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: uwagner@ph.tum.de
http://www.archaeometry.de
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.
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 socalled 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).
O
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.
RO
R
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.
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).
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.
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.