Wagner Precolumbian Pottery

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

Mössbauer Spectroscopy in Archaeology I:Precolumbian Pottery from Northern Peru

I. Shimada1, U. Wagner2 and F. E. Wagner2

1Department of Anthropology, Southern Illinois University, Carbondale, USA

2Physik Department E15, Technische Universität München, 85747 Garching, Germany

e-mail: [email protected]://www.archaeometry.de

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

57Fe Mössbauer spectroscopy has been used extensively to study archaeological pottery since it provides rather detailed and unique information on the chemical and physical state of the iron that is always present in clay-based ceramics. Iron in fired ceramics can occur in different valence states (Fe2+ and Fe3+) and different chemical environments depending on the raw materials and the firing conditions, e. g., the firing temperature and kiln atmosphere. Other methods, like X-ray diffractometry and optical thin section microscopy, often yield additional information on the mineral content of ceramics. Details on this can, for instance, be found in two special volumes of the journal Hyperfine Interactions (Mössbauer Spectroscopy in Archaeology, Volume I and II, Hyperfine Interactions 150 (2004) and 154 (2004)).

The power of Mössbauer spectroscopy is best brought to bear if one can compare the archaeological finds with specimens fired under controlled conditions in the laboratory or in replica pottery kilns. Ideally, in such model experiments one should use the same kind of pottery clay as the ancient potters, but such clay is rarely available. We here describe a favourable case where authentic unfired clay is available.

In 1999 and 2001, a large pottery workshop dating to the Middle Sicán period (AD 900-1100) was excavated by the Sicán Archaeological Project at the site of Huaca Sialupe on the northern coast of Peru. Remnants of kilns, ceramic moulds for shaping vessels, raw clay, unfired pottery as well as many fired sherds were found. This, together with pottery from the nearby pyramid of Huaca Loro, constitutes an ensemble of ceramic finds that presents ideal conditions for a comprehensive scientific study. The investigation is still ongoing; so far more than 400 specimens have been studied.

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

In the workshop at Huaca Sialupe (top left) both raw clay and unfired sherds (middle left) were found. This authentic clay could be used for laboratory test firings.

Among the fired pottery sherds found in the workshop and in the nearby pyramid of Huaca Loro, the so-called blackware (bottom left and middle) is abundant, typically with a glossy black surface and with a gray or even black interior of the sherds. The Mössbauer studies confirm the notion that this blackware was produced by a reducing firing step during which carbon (soot) was deposited on the surface and often also inside the sherds. The metallic sheen of the surfaces is produced by careful burnishing.

Besides blackware, reddish brown pottery (redware) is quite common in the Middle Sicán period (bottom right).

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

Firing pottery clay in varying kiln atmospheres and at different temperatures results in typical changes of the Mössbauer patterns. The spectra on the right are for a clay fired in the laboratory at increasing temperatures in oxidizing (O) and reducing (R) atmospheres and for refiring in air after a reduction at 800 ºC (RO). Firing cycles with changing atmospheres often occur in actual pottery kilns. The firing behavior is largely determined by the dehydroxilation of the clay minerals between 400 and 800 ºC, depending on the specific clay minerals making up the pottery clay. Outstanding features of oxidizing firing are the conversion of Fe2+ to Fe3+ between about 300 and 500 ºC, and the formation of hematite above 800 ºC, when the clay structure collapses. During reducing firing, Fe3+ in the clay becomes Fe2+ between 400 and 800 ºC. Around 800 ºC the Fe2+ in the amorphous aluminosilicate formed on dehydroxilation often recrystallizes to hercynite (FeAl2O4), which can easily be identified because it exhibits a magnetically split Mössbauer spectrum at 4.2 K.

O

RO

R

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

The quadrupole splitting (Q-Fe3+) and the fractional area (Anm) of the Fe3+ species that do not exhibit magnetic splitting at ambient temperature depend on the firing temperature. The behavior in oxidizing atmospheres and the underlying processes occurring in the clay at different temperatures are shown on the right. A comparison of the parameters of excavated sherds with such curves often allows a determination of firing temperatures.

The Mössbauer spectra of ancient sherds often cannot be reproduced by merely firing clay in oxidizing or reducing atmospheres. One then must find a firing cycle involving different atmospheres and temperatures that give rise to the spectra observed in the sherds.

The spectra of sherds of Sicán blackware shown on the right could be reproduced by laboratory firing cycles involving first a reducing and then an oxidizing firing or vice versa. Thin section micrographs (below) show the mineral composition of the sherds.

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

The field firing experiments succeeded in producing replica pottery (top right) with the glossy black surface and gray interior of the sherds typical for Sicán blackware when the kiln was fired with local hardwood and cow dung to a temperature of about 700 ºC with restricted admission of air. The Mössbauer spectra of the black surface and grey interior (below) are identical and typical for a partially reduced state, presumably because of partial re-oxidation during the cooling phase. Spectra of this type are also often found in ancient Sicán sherds.

To further test the ideas about the firing procedures in the Middle Sicán period, a replica kiln was built according to the excavated evidence and used for making replica pottery in Sicán style with the help of a local potter (left).

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

Ceramic finds from Huaca Loro and from the workshop of Huaca Sialupe can be sorted into 4 types (T1–T4) according to their Mössbauer spectra, here arranged in 3D plots, depending on their content of Fe2+ and Fe3+ and other features (above left). According to the Mössbauer patterns, T1 and T2 vessels were fired at around 400 ºC and T3 and T4 vessels around 900 ºC. The X-ray diffraction patterns confirm this by the presence or absence of the chlorite peak near 7 º2Θ, since the clay mineral chlorite is stable up to 650 ºC. The specimens marked in red and green stem from the West tomb of the pyramid of Huaca Loro and suggest that the pottery found in Huaca Loro was made in the workshop of Sialupe.

Many thanks to all who have contributed to this study.