Lead Chlorides in Paint on a Della Robbia Terracotta Sculpture.

Materials in the paint layers of an early 16th-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) was confirmed using Raman spectroscopy and electron backscatter diffraction. The 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 the 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 the alteration of materials in cultural heritage objects, emphasizes the importance of considering the role of treatments and environmental factors in changes in materials, and reinforces our awareness of the ongoing chemistry occurring in works of art.