Black earths from veneto and piedmont (Northern Italy): Origin, composition and potential use in different painting techniques

Abstract

Terminology used to define black earths is general, vague and unclear with regard to the correspondence between the name of the pigment used in different epochs and its specific mineralogical and chemical composition. Furthermore, no distinction is reported in distinguishing organic and inorganic sources. Here, French, Spanish, German and Italian treatises and modern and contemporary literature have been consulted to understanding the use of this uncommon black pigment in different pictorial techniques. Polarizing Light Microscopy (PLM), X-Ray Diffraction (XRD) and X-Ray Fluorescence (XRF) analytical techniques are used to characterize both the raw materials collected in Veneto and Piedmont (Italy) and the black earths available on Italian and German markets. Results indicate that the studied materials are clay minerals rich compounds, in which the colouring fraction is due to the presence of carbonaceous matter and amorphous iron oxides. Depending on the microstructure, black shale and black chalk were identified. The properties of these materials when used in a fresco and a secco techniques are generally very good, as reported in the written sources, even if fading is observed when replicas were exposed in outdoor environment.