La diffraction des rayons X au service de la conservation des bois archéologiques gorgés d'eau

In 1628, the swedish war ship “Vasa", who was designed to be the fastest and the most armed ship of her time, sank in the Stockhom's harbour, during her maiden voyage. After 333 years on the sea bottom, she was lifted out of water and treated. The Vasa is the biggest single object that has ever been preserved. A chemical resin called polyethylenglycol (PEG) is used to replace water in wooden objects to prevent them from shrinking when dried. But after some years these objects exhibit spots on their surface. Sulfur compounds (sulfates, sulfides, elementar sulfur) have been identified by Xray diffraction and absorption. The iron catalysed sulfur oxidation threatens the wood's wholeness. The challenge for the preservation labs, like ARC-Nucléart at CEA, is to measure the sufides content, to understand the sulfur accumulation phenomenon and to move a treatment to avoid the objects destruction. In spite of specific difficulties, we have shown that powder XRD is an efficient method to determine the current cristalline phases when coupled to monocristal diffraction which allows us to identify minor phases. Then it is possible to calculate the contents by the way of spectra simulation. It lets one for the first time distinguish between natural sulfates, already transformed sulfides and still dangerous sulfurs. For the future, the coupling with Xray fluorescence would improve the confidence of our answer to the curators