Chemical study of degradation processes in ancient metallic materials rescued from underwater medium

Underwater Cultural Heritage understands all traces of human existence having a cultural, historical, or archaeological character, which have been partially or totally underwater, in periodic or continuous forms for at least 100 years, according to UNESCO. This work is focused on two pieces extracted in 1999 from a shipwreck located in Bakio (Basque Country, Northern Spain). The two analyzed pieces were an iron anchor and a swivel gun, both exposed in Bakio's Town Hall after their restoration in 2005. The aim of this work is to study the elemental composition of archaeological metallic pieces extracted from underwater and the degradation processes that affect them. For that purpose, nondestructive analytical techniques were employed. Concretely, Raman spectroscopy and X-ray fluorescence spectroscopy were used to check the conservation state and to identify the raw materials that were used in the manufacture of those pieces. The analyses concluded that both artifacts were manufactured in cast iron, although the composition of the raw materials was not exactly the same. The main decaying compound was lepidocrocite (γ-FeO(OH)), a highly reactive iron phase that increases the corrosion rate of the artifacts, though other iron oxy-hydroxides were also detected in minor amounts. This compound together with the also found akaganeite (β-Fe₂(OH)₃Cl) were probably the responsible of the continuous oxidation of the metallic pieces, which are in a poor conservation state. Therefore, the applied treatment was not suitable for these pieces.