Surface Manipulation in Cu- and Ag-Based Pre-Columbian Artifacts

Abstract

Numerous metal artifacts of exceptional historical and artistic value from the Moche civilization (ca. 450 AD) were unearthed in the tomb of the Lady of Cao (El Brujo, Peru). The tomb yielded the tattooed, mummified remains of a young woman, who was approximately 25 years old at the time of her demise. The rich array of artifacts and insignia of power found within the tomb provides compelling evidence of her elevated status in the hierarchical Moche society. Among the artifacts, the gilded objects and intriguing apparently bimetallic nose ornaments, featuring adjacent gold and silver surfaces, are particularly noteworthy. These artifacts reveal the sophisticated craftsmanship of Moche metalworkers, who expertly produced and worked on Cu- and Ag-based alloys. Moche metalworkers, once they worked and shaped the alloys to a thickness of approximately 100–150 μm, in some artifacts meticulously formed localized, uniform, and thin (roughly 3–5 μm thick) gold- and silver-enriched surface layers by employing etching agents. This process involved the selective depletion of copper from Cu-based alloys and, in some regions, the removal of both copper and silver from a Ag–Cu–Au ternary alloy. The presence of epitaxially grown micrometric silver wires, which resemble the elongated architecture of naturally occurring silver curls, supports the hypothesis of a subtractive surface treatment. These findings demonstrate a pioneering, though empirical, capacity to produce specific Cu- and Ag-based alloys and to select suitable materials for surface manipulation. This capability led to the tailored chemical modification of the outermost layers, resulting in a fascinating monometallic or bimetallic appearance likely imbued with religious, symbolic, or shamanic values. It is noteworthy that the creation of such enthralling artistic masterpieces was uniquely enabled by this ability to manipulate matter at the micro- and nanoscale, combined with the goldsmiths’ artistic creativity.