Exploitation of IR statistical results as a guidance for the multi-technique study of synthetic and natural iron-based pigments

Ochres are natural deposits particularly enriched in iron oxides and hydroxides giving them their typical colours spanning from bright red to yellow or brown. For this reason, they have been widely exploited over millennia for artistic purposes, becoming among the most known pigments. A series of 16 iron-based powder pigments including both commercial and natural ochres was investigated. Specifically, natural materials came from the Røros (Norway) district, where they were historically extensively used for painting the wooden houses’ facades. The set of samples was studied by applying a multi-analytical approach involving colorimetry, X-ray fluorescence, infrared spectroscopy in multiple spectral ranges and configurations, Raman spectroscopy, and X-ray diffraction. The potentiality of coupling infrared spectroscopy with Principal Component Analysis (PCA) in characterising and discriminating iron-based pigments represents an innovative approach to guide the interpretation of results obtained by applying a variety of analytical techniques, thanks to the identified clusters. This allowed, in turn, for depicting an extremely consistent description of the samples and specifically of those used in the conservation intervention of the UNESCO World Heritage site of Røros (Norway) and whose composition has never been studied before.