Research on the darkening of drawings containing lead white
In a CATS-project supported by the Ministry of Culture (Kulturministeriet Forskningspulje), researchers from the School of Conservation (KADK) and the National Gallery of Denmark (SMK) with the Royal Library as an external partner, have undertaken a thorough investigation of old master drawings and early photography in order to determine the causes for the darkening of the highlights made in lead white.
The investigation at the SMK printroom showed that out of app. 900 art-works with lead white highlights or washes roughly 50 % of the drawings had already developed some degree of darkening caused mainly by external and internal air pollution.
The research has led to new knowledge of the necessity of: 1) filtering the air for hydrogen sulphide in areas containing objects with lead white and 2) to substitute old packaging materials with new ones. The survey of the collections has also located those parts of the buildings with the highest risk and thus made it possible to implement a qualified preventive conservation approach at the two institutions.
The research project also revealed that the mechanisms causing the darkening are much more complex than earlier anticipated, and that the phenomenon needs to be studied much more thoroughly in order to completely comprehend the interaction between historical materials and current air pollutants.
Researchers: Morten Ryhl-Svendsen (KADK), Niels Borring (SMK), Anna Vila (SMK), David Buti (SMK) and Birgit V. Hansen (The Royal Library, Denmark)