Crater-like holes extensively studied in Rembrandt van Rijn's "The Anatomy Lesson of Dr. Nicolaes Tulp" from 1632, were found to have formed as a result of the protrusion of firm partly transparent particles through to the surface.1 These particles were identified as a mixture of lead soaps and newly formed inorganic lead compounds in an oil-derived network most likely co-ordinated by lead.2 This form of degradation has since been recognised in another fourteen 17th century paintings in the Mauritshuis. Furthermore it is observed in numerous paintings elsewhere, ranging from the 16th to the 20th centuries, including schools outside the Netherlands.

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Journal ArtMatters
Noble, P, Boon, J. J, & Wadum, J. (2002). Dissolution aggregation and protrusion: lead soap formation in 17th century grounds and paint layers. ArtMatters, 1, 46–61.