Microanalysis of clay-based pigments in paintings by XRD techniques

Abstract

Despite considerable advantages of the method in the field of microanalysis of pigments, powder X-ray micro-diffraction has not yet been satisfactorily tested for correct and unambiguous discrimination of clay structures in colour layers of paintings and, eventually, for reliable quantitative analysis of their relative contents. To overcome this lack of knowledge, several sets of laboratory experiments with model mixtures and reference samples imitating real paint layers have been performed and general rules for proper discrimination of clay structures have been described. As an integral part of the research, clay–binder interactions were studied to avoid misleading interpretations. It was found out that various characteristic shifts of basal diffraction lines depend on the type of organic binder and the relative content of expandable layers in the clay structure. Based on these results, it was possible to differentiate pure smectites, interstratified illite/smectite structures and chlorites in paint and preparatory layers of actual paintings. The accuracy of quantitative microanalysis of clay minerals in mixtures with other minerals in Mid-European red earth pigments (typically containing kaolinite, illite, quartz, hematite, and anatase) was characterised by total bias values in the range of 3–10 wt.%. Finally, results were applied to differentiation of earth pigments of various provenances in grounds of Baroque paintings and Early Renaissance preparation layers for gilding.