lives at 20°C of -3x 103 and.- -ix 107 years. respectively2 ..

FA91

sicving, lhe 107 lo 175¡.Lrn size fraetion being used foe mose of [he expertAmcnts because ir contaios rhe large quauz graios which are of inrcres( foccnvironmental measurements of long-range gamma radiation. Most of rhe darkiron.rich particles in mis size fraction ("'-10 to 60% of (he sample) were removedby gently shaking the powdet on a glossy paper sheet between the poles ofa strong perrnanent magneto

Washing with ..••10% IICI, followed by washing with distilled waterand alcohol oc aceroue, and drying al 50-80° e inirially completed rhe basiecleaning procedurca There remained, however, apparently a layer of impuri-des on rhe partic1e surface which absorbs pan of rhe TL I¡ght. This canbe removed by etching for 0.5 to 2 minutes with diluted (1-2%) IIF. Etching¡ncreases [he light ourput in rhe large graios by up co a factor of (hree fOfboth the ancient and the newly induced TL signals (Fig. 1). In small grains,

XOUIC1250-950 BC107-1'75Jlm

10

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FA92 Moreno and Beeker

[he effect is even more pronounced (as an example, rhe effect of 0.5 mino 01etching in"" 1% UF al 20°C is given in Fig. 2 foc ehe

Thermolumi"escence measu,ements ••.

20 I S

FA94 Moreno and Hecker'

fluorescent laborawry light al50 produces a low-temperature peak, probablydue to a trap transíer process). Consequendy, an overnight (l8-24h) post.irradiation annealing al 800 was adopted fOf all further cxperimenrs, unl~,snoted otherwise. As can be seco in rhe following graphs, (his annealingproceJur:l;' ¡ndeed produces a glow-curve which is almase identical wirh rhe"natural" ancient signal.

Unlike aH orhcr TI. dating laborarorics who use beta radiation sourcesfOf low Linear Energy Transfcr (LEY) calibration irradiations, we employeda 6OCo gamma radiation soucee (close-rate 10 R/sec) fOf rhe un¡form exposureof OUT samples. le was confirmed in (he experiments rhar

a) [he inherent sensiriviry of rhe quartz graios varies froID sample tosample; and

b) the supralinearity of the radiadon response does not remaln thesame when an annealed sample is re" irradiated.

It can be seen in Fig. 4 that irradiadon (followed by the standard overnightannealing at 800 C) in addition to the ancient signal yields, by extrapolation,an estimate of the ancient radiation dose. If the same sample is re" irradiated

ORNL-DWG 73-12066

RE-EXPOSED AFTERANNEALlNG AT 500°C

300020001000EXPOSURE (Ryl

o

GUAMUCHALBLANCO DIFERENCIADO1250-950 BC107-175pm

IEXPOSUREOF UNANNEALEDMATERIAL

5

6

O-1000

en><

Thermo/umit'Jescence measuremet'Jls ••• FA95

after annealing at '5000 C, its supralinearity becomes more pronounced.Alpha irradiations of a few samples were carried out by coveri~a

'39pu layer electroplatcd on a metal disc by 10!-,m Kimfol (a polycarbonatefoil), on which (he grains are spread in a monolaycr and ocassionally movedslightly for homogeneous exposure. The inhcren( alpha activity of sampleswas measurcd by covering a ,hick (> 3 mm) laycr of lhc powdered samplewith a 4.'52 cm2 (24 mm diame(cr) ZnS:Ag scin(illa(or foil, enclosing i( with athin polycarbonatc foil, and counting overnight witll a photomultiplier in oneof the devices which arc useJ for roudne bioassay alpha counting at ORNL.In the Xquic matcrial for example, a nct count of 0.144 cpm would amount,according to thc calibradon factors given in reL 2, to an internal alpha radi-ation exposure of 1.17rad/year. In a modero sample (a small ceramic dogmade by highland indians in Chiapas) the count-rate was almost twice ashigh (0.26 nel cpm).

The internal exposure of the sample to beta radiadon was detcrminedwi,h a sandwich of a ,hin (- 0.5 mm) layer aboul 5 cm' in area of CaSO, :Dy,enclosed in a 0.05 mm (- 6 mg/cm') polye,hylenc foil'. ldenlical pOlsherdscovered both rides of this detector. The foil stops most of the alpha parti-eles, but permits the penetradon of almost all the beta radiation. (Accordingto ref. 4, a thicker layer of 30 rng/cm2 would be even beuer for this purposeand will be used in future experiments). A referencc sample of freshly an-nealed CaS04 :Dy was left in a plastic botde in order to subtract the environ-mental gamma radiadon exposure. Alter 3.75 months of exposure, theCaS04 :Dy was read out as described earlier8• '111einternal beta dose amountedro 95.6mR/ycar in rhc Xquic potshcrd, and 58.1 mR/ycar in the modernChiapas dog (aftcr subtraction of the external background radiadon of54.8 mR/year).

RESULTS AND DlSCUSSION

In Table 1, the samples which are discussed in this repon are listed.They were selected in order to c()ver a wide range of times and locations inMesoamerican archaeology, with reladvely well-established ages. Theyoungest sample(a.Tenochritlan Black 00 Oraogc Azrcc 11I, 1350-1500 A.D.from Lomas de Sanra Cruz, Puebla) has nor yet beco inc1uded in the study.In addition, numerous samples from rhe colJecdon ol one of the authors (K. B.)and his acquaintances were tcsted, wirh mosr samples giving a rarher clear"oId" or "new" resulto A few amoiguous cases may be expIainabIe with low

FA96 Moreno and Becker.

firing temperatures (insufficient annealing oí deep filled craps in the quartz,followed by retrapping), or abnormally high radioactivity in the clay. Re-fining oí old ceramics would, oí course, al50 make them look new. No ev.kdence for irradiated fakes has yet been found.

TAHLE 1

List oí Precolumbian Potsherds from Mexico Used in [his Study

No.

2

3

4

5

6

7

Designation Phase Location Date

Guamuchal Cuadros Aquiles Serdán. 1250-950 B. C.Blanco Diferenciado Chiapas

Xquic Cuadros Aquiles Scrdán, 1250-950 B.e.Chiapas

Michis Tuztlan Red Deos Aquiles Scrdán, 1350-1250 B.e.Chiapas

Thio Orange Teotihuacan La Ventilla, 350-500 A. D.11-11I México

Burnished Brown Teotihuacan La Ventilla, 400-

Thermol ••",irJescence ",eas ••re",enlS ...

GUAIIUCHAlIIUNCO OlFERENC1AOO12~O-950lIe107-175,_

ORNL-OWG 13~

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15

10

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NATURAL+600Rr

TEMPERA TURE

Fig.5. Guamuchal '-Blanco diferenciado" (Table 1, No. 1) l07-175,um largegrain fraction, magnetically c1eaoed and etched for 1 mino in 2% UF;TL signal duriog f¡rst heating of an unirradiated sample (natural TL),oC the background during the second reading oC the same sample, andof a pteviously unheated sample whichhas beeo exposed to 600R of6OCo gamma radiation for calibration purposes, and stored overnight

at BOOe in order to anneal the low-temperature peaks. Each curverepresents the average oC at least three individual readings oC- 20 mg samples, each.

FA98

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Moreno and Becker

Fig.6. Michis Tuztlan Red (Table 1, No. 3), treated as described in Fig. 5.

TlOfMJll,C,UlII-m--400-600"0lOT-I1!>I''''

..

o

Fig.7. Teotihuacan IJI-IV Burnished Brown (Table 1, No. 5). treated asdescribed in Fig. S.

Thermolumjnescence measu,emenls ...

CHOLUL.I. POlYCHl¡OlIlE1100-1200 .1.0101-11~ }1m

FA99

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.~.-~~5

o

Fig.8. Cholula Polychrome (Table 1, No. 7) I treated as described In Fig. 5.

TEQTltlJl(;,AN u-mTliIN OIW«;Em-m AO107 175}1m

"

~2••

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Fig.9. Thio Oraoge from Teotihuacan 11-111(Table 1, No. 4), ueated as lO Fig. 5.

FAIOO Moreno and Becker

10 TEOTIHUACAN lATE NBURNISHEO REO ON BUfF600-7'50 AOJ01-I75vm

oTEMP£RATUR£

Fig. 10. Burnished Red 00 Buff from late Teotihuacan IV (Table 1, No. 6),treated as in F ¡g. S.

TABLE 11

Doses and Dose.rates in Various Mesoamerican Potshercls

Sample No. • Method(2) Dose Estimated Date Dose- rateR Age (y) (y) (mR/y)

1 a 38S 3070 3223-2993 125b 380

2 a 653 3070 3223-2993 2133 a 565 3270 3323-3223 175

b S80a 133 1470 400~00 120b 220

6 b 324 1300 600-750 2S0• See Table ICor sample description.(2) a = peak height. b = peak area with maximum temperature .- 400.C.

The,moluminescence measu,ements ••• FAIOI

a. the etching procedure as used in dIese tests did not remove mostof the alpha radiation signal which is accumulated primarily inthe grain's surface ~extended etching and/or a higher UF concen.tration would be required to remove most oE the alpha.exposedsurface layer); and

b. the internal beta dose (mostly due to potassium) has to be sub.tracted, which would, for example, reduce the high reading oEsample No. 2 (Xquie) to a more realistie 117 mR/y.

lo Fig. 11, lhe results from a pieee of modelO 'Mexieao pOllery (a IOYdog from Chiapas made aboul lhree years ago) are giveo. Allhough lhere isa difference between the first and the second heating of the sample, no signalcan be observed in the peak region. In other cases of modern pottery, suchas a faked piece oE "ancient" pouery wlJich was purchased Jast year in Quito,Ecuador, not even a trace of a signal could be found in the unirradiated sample(Fig. 12). Other specimen which were assumed to be Eakes turned out, onlhe olher haod, to be aulheotie. Pieees of Cholula Polyehrome offered bychildren to tourists in the Cholula area were, for example, identical in theirTL to the authentic potsherds.

10

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CHIAPAS DOG 11l00ERN)107-173pm

TEIlPERATURE

ORM.-DWG 73-12063

Fig. 11. !L oE. a modern piece oE Mexican pottery, a toy dog made by Indiansin ChIapas three years ago (sample preparadon as described in Fig. S).

FA 102

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of lJ Of Th compounds ,,'ould, of course, be easil)~ detectable. Srill, rhepossibilit}, of enoes canDor be completely cIcluded in authenticit)O tests.Foe cIample, valuable old specimen are olteo bcin,e repaiced .¡lh moderoceramlc. lf a sample is ralíen from a rerJ.~r("d an"a. !his ;¡,ouM indic3U' amodero falce. Carerul interpretaríon of Tl~aurhenticiry checks ¡5, therefore,esscnlial.

SiOz (SEASANO.44-1061"'"A_",mAT5OO"C1

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15

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2 ••• 2'l1.HF

EltItilI7-30 •••5HF

Fig. 13. Glow cunes of 44-106p. quartz «rajos (sea s&Illd, pl"r-oeaaealcd al 500.C).lIec esposure lo Zl9Pu alplla radiarioa. (lf:ft) aocI6Deo£_. radiarioo •.• ithout aod aÍl:er elcbiDI .ith 2% UF f. diffn~t ti.es &1.- 25.C.

Preliminary results on me thermally süna.1ating ~oelectrm CIIIissioo(TSF.F.) 01

FAI04

ACKNOWLEDGMENTS

Moreno and Becker

We appreciatc discussions with numerous colleagues, su eh as~t.J. Aitken, Oxford University; 11. Barker, British Museum; and Kaplan.Arte Primitivo, New York. The samples, as well as valuable advice, wereprovided by J. Li,vak King, Depattment of Arehaeology of UNAM, andN. Castillo Tejero, Musco Nacional de Antropología, México .

•REFERENCESI. F. Daniels, C.A. Boyd, and n. F. Saunders, Science 117(953) 343.2. M. J. Aithen and S. J. f1eming, "Thermolumincsccnce Dosimctry In

Archacological nating" in F.IL Anix, ed., Tapies in RadíationDosimetry, (Acaclemic Prcss, New York, 1972).

3. II.N. Michael and E.K. Ralph, Daling Techniques lar Ihe Archae%gisl,(MIT Press, Cambridge, Mass., 1971l.

4. V. Mejdahl, Dosimetry Techniques in ThermoJuminescence Dating.Riso-Rep. 261, Danish AEC, Riso, Roskilde (972).

5. J. W. Miche1s, Daling ,\/ elhods in Archae%gy, (Seminar Press,New yo,k and London, (973).

6. K. Becker, Solid-Slale Dosimelry,(CRC Press, Cleveland, Ohio, 1973).7. M. C. lIan, University of Pennsylvania, personal correspondence.

(Dr. lIan also provided samples of his silicone oil).8. K. Ilecker, Nue!. lnstr. Me,h. 104 (972) 405.

RESUMEN

•For more complece bibliographies on TL dacing, see reference liscs in re£. 2-5.