Identification of materials in oil paintings through studies of correlations and ratios between the element line intensities during LIBS stratigraphy

Abstract

Small fragments of the oil paintings “Emmaus Dinner” by Bellini (XVII century) and “Annunciation” by Brughi (XVIII century) were taken to laboratory because of insufficient information collected in-situ. At first, the samples were analyzed by SEM-EDX and FTIR Spectroscopy in order to identify the elemental and molecular composition of the surface layers, mostly covered by Ca-based patina (calcium oxalate and carbonates). Afterwards, the depth profiling by LIBS made possible to distinguish surface contaminations and retouches from the original pictorial materials. Stratigraphic variations of the chemical composition were established by a proper normalization of the element line intensities, necessary due to large changes of overall plasma intensity during ablation of heterogeneous samples. So obtained layer separations were confirmed by Principal Component Analysis (PCA). Studies of the correlations and ratios between intensities of the analytical element lines were effective to identify pigments and priming layers and, in some cases, to indicate also a possible material provenance. In the Bellini's painting we identified Natural Umber, Azurite, Green Earth, Goethite, Lead-Tin Yellow and Lead White, the latter detected in all examined fragments. The surface contamination by debris and retouches by modern pigments containing Ba, Cr and Zn were also detected. The presence of K and Na in the priming layer suggests the use of glass, commonly employed in the XVII century. In the Brughi's painting we identified a thin layer of Ultramarine Blue placed over Green Earth layer while the priming layer was mainly constituted by gypsum. On this painting, we also detected surface restoration by Ba based modern pigments.