Abstract
Clay-based pigments are among the most traditional. Unlike other mineral pigments, they have never been fully replaced by synthetic analogues and are still used in painting today. Since their analysis requires a specific approach, detailed distinction of clay pigments has never been a part of routine chemical-technological research in fine arts—regardless of a great potential of clay minerals for determining regional provenance of the material. This review article maps and summarizes research on clay pigments in historical paintings that has been systematically pursued by authors since the beginning of this millennium. This rallying and interconnection of knowledge was an opportunity for a new reflection on the common aspects of these research projects, either methodological or interdisciplinary, since these findings are closely related to art-historical evaluation of artworks. It offers a comprehensive insight into the microanalysis of clay pigments with using powder X-ray micro-diffraction and complementary methods. Significant new findings come, for example, from research on the Italian Baroque. It becomes clear that cheap availability of raw material, pottery clays, could have played an important role in the change in painting technology at the turn of the 16th and 17th centuries.
Highlights
Laboratory of Painted Artworks (ALMA Laboratory), 1001 Husinec-Řež, 250 68 Řež, Czech Republic; Academy of Fine Arts in Prague, Academic Materials Research Laboratory of Painted Artworks
Clay and iron oxide pigments are among the longest-used mineral pigments
The analytical procedure usually involves optical microscopy followed by semi-quantitative elemental analysis (e.g., by scanning electron microscopy coupled with energy dispersive spectrometry (SEM-EDS) or micro X-ray fluorescence), and by spectroscopic analyses of pigments and binders by Raman and Fourier transform infrared micro-spectroscopy
Summary
Clay and iron oxide pigments are among the longest-used mineral pigments. They occur in prehistoric art and are still used up to the present days. The location of the dye molecules on the clay support, as well as the origin and variability of the colour, are still not fully understood, they have been studied extensively since the 1960s [8,9,10,11] In any case, this is one of the very first examples of targeted intercalation of clays by organic molecules in order to get hybrid materials-organoclays with new useful properties [12], such as stable colour which persists even at high temperatures and in an aggressive chemical environment [13]. Since the colouring agent in red, yellow and brown earths is, in most cases, goethite or hematite (much less frequently jarosite or other phases, Appendix A), the clay minerals remain rather overlooked— they can form the predominant part of the material. The diversity of clay minerals can be decisive for assessing the origin of the pigment, or even its regional source
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