Hyperfine Interactions 29 (1986) 1113-1116 1113

MOSSBAUER ANALYSIS OF RECENT CERAMIC FINDS FROM CHAVIN U. WAGNER, F.E. WAGNER, A. STOCKKLAUSER and R. SALAZAR Physik-Department, Technische Universitdt Munchen, D-8046 Garching, Germany J. RIEDERER Rathgen-Forschungslabor, Staatliche Museen Preuflischer Kulturbesitz, D-IO00 Berlin, Germany F. KAUFFMANN-DOIG Universidad Villareal, Lima, Peru

A t a r g e number o f c e r a ' n l c s h e r d s f r o m C h a v i n , P e r u , as w e l l as r e c e n t c l a y s a m p l e s f rOm the same r e g i o n have been s t u d i e d by M o s s b a u e r s p e c - t r o s c o p y . The changes o f t h e M o s s b a u e r s p e c t r a o f t he c t a y w e r e s t u d i e d as a f u n c t i o n o f f i r i n g t e m p e r a t u r e and a t m o s p h e r e . F rom a c o m p a r i s o n o f t h e s e data w i t h t h o s e f o r m t he s h e r d s one can make e s t ima tes o f t he p r e c o l u m b i a n f i r i ng c o n d i t i o n s .

I. INTRODUCTION

During the years 1980 and 1981, a large amount of ceramic sherds was excavated from an interior corridor of the pyramid Tello in the precolumbian ruins of the sanctuary of Chavin in Northern Peru. The trace element contents, determined by neutron activation analysis for 76 of these sherds and for local clay, indicate that most of these ceramics are of local origin, while about 20 percent are different /1/, presumably because they were brought to the sanctuary as offerings. The M0ssbauer spectra of these ceramics do, at least to some extent, confirm this classification /I/. We now report on first results of experiments undertaken in an effort to use Mossbauer data for the assessment of the firing conditions of the ceramics from Chavin.

2. EXPERIMENTS

To this end the firing behaviour of a recent clay collected near the excavation site was studied. Ceramics of the typical Chavin style often exhibit differently coloured layers, with brownish gray cores and orange or dull yellow surface layers. This indicates that Chavin ceramics were often fired under reducing conditions, at least during part of the firing cycle. In order to obtain reference data on the firing behaviour of clay from the chavin region, we fired samples of a clay collected near the excavation site in air as well as in a reducing atmosphere. The latter was produced by putting charcoal to- gether with the clay into a quartz crucible covered by a closely fit- ting lid. Samples were fired at temperatures increasing in 50~ steps between RT and 1200 ~ A new sample was used for each temperature. The firing time was 48 h for the firing in air and 3 h for the reducing treatment. The Mossbauer spectra were measured at 300 K and, in some cases, also at liquid nitrogen and liquid helium temperatures. To emulate the making of ceramics first fired in a reducing atmosphere and subjected to oxidation at the end of the firing cycle, clay was first fired under reducing conditions at 800~ for 3 h and then sub- jected to the oxidizing refiring procedure described above.

3 . RESULTS AND D I S C U S S I O N

The results of these experiments are shown in Fig. I. The most relevant Mossbauer parameters are the quadrupole splitting of the Fe 3" doublet (QQI), the nonmangetic fraction in the spectra (A~), and the fraction of Fe 2" (A~..2). The latter is about 15 % in the fresh clay and

�9 J.C. Baltzer A.G., Scientific Publishing Company

114 U. Wagner, et al., MOssbauer analysis o f recent ceramic finds from Chavin

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F~g. 1. F ~ r ~ n 9 b e h a v t o u r o f a c l a y f r o m the r e g i o n o f C h a v i n . On t he r ~ g h t the M o s s b a u e r p a t t e r n s o b t a i n e d a t 3 0 0 K a f t e r f i r i n g a t d i f f e r e n t t e m p e r a t u r e s a r e shown. On the t e f t t he s p l i t t i n g QQ1 o f t he Fe 3§ d o u b - l e t and the r e t a t ~ v e i n t e n s i t y o f t he n o n m a g n e t i c f r a c t i o n ( A ~ ) and o f the o f t he F ~ § s p e c i e s (Art,-2) a r e p l o t t e d as a f u n c t i o n o f t he f i r i n g t e m p e r a t u r e . F~r Jng ~n a i r Js shown on t o p , r e f i r i n 9 in a i r a f t e r r e d u c i n g at 8 0 0 ~ in the m l d d t e , and f i r i n g unde r r e d u c i n g c o n d i t i o n s b e l o w . (2,Q2, G~3 and ( ~ 4 a r e the q u a d r u p o l e s p [ i t t i n g s o f t he t h r e e F ~ § q u a d r u - p o l e d o u b l e t s t h a t can be d i s t i n g u i s h e d in some o f t he s p e c t r a o b t a i n e d a f t e r f ~ r r n 9 ~n a r e d u c i n g a t m o s p h e r e .

U. Wagner, et al., Mdssbauer analysis of recent ceramic finds from Chavin 1115

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F i g . 2. P t o t o f the Fe =~ q u a d r u p o [ e s p l i t t i n g ~ 1 v e r s u s the nonmagne t i c f r a c t i o n A ~ f o r a v i r g i n c t a y f rom Chav in (upper l e f t ) and f o r the same c t a y p r e v i o u s l y f i r e d r e d u c i n g t y at 800~ (upper r i g h t ) . The da ta were o b - t a i n e d a f t e r f i r i n g in a i r at the t e m p e r a t u r e s (~ n o t e d at the p o i n t s . S1mi ta r d a t a f o r a s - f o u n d Chav ln c e r a m i c s a re shown on the tower t e f t , w i t h the nurr~ers g i v e n them d u r i n g the e x c a v a t i o n n o t e d at the p o i n t s . These d a t a a re compared w i t h those f r cm the l a b o r a t o r y f i r i n g s on the tower r i g h t .

disappears during firing in air between 200 and 30O~ A,. also drops sharply between 200 and 300~ obviously because the Fe 3§ in the clay is mainly present in the form of hydroxides, which loose water at these temperatures. AM again decreases above 800~ as is normally observed /1,2/. The increase of QQ1 between 200 and 400~ occurs ear- lier than it commonly does when clay is first fired. The further behaviour of QQI follows the usual pattern /1,2/. On firing under reducing conditions, the Fe "§ doublet largely disappears near 350~ magnetite is formed and disappears again above 600~ when a com- plicated pattern of at least three Fe 2§ doublets emerges, with only

1116 U. Wagner, et al., MOssbauer analysis o f recent ceramic f inds f rom Chavin

very little Fe =" left. At 1100~ some iron is reduced to the metal. Hossbauer patterns consisting of several Fe 2. quadrupole doublets appear to be typical of ceramics from the Andean region /I-3/, When clay previously fired reducingly at 800~ is refired in air, these doublets disappear largely between 400 and 600~ mainly feeding the Fe ~" doublet, but also forming some magnetic species with rather small fields. Hematite is formed strongly only above 850~

From the point of view of archaeometry, the aim of these inves- tigations is to use the Mossbauer data for an assessment of the pre- columbian firing technologies. For ceramics containing mainly Fe" because they were fired in reducing conditions, the structure of the Fe 2" doublets depends on the firing temperature, but further work is needed before these properties can be exploited to asses the firing conditions. The splittings of these doublets depend on the tempera- ture of measurement. Below 20 K, some of them split magnetically with hyperfine fields of the order of 13 T. Once these properties are under- stood in detail, they may be archaeometrically useful. In the mean- time, we restrict our considerations to ceramics that contain mainly Fe 3" and thus appear to have been fired in oxidizing conditions. For these, QQI and A,. are the most relevant parameters. In Fig. 2 these quantities are plotted for a number of samples and compared with the reference data obtained by the laboratory firing experiments. It turns out that they fall closer to the reference points for the clay first fired under reducing conditions and then refired in air, than to the points for clay directly fired in air. We consider this as proof that these ceramics were also fired reducingly, as seems nor- mal for the Chavin pottery, and were exposed to oxidizing conditions only at the end of the firing cycle. The firing temperatures can also be estimated from Fig. 2: they are quite high, with the highest values near I000~ This assignment is confirmed by the apearance of small amounts of metallic iron in the Mossbauer patterns of some of the sherds. In the reducing laboratory firing, metallic iron also appears only at very high temperatures (Fig. I).

4. CONCLUSIONS

The behaviour of Chavin clay on reducing firing, in particular, is complex and as yet not completely understood. The proposed method of assessment of the ancient firing conditions may appear rather crude, hut it is easily applicable when large numbers of samples have to be classified, which virtualy rules out lengthy laboratory refiring experiments for every single one of them.

ACKNOWLEDGEMENT

We wish to thank Mrs. M. Bartel and Hr. J.E. Punsch for their help in preparing the samples and performing the experiments. Support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.

REFERENCES

1. R. S a L a z a r , U. Wagner, F .E . Wagner, W. K o r s c h i n s k y , M. Zahn, J. R i e d e r e r and F. K a u f f m a n n - D o i g , in : P r o c e e d i n g s o f the 1984 symposiurn on a r c h a e o r n e t r y , Wash ing ton 1984 ( S m i t h s o n i a n I n s t i t u t i o n , W a s h i n g t o n , 1985) in p r i n t .

2. U. Wagner, F .E. Wagner and J. R i e d e r e r , i b i d . 3. U. Wagner, F .E. Wagner, B. M a r t i c o r e n a , R. S a l a z a r , R. Schwabe and J. R i e d e r e r ,

Hyp. I n t . , t h i s vo t ume.