High energy X-ray tomography of Bronze Age copper ingots

A few samples from the Bronze Age settlement of Santa Caterina Tredossi, Cremona, Italy generically identified during the excavation as metallurgical slags have been investigated by high energy X-ray computed tomography (XCT) in order to explore the possibility of using XCT as a non-invasive diagnostic tool for metal-related materials, and eventually of calibrating the absorption signal toward the actual composition of the object. The experiments were performed using the high energy X-ray source available at the Museum Research Laboratory of the Getty Conservation Institute, Los Angeles (GCI) and the experimental setup developed through by a collaboration between the GCI and the University of Bologna. By changing the distance between the sample, the GOS screen, and camera the setup makes it possible to optimize the resolution of the measured images even for large objects. The objects turned out to be ingots of pure copper with a very thick alteration layer. The experiments showed that through XCT it is possible to clearly identify the core of pristine copper metal and its shape underneath the thick layers of cuprite and secondary copper minerals, mainly malachite and brochantite. The grey-scale segmentation of the layers based on the absorption contrast was successively confirmed by sacrificing a few samples and by direct chemical check of the layer compositions with electron probe microanalysis, in order to carefully calibrate the absorption contrast vs the copper content of the material. The interlayer surfaces can be used to image in 3D the shape of the object and the depth of alteration