Degradation studies of cultural heritage materials using mu-XANES and mu-XRD

The mission of cultural heritage institutions is to preserve and protect artifacts from the distant or more recent past for the enjoyment and education of current and future generations. In order to fulfill this mission in a professional manner, detailed knowledge on alteration phenomena of various kinds that gradually and unobtrusively are taking place at or below the surface of these objects is required. In order to be able to investigate the nature of these chemical transformations, that sometimes lead to the formation of microscopically thin alteration layers, the use of state-of-the-art microanalytical methods is required. Next to being able to provide information on the composition of various materials at or just below the surface, these methods also must be able to deliver highly specific information on the nature of the chemical compounds that are locally encountered. In this respect, our recent experience shows that the use of a combination of synchrotron X-ray based spectroscopic and imaging methods such as X-ray fluorescence analysis, X-ray absorption spectroscopy and X-ray diffraction can reveal significantly new information of certain alteration processes that have remained enigmatic for a long time. Concrete examples to be discussed is the darkening of originally yellow lead chromate paint layers, as encountered in paintings of V. Van Gogh and the blackening of red cinnabar-based paint layers in works of Rubens. In both cases, one or more microscopically thin alteration layers were encountered that are responsible for the colour and that contain resp. the metals Cr and Hg in other chemical environments that what they were originally.