Determination of clay pigment provenance rased on crystal structure characterisation

Application of X-ray diffraction techniques (XRD) in material analysis of artworks is beneficial in several aspects. Primarily it provides indisputable identification of crystal components found in colour layers, further it makes quantification of identified phases possible and last but not least it can also bring insight in material crystal structure. The crystal structure of materials is often affected by conditions which have been formed under, e.g. geological genesis, manufacturing procedure or secondary changes for example resulted from corrosive action of environment. The knowledge of the relation between the crystal structure of the material and its formation can be useful not only for art historic evaluation of the artwork but also for the artwork restoration and/or conservation. Within the example of clay mineral kaolinite, we show that its crystal order evaluation (based on XRD) can serve well to find its natural origin. We found kaolinite, the main component of kaolin (white earth), in a preparation layer of a Gothic wall painting in St. Maria-Magdalena Church in Bor near Karlovy Vary, where important kaolin deposits are situated. Comparing eight reference kaolins, we demonstrated that these can be differentiated just according to the kaolinite crystallinity (crystal structure ordering). Within this study we compared laboratory powder X-ray micro-diffraction (micro-XRD) with synchrotron radiation X-ray diffraction (SR-XRD). We found that both techniques led to the same results