Abstract
In the present work, a fragment of a stone mould recently found in Galicia (NW Spain) was studied by multiple analytical techniques approach involving 3D optical imaging reconstruction to obtain data about the shape of the mould, typology of artefact produced, and distribution of a black residue at the surface of the mould and pXRF, SEM-EDS, micro-FTIR, and micro-Raman to investigate the nature of the black residue. The study shows that the mould was likely used for socketed axes with a side loop, was originally composed of two valves and one core, and that it might have been subjected to a repair during use. The black residue is distributed in the carved surface and spreads to nearby surfaces as a result of the use of the mould. The alloy cast in the mould was a ternary bronze (Cu + Sn + Pb). The analyses by SEM-EDS of black residue covering the surface did show the presence of scattered micro particles with P and Ca, and micro-Raman analysis detected the presence of a carbon black of animal source, while micro-FTIR analysis detected remains of proteins, oxalates, and hydroxyapatite. These results are amongst the very few studies made on black residues of ancient moulds and suggest that the mould was dressed with a carbon black of animal origin, such as burned bones, prior to metal casting, probably used as a coating agent to improve the casting and artefact recovery.
Highlights
Ancient metal artefacts were often obtained in a mould
This dark residue, which can be described as a black greasy powder that sticks to its surface, spreads out of the carved area and accumulates in a frame-like shape at the parting face of the valve and to the top surface, where it is present in a circular shape
Analyses by Portable XRF (pXRF) were made on the surface of the mould at different areas, including areas with the black residue and areas without it
Summary
Ancient metal artefacts were often obtained in a mould. Simple moulds, such as those composed by only one piece, have been known since the Chalcolithic (4th and 3rd millennium BCE). Some emblematic artefacts found during archaeological excavations include two binary alloy bronze awls considered to be amongst the earliest bronze objects from Iberia [17,18], ceramic sherds of Bell–Beaker Pan-European typology (Late Chalcolithic/Early Bronze Age), and Cogotas I typology, typical from the Mesetas (mainland central Spain) [19] The presence of these artefacts suggests the integration of this small islet in a large network of contacts. The use of a dressing or parting layer on the inner surface of the moulds is sometimes proposed for archaeological examples since it could improve the casting process and artefact detachment from the mould
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