Blackening of lead white: Study of model paintings

This study is aimed at clarifying the causes of lead white darkening by studying the behaviour of its two components: cerussite and hydrocerussite. Their degradation processes have been investigated by micro-Raman spectroscopy both in an alkaline environment generated from a fresco conditions and in the presence of NaClO as an external oxidizing agent. It was confirmed that no darkening of mural paintings components occurs in the alkaline environment, highlighting that any blackening process must be due to an external agent. On the contrary, a markedly different behaviour was observed for cerussite and hydrocerussite in the alkaline conditions generated by a fresco technique, where formation of different partially oxidized products was observed. Furthermore, the conditions required for blackening of cerussite and hydrocerussite with NaClO were in-depth investigated by X-ray diffraction analyses, finding that the reaction products consist of lead (IV) in plattnerite and scrutinyite forms, produced in different amounts. Hydrocerussite turned out to be the more reactive component, giving rise mainly to the more kinetically favoured scrutinyite, whereas, in the case of cerussite, plattnerite was prevalently formed owing to its larger thermodynamical stability.