Lead soap protrusions in oil paintings: a perspective on nucleation pathways

Lead soap protrusions are a well-known degradation phenomenon that appears on the surface of historic oil paintings. Despite recent progress in characterising lead soap structures and crystallisation behaviour, the physical and chemical processes that cause them to aggregate in certain locations in an oil paint and break through paint layers remain poorly understood. Based on a thorough investigation of documented lead soap protrusions in historic paintings and a review of literature on the chemistry of lead-containing oil paint layers, we propose four mechanistic scenarios for the nucleation of lead soaps. They consider the key influences of reactive lead minerals, phase separation and reverse micelles. It is anticipated that more than one of these scenarios may be relevant for historic oil paintings, and that observed differences in the phenomenology of lead soaps and protrusions in paintings may be attributed to these nucleation mechanisms and the conditions that trigger them, both in terms of paint composition and environmental conditions.