Decorative coating or corrosion product? TEM and SEM Study on a Late Neolithic Axe to find origins of silver metal on the surface

TL;DR

This study investigates a Late Neolithic copper axe with unusual silver enrichment, revealing through microscopy and corrosion experiments that the silver is a noble corrosion product rather than decorative.

Contribution

It provides new insights into the corrosion processes and surface composition of ancient copper artifacts with silver enrichment, using advanced microscopy and experimental replication.

Findings

Silver enrichment is due to corrosion, not decoration.

The surface contains copper oxide with embedded silver particles.

Silver remains on the surface because of its noble character.

Abstract

Here we report on the analysis of a metallic Late Neolithic copper axe by means of Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The axe was found at Eskilstorp, south-west Scania during archaeological excavations in autumn of 2015. It showed a hint of silver en-richment on the surface which is unusual for Late Neolithic axes from Pile type hoard. To identify the origin of the silver content, we extracted a thin lamella of the axe interior using Focused Ion Beam (FIB) technology to reach the internal structure to keep the process extremely minimally invasive. The results revealed the presence of porous external layer of copper oxide enriched with silver particles. It is shown with the corrosion test performed on copper-silver experimental replica that the silver en-richment is attributed to the selective dissolution of copper metal as a resut of corrosion process on the axe. The corrosion experiment is performed in the presence of an organic electrolyte such as Hemic Acid (HA) which replicates soil and natural water environment. In the scope of this research work, we found a strong evidence that the silver on the surface of the axe was not decrative layer and instead remained on the surface due to its more noble character while copper leached out into the soil due to corrosion.