Identifying lead compounds and why they darken in white highlights in master drawings: provenance, production, trade, application and degradation

A study of drawings housed at the Royal Collection of Graphic Art at the National Gallery of Denmark (SMK) has shown that unique works of art containing highlights created with lead white pigments change their colour, becoming dark brown or black, due to unknown degradation processes. Research conducted so far indicates that this degradation can be attributed to a combination of adverse storage conditions and air pollution, but a range of other suspected and still unknown factors should be examined.

Understanding why some of the lead white highlights in old master drawings maintain their light tones while others grow dark is crucial for preserving this type of artworks in museum collections and retaining their suitability for public display.

The aim of the project is to advance preventive conservation of drawings (as well as other objects such as photographs) with intact white highlights by using a multi-analytical approach for identifying the physicochemical properties of lead white (e.g., particle size, chemical composition and presence of impurities) and the storage conditions (e.g., relative humidity and concentrations of airborne pollutants) that make the pigment more prone to darken.

blyhvidt, mørkning, H2S, svovlforbindelser