Laboratory Powder X-ray Microdiffraction – the Use for Pigments and Secondary Salts Identification in Frescoes

Laboratory X-ray powder microdiffraction (micro-pXRD) is a very effective non-destructive method for direct phase analysis of samples smaller than 1 mm containing crystal constituents with no need of their pre-treatment. It is suitable for identification of pigments in fragments of colour layers of artworks, or their cross-sections, as well as for examination of secondary degradation products. X-ray powder diffraction is indispensable in the identification of pigments that are variable in mineralogical composition and similar in elemental composition, such as copper-based or earthy pigments. The micro-pXRD with a conventional X-ray tube, monocapillary with an exit diameter of 0.1 mm and position sensitive detector permits us to deal with samples routinely prepared for microscopy. Contrarily to the microdiffraction using synchrotron beams the presented technique is suitable for routine laboratory analysis. In the case of wall paintings, compared to easel paintings, inorganic components are predominating. Lower amounts or even absence of organic stuff facilitate the measurements using XRD. We used micro-pXRD for distinguishing of different cooper-based compounds, basic copper chlorides, carbonates, sulphates and copper acetates, used as green pigments, e.g. atacamite, malachite, verdigris, in fresco-secco paintings from the 15 century found in monasteries and churches in the Czech Republic. We found that chlorides are much more frequent in frescoes than in easel paintings from the same period, probably because of the conversion of azurite to atacamite. Micro-pXRD was also successfully used to distinguish the structural type of the lead tin yellow usually present in Gothic wall paintings and for the identification of secondary salt efflorescences on their surface. To decide how much the colour layers are affected by secondary alterations laboratory experiments focused on copperand lead-based pigments stability were carried out. INTRODUCTION Materials investigation of artworks by modern methods should be integrated into their restoration or artistic evaluation. It is based on the identification of materials and painting techniques. The success of that identification depends on the nature of materials sampled and the correct choice of analytical methods. In the case of easel and wall paintings, analytical methods used routinely in laboratories associated with galleries and restorations ateliers are optical microscopy and scanning electron microscopy supplemented with X-ray microanalysis (SEM/EDX) . Microscopic methods provide the information about the stratigraphy of painting layers that are distinguished according to their different colour, luminescence and chemical contrast by observing the cross sectioned samples in white and UV light and in backscattered electrons, respectively. On base of chemical composition determined by EDX analysis, the pigments present in the painting layer can be estimated. However, in many cases the mentioned traditional techniques are insufficient for proper pigments identification and thus an additional method has to be employed. Very effective tool for indisputable determination of mineral, thus practically all inorganic crystalline pigments and materials, is powder X-ray diffraction (XRD) 1, 9, . The essential problems for XRD phase analysis are the cases when some substances having the same or similar chemical composition occur in more crystal modifications, e.g. aluminosilicates in earthy pigments or copper-based pigments. 1 9th International Conference on NDT of Art, Jerusalem Israel, 25-30 May 2008 For more papers of this publication click: www.ndt.net/search/docs.php3?MainSource=65