Naples yellow and Pb-Sn-Sb yellow: characterization and evaluation of their stability through a multi-analytical approach

This work is focused on the characterization and evaluation of the stability under different aging conditions of Naples yellow (Pb2Sb2O7 – LAY) and the less known lead-tin-antimony yellow (Pb2SbSnO6.5 - LTAY) pigments. In the last years a renewed interest on the production and identification of these pigments raised from the scientific community, but still little is known about their degradation. Light-induced degradation was first studied. UV-DR spectra show an absorption edge typical of semiconductor compounds, therefore photoactivity (and related degradation) was expected, as demonstrated in literature for others yellow semiconductor pigments (e.g. PbCrO4 and CdS). Photoelectrochemical measurements and illumination experiments on the samples with green and blue lasers were carried out, but no or very low photoactivity was observed. To better understand the electronic structure of the pigments preliminary DFT calculations were performed for LAY, suggesting that this compound presents a metallic and not semiconducting behavior, therefore in good agreement with experimental results. To test the chemical stability, mock-up samples were exposed to acetic and formic acid, two common pollutants in museum environment. After these tests, samples were characterized with µRaman spectroscopy, XRD and SEM-EDX. A possible color change was evaluated by colorimetric measurements. Results indicated the probable formation of Pb-soaps, and Pb-Sn oxide hydrate was detected on one sample. Further analyses need to be done for a better understanding of the degradation phenomena.