Archacometry 14, 1 (19721, 119-126. Printed in Great Britain

THERMOLUMINESCENT DATING OF COLOMBIAN POTTERY IN THE YOTOCO STYLE

E. H. SAMPSON, S . J . F L E M I N G Research Laboratory for Archaeology and the History of Art, Oxford University, England

and W. BRAY Institute of Archaeology, Gordon Square, London, England

1. THE A R C H A E O L O G I C A L P R O B L E M S

(w. Bray)

The archaeological material discussed in this paper was collected in Colombia, South America, during the summer of 1964. The sherds submitted for TL dating were selected because the method promised a solution to two, unrelated, chronological problems.

a. The date of the Yotoco phase The Yotoco pottery style was first recognized in excavations and surface collections from the floodplain of the Cauca Valley, in the Colombian Andes close to the city of Buga (figure I), and has been described in a preliminary report (Bray and Moseley 1971).

In the present state of knowledge, the Yotoco phase represents the earliest occupation of the valley floor by groups of farmers living in villages of wooden houses. The presence of grinding stones suggests that maize was the staple crop, but there is no direct evidence for foodstuffs in the archaeological record. Monumental architecture was absent, but metal- lurgy was practised and pottery-manufacture attained a high standard. The most diagnostic ware, found in both domestic and funerary contexts, is decorated with black resist-painted designs over a red, orange, or cream background. The elaborate patterns are based on dot- filled circles, rosettes, arch-shaped designs, spirals, stepped panels and zigzags, triangles placed point to point, and narrow bands with small reserved dots. Vessel forms, colour scheme and decorative motives together add up to a distinctive and easily recognizable ceramic style (see plate 1).

There are no dated pottery sequences from neighbouring areas of the highlands, and no close parallels for the characteristic Yotoco ware. A few Yotoco-style vessels are known to have been found in the Quindio to the north, but their local context is unknown and the detailed chronology of the various Quindio (or Quimbaya) styles has yet to be worked out (Bruhns 1967).

If the makers of Yotoco pottery were the first people to cultivate this part of the Cauca floodplain, the chronology of the Yotoco phase is a matter of considerable interest.

Its stratigraphic position is unambiguous. In excavations at the site of Moralba, the level containing Yotoco sherds was overlain by sterile silt, above which was pottery of a later style which has been named Sonso. From Moralba and other localities in the Cauca and Calima Valleys there are now seven radiocarbon determinations referring to Sonso pottery

I20 E. H. Sampson, S. J . Fleming and W. Bray

and ranging from A.D. 1235k 50 (IVIC-160) to 1580f 70 (GrN-4695). These figures provide a terminus ante quem for the Yotoco style.

The Yotoco stratum at Moralba produced radiocarbon dates of 890 B.C. k270 (IVIC-597) and 800 B.C. & 50 (GrN-5762). These measurements are consistent with each other but are incompatible with a pair of determinations for identical pottery from excavations at the nearby Yotoco Ferry site. The Yotoco Ferry dates were A.D. 1175f65 (GrN-4694, from 1.60-1.70 m below ground surface) and A.D. 1 l 0 0 ~ 1 4 0 (IVIC-598, from 1.40-1.50 m) (see figures 2, 3).

There is no obvious explanation for the great discrepancy between the dates from the

Figure 1 Map of the Buga district showing the archaeological sites of Moralba (1) and Yotoco Ferry (2).

two sites. Experimental error is unlikely, since similar results were obtained from two different laboratories. The excavators were unable to recognize any source of contamination at either Moralba or Yotoco Ferry, but one set of dates must clearly be incorrect. On purely archaeological grounds the dates from Moralba seemed too old, while three radio- carbon dates for the Sonso occupation at the site came out just as expected. The twelfth century dates from Yotoco Ferry were archaeologically acceptable, but, in the absence of dated comparative material, there still remained an element of doubt.

The sherds submitted to the Oxford laboratory included all the most typical Yotoco types. Out of the eleven fragments sent for examination, five proved suitable for TL dating. The Moralba specimens came from the middle level of the Yotoco occupation, between 20 and 30 cm above the point which provided the radiocarbon samples, and the three datable

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Thermoluminescenf dating of Colombian pottery I21

sherds from the Yotoco Ferry excavations were from various depths between 1.50 m and 1.70 m below ground surface.

b. The date of cruciblesfiom La Quebrada The analyzed crucible is one of a pair said to have been found in a grave at Finca La Quebrada, Valledupar, Alto Barranco, south-east of Cali (figure 1) in the foothills of the Cordillera Central (plate 2). The crucibles are of different types. The example submitted for TL dating is a plain hemispherical vessel, 11 cms deep, and with the lip drawn out into a

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Figure 2 Simpl$ed archaeological section from excavation at Yotoco Ferry. The source location of sherds a 2, a 3 and a 7 used for TL analysis are as marked. An additional radiocarbon determination was made in the context 2a Of A.D. 1770 associated with later development at the site.

pouring spout. The second crucible, now in the Museo del Oro, Bogoth, lacks the spout but has a nick in the rim to direct the flow of metal. This specimen also has a horizontal groove 3 crns below the rim, designed presumably to take a handle made from a bent twig. Both crucibles had been used for the smelting of non-ferrous metals. The association of crucibles with other vessels in prehispanic tombs has been recorded elsewhere in Colombia (Arango 1924, I, p. 166; Restrepo 1892, p. 56), and the spouted example has a good parallel in an unpublished and unprovenanced crucible in the collection of the Museum of Bar- ranquilla (no. 51-120).

In the absence of associated pottery or material for radiocarbon dating, the TL method seemed to offer the best possibility for obtaining a date. Since the spouted crucible was already broken, a small sample was detached before restoration.

I22 E. H. Sampson, S. J. Fleming and W. Bray

The expected age was anywhere between A.D. 1 (by which date there is evidence of casting at San Agustfn in southern Colombia, Duque G6mez 1964) and mid-16th century, when Spanish influence made itself felt in this part of the Cauca Valley. A slightly later date would still have been acceptable, since the native technological tradition did not die out immediately.

0 1

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Figure 3 Simplified archaeological section from excavation at Moralba. The source location of sherds b 1 and b 3 used for TL analysis are as marked. Additional radiocarbon determinations were made in the context 6 of A.D. 1240, A.D. 1480 and A.D. 1550, associated with later development at the site.

2. THERMOLUMINESCENT DATING RESULTS

(E. H. Sampson and S. J. Fleming)

Six sherds were available from the Y otoco Ferry context, shown in figure 2(a 2-7). The TL characteristics of each were initially checked by a sample drilling. This led to sherds a 5 and a 6 being excluded from the programme, due to their very low natural TL light levels and insensitivity to radiation; also sherd a 4 was omitted since its behaviour was indistinguish- able from that of sherd a 2. Of the five sherds available from the Moralba context of figure 3 (b 1-5), sherds b 2 and b 3 showed negligible TL, while the low natural TL level of sherd b 5 meant that an upper limit only, of 450 rad, could be set on its archaeological dosage. This data is consistent with that obtained from the two remaining sherds b 1 and b 3 which received full TL analysis (see table 1).

Thermoluminescent dating of Colombian pottery 123

The TL dates obtained using the fine-grain method of sample preparation (Zimmerman 1971) are listed in table 1. The errors in age determination for each sherd were assessed following the approach of Aitken and Alldred (1972), assuming a water retention of the sherd and its burial soil at 80% of saturation level, and allowing uncertainty in that uptake of +20%. The k-value for all five sherds is notably high (compare with a typical level for Roman pottery of around 0.13), stressing the importance of the alpha radiation dosage component in the internal dose-rate estimate. This in turn puts particular weight upon the determination of the uranium and thorium content of the pottery and their radon retention properties. All the samples were stored for a fortnight in preparation for alpha-particle analysis (see Aitken and Alldred 1972), to allow build-up of radon towards saturation level in the pottery fabric. This storage revealed high levels of emanation for sherds a 2 (35%) and b 3 (60%). However, even these loss levels, which would be expected to occur in dry environmental conditions and which would influence the maximum possible age of those sherds, are unable to affect the overall conclusion that the TL dates for the Moralba sherds

Table 1 TL dating results for pottery in the Yotoco style

Sherd Saturation ref water uptake k*

(% of dry weight of

sherd)

Yotoco Ferry a 2 12.5 0.29 a 3 15.1 0.21 a 7 11.6 0.27 Moralba b l 13.0 0.27 b 3 20.8 0.31

Annual dose-rates ( m radiyr) Archaeological TL Age Effective Beta Beta Environmental f dosage (rad) (yr B.P.)

alpha (U,Th) (‘OK)

208 33 39 81 380 1050k 85 189 34 41 81 390 1130+ I 0 0 240 38 17 73 3 70 1010+ 85

238 38 51 72 470 1180+ 100 288 42 6 72 420 1030+ 100

k expresses the relative inefficiency of alpha radiation compared to beta or gamma radiation to induce

t Including a cosmic-ray dose component of 14 mrad/yr. therrnoluminescence.

are quite inconsistent with the radiocarbon dates for that site previously obtained (see figures 2 and 3). For sherd b 3 recalculation of a TL age, with the assumption of dryness and radon-lossy conditions, would increase the quoted 1040 yr of table 1 by only 130 yr.

In the case of the TL ages the most probable cause of a systematic error would be long- term variations of environmental conditions with changes of water retention and radon movement in the burial soil. However, it is possible to rule out extreme changes in the past through the TL study of quartz extracted from sherd a 7, using the inclusion method of sample preparation (Fleming 1970). Glow curve analysis of this extract (figure 4) gave an archaeological radiation dosage of 113 rad, due to the internal beta radiation dosage and the external environmental dosage. As the former is quite low (only 0.05 rad/year if the fine-grain age estimate is used to obtain a first approximation of that contribution) the quartz’s archaeological dosage strongly reflects the external environmental level and sets it close to 0.07 rad/year. Similar analysis of the quartz extract from sherd a 3 yielded a similar environmental dose-rate level of 0.09 rad/year. This data is consistent with the estimate of soil radioactivity obtained in laboratory analysis and used in table 1. Again it is not possible

E. H . Sampson, S . J . Fleming and W. Bray

3 o o \ ~ o w temp. scale f High temp. scale 120,

250

CI 2 200- c 1 0

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Figure 4 TL glow curves for the quartz extractedfrom sherd a 7: (a) Natural TL, (b) Natural TL + TL induced by an additional 220 rad of laboratory-applied beta radiation, (c) background red-hot glow.

100 1

0 100 200 300 400 500 Temperature ( " C )

Figure 5 TL glow curves for the La Quebrada crucible: (a) Natural TL, (b) Natural TL + TL induced by an additional 1 10 rad of laboratory-applied beta radiation, (c) background red-hot glow.

Thermoluminescent dating of Colombian pottery 125

to introduce enough variation in the environmental level into the TL age calculations to approach the disturbing 800-900 B.C. radiocarbon dates of Moralba that initiated this dating study. Unfortunately it was not possible to extract sufficient quartz from the re- maining three sherds (a 2, b 1, b 3), so a check of the environmental estimates used was not possible.

Finally, TL analysis was also carried out on the smelting crucible discussed in section 1 b. The archaeological dosage was estimated at 35 rad (figure 5). A minimum dose-rate estimate of 0.3 rad per yr (setting the unknown environmental component equal to the cosmic dose-rate alone) obtained from radioactive analysis of the crucible's clay, limits its period of last firing to the lust 120 yr. TL analysis suggests that that artefact is not relevant to the archaeological period under discussion.

3. C O N C L U S I O N S

(i) The TL dates for the three Yotoco Ferry sherds and the two Moralba sherds are strongly internally consistent at each site and between the two sites, in support of the archaeological evidence discussed earlier. Both sets of dates come out rather older than the radiocarbon results for Yotoco Ferry (which were archaeologically acceptable). However, the difference between the mean TL age of Yotoco Ferry (A.D. 900) and the older radiocarbon determina- tion (A.D. 1100) is adequately covered by the dating errors of the two techniques. (ii) The very recent TL date obtained for the crucible comes as an unwelcome surprise. The craft of metal-casting is not practised in the Ginebra region a t the present day, and there is no obvious reason why an ancient crucible should have been refired. Unfortunately the specimens were collected without TL dating in mind, and no information was obtained about the history of the crucibles after their alleged discovery. We were told that the grave was discovered and excavated by a farmer some time before 1964, and apparently contained other broken pots which were thrown away. Under the circumstances, there was no pos- sibility of checking this information, although it appeared reliable at the time. The material was acquired at the farmhouse and had not passed through the hands of dealers, nor is the Ginebra region a centre for commercial grave-robbing. Deliberate faking can probably be ruled out. So the TL information is at odds with the archaeological data-admittedly slight and unsubstantiated, and raises doubts about the antiquity of at least one of the crucibles.

A C K N O W L E D G E M E N T S

We wish to thank the Nuffield Foundation for financial assistance which was greatly appreciated.

R E F E R E N C E S

Aitken, M. J. and Alldred, J . , 1972, The assessment of error limits in thermoluminescent dating, Archaeo- merry, 14 (2 ) .

Arango, C. L. , 1924, Recuerdos de la Guaqueria en el Quindio, 2 vols. Editorial de Cromos, Luis Tamayo, BogotB.

Bray, W. and Moseley, M. E., 1971, An Archaeological Sequence from the Vincinity of Buga, Colombia. &uwpa Pucha 7-8, 1969-70, 85-103.

Bruhns, K. O., 1967, Ancient pottery of the middle Cauca Valley, Colombia. Unpublished Ph.D. dissertation, Department of Anthropology, University of California, Berkeley.

Duque Gbmez, L., 1964, Exploraciones Arqueolbgicas en San Agustln. Instituto Colombiano de Antro- pologla, Revista Colombiana de Antropologia Suplemenro No 1, 408, 242. Imprenta Nacional, BogotA.

I 26 E. H. Sampson, S. J. Fleming and W. Bray Fleming, S. J., 1970, Thermoluminescence dating: refinement of the quartz inclusion method, Archueomerry

Restrepo Tirado, E., 1892, Ensayo Etnogrhfico y Arqueoldgico de la Provincia de 10s Quimbayas en el

Zimmerman, D. W., 1971, Thennoluminescent dating using h e grains from pottery, Archaeometry 13 (l) ,

12 (2), 135-146.

Nuevo Reino de Granada. Imprenta de La Luz, Bogota.

29-52.