Mechanical and chemical changes in Old Master paintings: dissolution, metal soap formation and remineralization processes in lead pigmented ground/intermediate paint layers of 17th century paintings

Thirteen paintings in the collection of the Mauritshuis and many others elsewhere show the phenomenon of protrusion-formation as identified earlier in Rembrandt's The Anatomy Lesson of Dr Nicolaes Tulp. Protrusions and related crater-like structures are a result of chemical changes in the composition of the underlying ground/intermediate paint layers, leading to mechanical expansion and protrusion of the newly formed substances at the surface. Different stages can be observed: early stages with increased transparency, dissolution of lead white and aggregation of organic substances in the paint; expansion of the occluded mass eventually leading to eruption through paint surface; and remineralisation of mature protrusions with lamellar precipitates of lead containing minerals. Moisture and accompanying pH excursions towards high basicity are thought to be the main driving forces for the chemical changes. Poor quality of paint materials may be an underlying cause.