The evolution of metallurgy is a fundamental aspect related to the knowledge of the technological level of ancient civilizations, for which the information was mostly part of an oral tradition. The ancient, preserved artefacts are the only keepers of this long gone knowledge. Most advanced non-invasive techniques provide us the key to access it. Neutron techniques are nowadays the only available approach for revealing, non-destructively and with good spatial resolution, the morphological and microstructural properties within the whole volume of densely composed artefacts such as bronze statues. Application of neutron methods allows us to learn about ancient artefact manufacturing methods and to study at a very detailed level the current conservation status in their different parts. As part of a research project dedicated to the study of ancient Asian bronzes led by the Rijksmuseum Metal Conservation Department, four statues from the Rijksmuseum Asian collection were analysed using non-invasive neutron techniques. In this work, we present the investigation of a South Indian bronze statuette depicting Shiva in the form of Chandrasekhara (AK-MAK-1291, c. 1000–1200 A.D.) by means of white beam tomography, energy-selective neutron imaging (performed on CONRAD-2 at HZB, DE, and on FISH at TU-Delft, NL), and neutron diffraction (on ENGIN-X at ISIS, UK). The application of neutron imaging revealed the inner structure of the statue and allowed us to investigate the conservation state and potential cracking on the surface and in the bulk, to understand the interconnection of the different sections of the statue, and to obtain clues about the manufacturing processes. These morphological and microstructural results were employed to guide neutron diffraction analyses that allowed us to precisely characterize compositional differences, the presence of dendrites and columnar growth peak structures related to casting. This work is a complete non-invasive analytical investigation on an archaeological bronze artefact, providing outstanding results: from a quantitative analysis of the composition and microstructure to an in-depth morphological analysis capable of unveiling details on the ancient casting methods of the statue.

Neutron tomography is gaining popularity particularly in cultural heritage research, for non-destructively analysing the inner structure of bulk metal artefacts, such as bronzes, but the induced temporary decay radiation is often considered as a drawback. However, this delayed gamma-emission can be put to good use: by performing gamma spectroscopy after neutron tomography, the interior elemental composition of artefacts can be obtained “for free”. Inspired by this, we propose a ray-tracing approach to non-invasively quantify both interior geometry and elemental composition using only a single neutron tomography experiment. This strategy aligns well with both the aim for efficient use of neutron beam time and the expectation from curators and conservators for minimal neutron irradiation. Here, we outline the core principle of this method, demonstrate the extent of its quantification capability on bulk objects of known composition by fusing neutron tomography and delayed-gamma spectroscopy data sets. We also showcase its practical application on an ancient solid-cast Indonesian bronze statuette, by which we gain insights into how the pristine inner bronze segregated into a different composition than the surrounding shell. Similarly, the method allows us to quantify the composition of a hidden offering in the statuette that consecrates the bronze for worship purposes.