The identification, approaches to cleaning and removal of a lead-rich salt crust from the surface of an 18th century oil painting

Abstract

A thin, grey surface layer was noticed after removing the varnish of an eighteenth century Dutch oil painting, by an unknown artist titled Vision of Saint Lutgard of Tongeren. This layer masked the paint layer almost entirely, casting a greyish veil that greatly influenced the painting’s appearance, colour, and saturation. Using scanning electron microscopy coupled with elemental analysis (SEM–EDX), this layer was identified as a lead-rich salt crust. The crust was a distinct layer on top of the paint layer, closely following the topography of the layer underneath but not embedded in it. The article discusses the possible theories behind the formation of this lead-rich salt crust by characterizing the painting’s material, past treatments, and environmental history. Furthermore, the article reports the decision-making process behind the removal of the crust, focusing specifically on the ethical and technical challenges of the cleaning treatment. Tests with different cleaning agents targeting separate components of the crust to remove it and different application techniques are described. The chelating agent Ethylenediaminetetraacetic acid (EDTA) was found to be the most effective in the removal of the crust and was chosen for its removal. The pH of the EDTA cleaning solution and its application had to be modified according to the colour of the paint the crust lay upon. Visual observations and analytical assessments using optical microscopy and SEM after cleaning indicate a successful reduction of the crust resulting in a significant improvement in the painting’s appearance.