ANIKO BEZUR

DEPARTMENT OF MATERIALS, SCIENCE AND ENGINEERING

UNIVERSITY OF ARIZONA

BRUCE OWEN

SONOMA STATE UNlVERSITY

|Abstract: The copper artifacs exceavated by the Upper Mantaro Archaeological Research Project (UMARP) demonstrate that arsenical copper was steadily replaced by tin bronze after the Inca invasion of the Mantaro Valley. This paper discusses some of the technological issues involved in the switch from one alloy to the other and explores possible social and political forces behind the change.

Resumen: Los artefactos de cobre excavados por el Proyecto Arqueológico del Alto Alto de Mantaro (UMARP) en el Perú muestran que el bronce arsenical the uniformente reemplazado por bronce estañífero luego de la invasión inca del valle del Mantaro. Los objetos del primer período estudiado (1000 a 1350 d.c.) son esencialmente de cobre. En el segundo período (1350 a 1450 d.C.) se observa el uso de arsénico, aunque en porcentajes menores de los esperados para modificar las propiedades del metal. Tras la conquista inca (1450 a 1533 d.C.) persiste el uso de cobre pero aparecen piezas que contienen estaño en porcentajes que modifican sus características (agujas más delgadas y largas, bordes más cortantes, tupus brillantes, piezas mejor fundidas) sin que sea notoria su utilización como bienes de prestigio.

The Upper Mantaro Valley lies in the central highlands of Perú. Researchers of the Upper Mantaro Archaeological Research Project (UMARP) surveyed sites at altitudes ranging from 3100 meters on the valley bottom to 3900 meters on rolling uplands. The research was designed to evaluate theories regarding social, political, and economic changes experienced by Late Intermediate Period Xauxa chiefdoms upon their incorporation into the Inca state around AD 1450, and the strategies of the Inca state to integrate the Xauxa chiefdoms into its expanding imperial system. To achieve these goals, UMARP collected statistically comparable quantitative data from "commoner" and "elite" households of the Inca and the immediately pre-Inca periods. Elite and commoner contexts were established by architectural criteria and subsequently confirmed in almost every case by differential distributions of many categories of goods. These included decorated and imported ceramics, preferred diets indicated by animal bone, shell artifacts and artifacts made of copper, silver, and lead (Costin and Earle 1989; DJAltroy 1992; DJAltroy and Earle 1985; Owen 1986). UMARP defined two pre-Incaperiods: Wanka I (AD 1000-1350) and Wanka II (AD 1350-1450). The.period of Inca rule was designated as Wanka III (AD 1450-1533) and Wanka IV refers to the beginning of the colonial occupation. In Wanka I, small agricultural and pastoral settlements were dispersed from valley bottoms to higher elevation hilltops. During Wanka II settlements tended to be larger, and located on higher, more defensible hilltops. Many sites were fortified, corroborating early historical accounts of widespread conflict during this period. The Inca conquered and pacified the region in Wanka III, resettling much of the population to smaller, valley bottom communities more suited to intensive maize production and imperial control (Costin and Earle 1989; DJ´Altroy 1992; Earle 1987; Earle et al. 1980).

Owen has compared the distribution of metal goods before and after the Inca conquest in contexts established as elite and commoner. He found that before the Inca conquest metal goods were present predominantly in elite contexts and were relatively rare. After the Inca conquest, the overall amount of metal goods in circulation increased and differences between their distribution in elite and commoner contexts decreased. Similar patterns were observed for shell and ceramic objects, suggesting that incorporation into the Inca state was accompanied by a democratization of access to a variety of valued status goods. The presence of few Inca-style cast tin bronze objects in elite contexts after the Inca conquest mar indicate that the Xauxa elite now needed new kinds of metal objects to serve as material symbols of status or power. Since access to Inca-style cast objects was probably through state channels, the Xauxa elite were probably becoming more dependent upon the Inca for legitimation of their status. At the same time, objects with traced or graved decoration began to appear in elite households, with designs in a new, local style. Since Xauxa elite mar have been able to control the production of these locally-made objects, they mar reflect an effort by the elite to develop their own metal symbols of status, access to which would have been independent of the Inca state (DJ´Altroy 1992; Owen 1986).

Owen drew his conclusions primarily on the basis of visual characteristics and distributional information, although he did have access to some compositional information. To find further support for his hypotheses and to be able assess technological changes between pre- Inca and Inca contexts a technical study of the copper objects is being carried out. A preliminary study of the composition and structure of the copper and copper alloy objects excavated by the UMARP was conducted at the Museum Services laboratory of the Getty Conservation Institute, Los Angeles, in 1995-96. The objects were examined by x-ray fluorescence spectroscopy (XRF) to establish whether they contained arsenic, tin, lead, iron, silver or any other metal in addition to copper. ^{| 1} The analyses did not require the sampling of objects and were therefore carried out on all objects. Objects designated as fragments or lugs were sampled for inductively coupled plasma - optical emission spectroscopy (ICP-OES). ^{| 2} This destructive analytical technique yielded weight percent elemental compositions which were normalized to 100% to allow comparison between objects. The trends observed in the data acquired by the two methods are identical and are outlined in this paper.

Heather Lechtman has studied the connection between the expansion of the Inca empire and the availability and use of tin in places far removed from potential tin sources in northern Bolivia (1975; 1980; 1984). In order to evaluate this "tin bronze horizon" it is necessary to be able to compare objects that are from securely assigned pre- Inca and Inca contexts from the same regíon. UMARP offers just this, as research design from the very beginning emphasized the clear separation of pre- Inca and Inca contexts as well as the collection of statisticaliy comparable data from each type of context. Furthermore, excavations concentrated on representative domes tic sites as opposed to unique, special-purpose administrative or religious site. Very few collections satisfy these prerequisites for research on technological change; in fact the UMARP collection mar be unique in this respect. This collection allows us to address tour questions:

1. Was tin bronze really absent before the Inca conquest?

2. How prevalent was the new tin bronze technology after the Inca conquest?

3. How was tin incorporated to produce bronze?

4. What were some technical and ideological reasons for the adoption of tin bronzes?