Chemical changes in lead-pigmented oil paints: on the early stage of formation of protrusions

Small craters-like holes with a diameter about 200 µm, first observed on the surface of Rembrandt’s Anatomy Lesson of Dr. Nicolas Tulp are now found to occur in many other paintings. These small crater-like holes are filled with a whitish material lthat sometimes ‘protrudes’ out of the surface. Different stages in protrusion formation have been recognized in paint cross sections by light microscopy. Protrusion masses are being characterized with optical microscopy, imaging-FTIR and imaging-SIMS and SEM/EDX. In early stages, the increased transparency of the lead white- containing layer is the most characteristic feature. This study on an early stage of protrusion formation in the Anatomy Lesson of Dr. Nicolas Tulp confirms the existence of oil- derived, organic- rich, transparent materials replacing a seemingly unstable form of lead white used in the ground. Excess lead is present as lead soaps and minium crystals. The chloride map obtained with SIMS shows that chloride is dispersed in a finely divided form contrary to its presence as fiedlerite crystals in mature protrusions of this painting.