Technological and material features of the gold work of Mesoamerica Caractéristiques technologiques et matérielles du travail de l'or en Mésoamérique

During the excavations made at a burial of the post-Classic Mayan period (1220-1521 A.D.) in the pyramid number 2 of the pre-Hispanic site of Lagartero, Chiapas, Mexico, a set of four small metallic artifacts depicting reptile's heads, were recovered. The objects were in poor conservation conditions and were taken to the Metal Conservation Laboratory of the National School of Conservation (ENCRyM-INAH) for suitable cleaning and conservation treatments.Analyses allowed identifying important technological features such as gilding remains. The analytical techniques included optical microscopy followed by X-Ray Fluorescence Spectroscopy (XRF), and Scanning Electron Microscope-coupled with Energy Dispersive Spectrometer (MEB-EDS). For the elemental depth profile a combined Particle X-Ray Emission Spectroscopy and Rutherford Backscattering Spectrometry (PIXE-RBS) analysis was carried out.The results indicate that the objects are made of a copper alloy, and then gilded probably using electrochemical replacement gilding (thickness has less than 1 m). This technology has been observed in other metallic objects recovered from the Chichen-Itza Cenote in Yucatan, but it was not available in Mesoamerican areas so far. So, it could imply that these artifacts might have been imported from South American areas.

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One of the most remarkable examples of metalwork from Mesoamerica is the collection from Monte Alban’s Tomb 7. Among its items, we encounter extraordinary examples of bimetallic artefacts; a pair of solar representations whose main particularity is that they are formed by two different alloy sheets: one rich in silver and the other rich in gold. The silver-rich zones show deterioration characterized by brittleness, fractures and the loss of some fragments; this fragility is apparently due to dislocations caused by repeated hammering and insufficient annealing during the manufacturing process. Because sampling of the original items was not allowed, test specimens were prepared replicating the manufacturing techniques and alloy compositions, based on data previously obtained by X-ray Fluorescence Analysis of the original pieces. Examination of these test specimens was undertaken in order to understand the thermodynamic behaviour of the metallic microstructure and its relation to the alteration processes. Samples of each alloy were then taken and analyzed by metallographic methods.