Oldest proof of metal surface alteration? Characterization of Cu3As formation of prehistoric and recent Cu-As alloys

Arsenical copper artifacts from prehistory often display silvery Cu₃As surface layers whose origin has been debated for decades. To clarify the mechanisms, we reproduced the phenomenon experimentally by exposing Cu–As alloys (6–11 wt.% As) to acetic acid and vinegar–salt solutions, mimicking accessible prehistoric materials. Surface and microstructural characterization (LOM, SEM-EDXS, XRD) combined with ICP-AES analysis demonstrates that these conditions trigger selective copper leaching and progressive surface enrichment of Cu₃As. The resulting layers form banded morphologies with penetration depths up to 400 μm, closely resembling archaeological specimens, and enhance corrosion resistance. We introduce the term arsenification for this process of Cu₃As surface enrichment. Beyond providing insights into the earliest known case of intentional surface modification, these results establish a model for alloy surface engineering via controlled dealloying under mild acidic and saline conditions.