In-depth study of a speiss/matte sample from Castillo de Huarmey, North Coast of Peru, and its implications for the pre-Columbian production of arsenic bronze in the Central Andes

Abstract

This study aims to characterize the phase composition and chemistry of the speiss/matte sample from the Metallurgist's Burial at Castillo de Huarmey and to use the information derived from these analyses to infer the temperatures, furnace conditions, and ores associated with the smelting processes, which created the speiss/matte sample. For this purpose, a number of geochemical analyses were performed on the spies/matte fragment: analysis of the general chemical composition (handheld X-ray fluorescence spectrometry [hhXRF], X-ray photoelectron spectroscopy [XPS]), analysis of the chemical composition in the micro area (field emission scanning electron microscope with an energy dispersive spectroscopy detector [FE-SEM-EDS], field emission electron probe microanalysis [FE-EPMA]), analysis of the mineral composition (X-ray diffraction [XRD]), and analysis of the phase composition (Raman spectroscopy). Chemical and mineralogical analyses of the speiss/matte specimen determined that the specimen is composed of distinct arsenide, arsenate, sulfide, and glass phases. During the smelting process, the charge material consisted mainly of Cu, Fe, and As sulfides. Arsenopyrite is the most likely candidate as the mineral source of arsenic. In addition, temperatures of at least 1200°C were achieved during the smelting process, with smelting occurring over a relatively short timeframe given that effective density separation of speiss and matte phases was not achieved.