Environmental effects on surface lead–calcium apatite formation on Roman wall paintings

The formation of lead apatites in heritage objects is increasingly recognized as a recurrent phenomenon, however their origins and formation mechanisms remain poorly understood. This study explores the process behind the darkening and surface clouding in Roman fresco paintings, providing novel insights into the formation of lead-calcium phosphates, Pb_xCa_(5-x)(PO₄)₃Cl_y(OH)_(1-y), and plattnerite, β-PbO₂, from the pigment minium, Pb₃O₄, linked to interactions with environmental factors and dust deposition. A combination of microanalytical techniques was used, including optical microscopy, scanning electron microscopy and elemental analysis, synchrotron-based X-ray diffraction and Raman and FTIR spectroscopy. The results show that slightly acidic rainwater or atmospheric moisture reacts with the minium and the calcium carbonate present in the wall painting's binder, resulting in the formation of plattnerite and the release of Pb²⁺ and Ca²⁺ ions, while the environment supplies chloride and phosphate ions, present in the infiltrating water. This process culminates in the precipitation of the lead-calcium apatite, due to its high stability. This apatite acts as a cement that incorporates dust and organic residues, forming a layer several tens of micrometres thick that tends to expand outwards. These findings contribute to a better understanding of pigment degradation mechanisms and will help inform conservation strategies.