Le pigment d’aérinite dans deux peintures murales romanes du Sud-Ouest de la France

Abstract

The inventory of the mural paintings where the pigment of aerinite was employed, corresponds chronologically to the period of the Romanesque art, and geographically, to a zone of the south of the Pyrenees, which coincides with the local layers of this argillaceous mineral in Catalonia and Aragon. The discovery in France, for the first time, of this pigment in paintings of the vault of the old abbey home of Moissac (12th Century, Tarn-et-Garonne, France), and in those of the church Saint-Nicolas of Nogaro (end 11th Century, Gers, France), as well as the iconographic relations and stylistics of these sites with the Spanish sites, are strong indices of the circulation of the workshops and/or raw materials between the north of Spain and the south of France at the medieval time. Moreover, this pigment appears under nuances varying from major blue to green, as in the case of Saint-Nicolas of Nogaro, which often leads to a confusion with the use of green earths. Several factors can explain these differences in hue : the origin of mineral, the granulometry, the state of oxidation of the constitutive iron of this clay, the state of dehydration… We, on the one hand, compared and analyzed by various physicochemical methods the samples resulting from geological layers in France, Spain and Morocco. This characterization, in particular by Raman micro-spectrometry, was used as reference for the identification of this pigment in the two French medieval mural paintings. In addition, measurements of color after heating the aerinite in oxidizing atmosphere highlighted that the process of dehydration involves the variations of color of this mineral. It proves that the green or greenish color of the aerinite can be either of natural origin (and depend on the layer), or caused by the voluntary heating of the mineral to 400°C.