Study of the mechanism of formation of calcium soaps in an early 20th century easel painting with correlative 2D and 3D microscopy

This paper describes the identification of a whitish insoluble surface crust that covers most of the dark areas in Rembrandt’s Homer (1663) in the collection of the Mauritshuis, and the consequences of its presence for the interpretation and treatment of the painting. Analysis demonstrated that lead and potassium from the smalt-rich paint and the lead white-containing upper ground have migrated to the surface, possibly in the form of soaps. The deposits are considered to have undergone further reactions with atmospheric compounds to form insoluble complex salt mixtures rich in lead, potassium and sulphur. Recent identifications of comparable surface crusts in other Old Master paintings suggest that lead potassium sulphate is a common degradation product in oil paints that can have different origins.