4. Difficulties encountered when restoring archaeological timbers

4.1 Pyrite problem

Among archaeological collections, it is relatively common to find composite objects containing both wood and mineral matter in the form of concretions. In most cases, these concretions consist of the corrosion products of metallic materials that were present in or in contact with the objects. Active corrosion of metallic materials (Fe, Cu...) generally occurs before the burial phase, in the presence of atmospheric oxygen. During burial in sediments, the corrosion products, generally in the form of more or less complex metal oxides, undergo chemical transformation in the absence of air, through reduction reactions, in particular thanks to the soil's bacterial flora. For example, the most classic case is the transformation of iron oxide into iron sulfide (or FeS...