Clay and iron oxide pigments in the history of painting

Clay minerals and iron oxides are intimately related in the process of their natural formation. Their mineralogical composition and physical properties correspond to the physical–chemical conditions of weathering, sedimentation and alteration processes by means of which these minerals are associated giving different types of laterites, ferrolites, ochres, and coloured clays and soils. Very early in human history, these and other clay materials were adopted as mineral pigments. Their structural and mineralogical features are directly related with their natural genesis and provenance and help us in the study of historical painting techniques and materials. This paper gives general information about geological sources and their characteristics, the literary evidence of use of different forms of earthy pigments on historical paintings, about analytical methods suitable in their identification within the ground and colour layers of the painting, and handling with the samples of works of art. The examples focused preferentially on the period of European mediaeval and baroque painting.