In 1992, two helmets were discovered on the Mediterranean seabed of Capo San Vito (Trapani, Italy) alongside Spanish galleon, guns and other finds, thought to be there due to a shipwreck in 1526. These helmets appeared to be Montefortino-type helmets but had not degraded in the same way as other examples previously discovered underwater. They are kept on display at the Parco Archeologico di Lilibeo in Marsala (Trapani, Italy).
Usually after this long in the sea, the metals in a bronze artefact, such as these helmets, degrade to the extent that the shape is distorted and the walls become thicker, making the helmet lighter. However, these helmets were heavier and with thicker walls than other Montefortino helmets.
In this study, published in Scientific Reports, the Italian researchers from STEBICEF department of University of Palermo looked at how the bronze had corroded over time. Using a range of techniques, including Neutron Resonance Capture Analysis (NRCA) as well as Neutron Diffraction (ND) on the INES beamline, they were able to determine the bulk elemental composition and study the crystalline phases present in the helmets. Using neutron diffraction enabled the researchers to study the samples without causing them any damage.
By measuring which elements were present in the helmets, they gained an insight into the manufacturing methods and how the helmets were used. For example, the special forces wore helmets with plumage, requiring the hollow apex to be filled with lead to hold the crest pin securely in place. As no lead was present, the helmets must have belonged to common soldiers.
Looking in more detail at the copper-containing phases enabled the researchers to determine the corrosion processes that had happened while the helmets were underwater. Interestingly, they showed that the initial degradation occurred in conditions with no oxygen. Usually this is the case only in very polluted waters, but as the shipwreck was in clean water, it suggests the samples spent a lot of time buried in the sand.
This initial mechanism formed copper species that retained the shape of the helmet, before a second corrosion mechanism occurred once the helmets had been exposed to the clean sea water. By comparing the two helmets, they could see that one had been uncovered earlier than the other.
This explanation of the corrosion that took place combined with the insight into the manufacturing process confirms that the helmets are Montefortino-type.
The full paper can be found at DOI: 10.1038/s41598-021-02425-6