Study of Fe(II) sulphides in waterlogged archaeological wood

Abstract

Wet organic archaeological materials extracted from seawater may suffer damage as a result of degradation influenced by micro-organisms. One of the most common phenomena is indirectly induced by sulphate-reducing bacteria (SRB). Due to their metabolic activity in anoxic conditions, SRB generate hydrogen sulphide from sulphate ions present in seawater. When steel items are in contact with organic matter in presence of sulphides, corrosion of the metal leads to the precipitation of Fe(II) sulphides. These phases are responsible for dramatic post-excavation damage: their oxidation during storage or exhibition in museums leads to the formation of voluminous crystals, which may cause cracking and crumbling, and lead to the production of sulphuric acid. In order to characterize Fe(II) sulphides and their by-products, 13 waterlogged samples were analysed by environmental scanning electron microscopy, micro-Raman spectroscopy, and X-ray diffraction. Experiments were performed on untreated wood fragments, on a fragment of rope, and on mineral concretions scratched from the surface of wood remains, all extracted from different shipwrecks. Mackinawite was detected inside the fragments and between the fibres of the rope. Greigite was detected in scattered locations. Pyrite and sulphated phases, like gypsum and iron sulphates, were identified at the surface of the wood fragments and in the mineral concretions.