Chrysocolla at high temperatures: understanding Pre-Hispanic metallurgy through Collahuasi slag analysis (Tarapaca, Chile)

This paper was presented at the YRA seminar series in December 2024.

Ujina-Collahuasi is an archaeological site located in the Atacama Desert in Chile, renowned as a major center for copper production during the pre-Inca and Inca periods. Pre-Hispanic metallurgists utilized bench-shaped granodiorite furnaces to smelt ore, leveraging the wind to achieve high temperatures and extract copper in the form of metallic beads. The resulting slag, rich in silicon, exhibits various mineralogical phases, notably yellow-green or yellow-orange phases that are of particular interest. These phases, rich in copper and silicon, may be attributed to the prevalence of chrysocolla among the ores used.
This study focuses on a characterization of chrysocolla, a copper silicate with an amorphous and controversial structure. At ambient temperature, several analytical techniques (PIXE, SEM-EDS, and solid-state NMR) were employed to overcome challenges related to its variable chemical composition and disordered crystalline structure. At high temperatures, the thermal decomposition of chrysocolla was investigated by heating several samples to different temperatures and characterizing them using various techniques (optical microscopy, XRD, IR spectroscopy, SEM-EDS). Additionally, an in-situ study was conducted, providing XRD scans of a sample subjected to increasing temperatures.
The results obtained enhance our understanding of the transformation of chrysocolla observed in the pre-Hispanic slag from Collahuasi and the evolution of metallurgical techniques. Previous studies have been supplemented by analyzing slags from excavations in the Ujina 10 sector, including three furnaces dated to different periods. The objective was to identify any changes in copper reduction techniques between the pre-Inca and the Inca occupation.

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