Contribution of magnetic measurement methods to the analysis of iron sulfides in archaeological waterlogged wood-iron assemblies

Pyrite and greigite were identified in the wood of two ancient shipwrecks using an original multi-technique analytical approach. Structural characterization methods such as environmental scanning electron microscopy, micro-Raman spectroscopy and X-ray diffraction were combined with magnetic measurement methods, such as magnetic susceptibility measurements and isothermal remanent magnetization acquisition curves. This is the first time that magnetic measurement methods are used in the field of cultural heritage to study wet organic archaeological materials. They proved to be particularly suitable to detect with a very high sensitivity ferromagnetic s.l mineral phases inside waterlogged wooden samples, i.e. in the bulk. The occurrence of iron sulfides in archaeological shipwrecks extracted from waterlogged environments is usually attributed to microbiologically influenced corrosion of iron fasteners. This study demonstrates that the nature of the identified iron sulfides is consistent with a step-by-step in situ anoxic oxidation process of mackinawite.