Metallurgical analysis of a bronze powder chamber retrieved from an underwater excavation in Akko (Israel): an application of novel minimally destructive field multi-focal metallography

During the 2016 underwater excavation season of the nineteenth-century Akko Tower shipwreck, a cylindrical powder chamber was retrieved from a trench 2 m north of the north-eastern end of the site. Powder chambers of similar design and composition were common in Mediterranean fleets during the sixteenth and seventeenth centuries. The surface of the powder chamber was cleaned by abrasive brushing, and then examined by several metallurgical methods. These included X-ray fluorescence (XRF), optical emission spectroscopy (OES), destructive metallography, light microscopy, microhardness measurements, scanning electron microscope–energy-dispersive X-ray spectroscopy (SEM–EDS) analysis, and lead isotope analysis, in order to reveal information concerning the composition, microstructure, and manufacturing process of the powder chamber, its uses, and the origin of the raw material. Additionally, the powder chamber was examined by novel minimally destructive field multi-focal metallography (FMM), and the results were compared to conventional destructive metallography. A dendritic microstructure with entrapped gas bubbles and interdendritic shrinkage porosity was observed. Based on the analyses, the powder chamber was made of as-cast bronze, with about 10 wt% Sn and about 2 wt% Pb. The results of the FMM agree well with those of conventional destructive metallography. Samples taken from different parts of the powder chamber for lead isotope analysis gave a homogeneous signature, indicating the use of metal from the same source for the entire object. The results demonstrate that the novel use of FMM in metallurgical analysis was an improvement to present methods. This study also proved that the bronze powder chamber did not originate in the nearby Akko Tower shipwreck.