LAUSR

Materials in the paint layers of an early sixteenth-century painted and glazed terracotta sculpture by Giovanni della Robbia, Pietà were identified. The presence of the rare lead chloride salts, cotunnite (PbCl2) and challacolloite (KPb2Cl5), were confirmed using Raman spectroscopy and electron back scatter diffraction. Well-formed habit of large crystals of challacolloite indicates slow growth from solution. The stability and potential pathways for the in-situ formation of lead chloride compounds are discussed. The presence of lead chloride phases in the Pietà indicates conditions of high chloride ion concentrations (i.e., activities) and/or low pH led to the alteration of the pigment lead white (hydrocerussite, 2PbCO3·Pb(OH)2). This work highlights the role of ion transport in alteration of artists' materials and emphasizes the importance of considering the role of treatments and envi-ronmental factors in changes in materials, and reinforces our awareness of the on-going chemistry occurring in works of art.