On unexpected colour of lead sculptures in Queluz: Degradation of lead white

Abstract

Although the usual aspect of lead surfaces is light grey, in the gardens of the ‘Palácio Nacional de Queluz’, Portugal, some of the sculptures present an atypical red brown appearance. In order to investigate this phenomenon, lead coupons previously prepared with different surface finishes have been exposed in nearby statues presenting both surface aspects. After 15 months, visible surface transformations have been noticed for certain locations, and Raman microscopy has been used to characterise the surface products. Lead(II) oxide near the metal and salt layer at the outer part has been found for the white patina. For the red patina, a mixture of lead(IV) oxide and an uncommon lead oxycarbonate, probably shannonite (PbCO₃.PbO), has been found. Complementary in situ Raman analyses performed during electrochemical oxidation of lead, lead covered with hydrocerussite and pure hydrocerussite have shown that the presence of both lead and hydrocerussite is required to reproduce the field experiment.