By
Laura Arcidiacono (presenter)
Giulia Festa (presenter)
Carla Andreani (presenter)
Marcos Martinón-Torres (presenter)
Summary
Bronze artefacts were buried in large quantities in isolated hoards in the Atlantic façade of Europe, between the Bronze Age and the beginning of Iron Age. This practice was particularly intense, and despite many years of research, even today we do not understand what motivated such disparate and widespread communities to express themselves in this way (Armada and Martinón-Torres, 2016; Montero Ruiz et al, 2015). This presentation is focused on the totally non-invasive characterisation of leaded bronze unused (as-cast) axes from NW Iberia through neutron tomography (NT), radiography and prompt gamma activation analysis (PGAA). Our main focus is to unveil the presence and distribution of high quantities of lead in the axes cores, which raises interesting questions about their manufacture as well as challenges for conventional x-ray based analytical techniques (Harrison et al. 1981; Gutierrez Neira et al, 2011). The samples include a variety of compositions and states of preservation, as well as an enigmatic palstave that has a hidden thick lead core as well as a large lead ball inside the casting sprue. Our specific objectives are (1) study the manufacturing processes of these axes; (2) study the inner morphology; (3) quantify their bulk chemical composition without analytical biases derived from sampling uncertainty. The experiments were performed at ISIS pulsed neutron and muon source in Oxfordshire UK at the IMAT beamline where ad hoc instrumentation has been installed to allow the simultaneous acquisition of NT and PGAA spectra. Neutrons are a unique probe for the investigation of metal objects because of their weak interaction with matter and high penetration power allowing the study of hidden features in the bulk of the artefacts in a non-destructive and non-invasive way (Festa et al, 2018). Results from the NT study unveil the inner morphology of the axes, allowing the segmentation of their various inner parts and materials, leading to new hypotheses about their manufacturing techniques. The presence of a sphere of pure lead in the core of one of the axes opens multiple scenarios on the manufacturing processes of this object, suggesting the presence of a double casting or the occurrence of a massive lead segregation. Through prompt gamma activation analysis, it is possible to perform a thorough chemical characterisation in the bulk of the objects.
Keywords
Archaeometallurgy. Archaeometallurgy. Leaded Bronzes. Palstaves. Neutron Tomography.