Impact of Lead Soaps on the Formation of Age Craquelure

Abstract

The painted wall hanging (Andries Warmoes, signed and dated 1778) in the rear salon of the eighteenth-century Hofkeshuis in Almelo, Netherlands, offers a unique opportunity to study the relationship between lead soap formation and age craquelure in the oil paint on these canvasses. The proportion of intact lead white pigments to lead soap-rich areas (LW/LS ratio) was deduced from backscattered electron images of paint cross sections. The LW/LS ratio was determined with a newly developed computational imaging analysis method.This paper presents insights regarding two craquelure phenomena related to lead soap formation. The first concerns areas of paint with pronounced age craquelure alternating with areas with minimal age craquelure. These areas are only a few centimeters apart. The paint with extreme craquelure shows considerable lead soap formation, while the lead white in the paint film with minimal craquelure is much better preserved. This difference can be linked to the preparation of the canvasses. The second phenomenon occurs in paint that is partially covered by oak panelling that was installed in 1956 to encase the original marble mantelpiece. The paint behind the oak panelling shows less cracking and more lead soap formation, in contrast to the uncovered paint. The saponification is explained by the local humid environment created by the panelling. The low degree of craquelure is ascribed to small fluctuations in relative humidity and temperature behind the wood panelling.