Metal soap degradation of oil paintings : aggregates, increased transparency and efflorescence

Previous studies have revealed that lead- and zinc-containing oil paint layers in paintings and objects from different time periods, schools and collections, on varying supports, and including both treated and non-treated works, can undergo metal soap-related degradation that may affect their stability, appearance and interpretation. Depending on the composition and build-up of the paint layers this can be manifested on the surface in various ways: as protruding aggregates, increased transparency or metal soap efflorescence. These alterations are now recognized in thousands of paintings. This paper will give an overview of the surface defects that conservators can encounter. While the exact conditions leading to these changes are still not fully determined, many factors are now better understood.