Does the lead white on graphic art have a dark future?

The darkening of lead white in artworks has been previously observed and reported. This process has been attributed to the action of H2S present in the atmosphere as well as from the breakdown of the organic matter by microorganisms [1, 2], which reacts with 2PbCO3·Pb(OH)2, resulting in the formation of lead sulphide (galena, PbS) [3]. Although the galena is the most frequently found degradation product associated to lead carbonate, brown lead dioxide (plattnerite, PbO2) can also be detected.

Records from the last ten years on the Statens Museum for Kunst (SMK) graphic art collection revealed the formation, in a very short time, of different types of dark spots on the lead white highlights. A recent systematic survey at SMK and the Danish Royal Library (RL) mapped the occurrence and degree of degradation on graphic art and photography and allowed to classify the lead white phenomena.

Watercolor mock-ups samples prepared with lead carbonate (basic and neutral) and Arabic gum on paper, parchment, and photographic supports have been monitored for blackening while exposed to natural (galleries and storage areas) and artificial atmospheres (elevated doses of H2S). The level of H2S in collections areas at SMK and the RL are being monitored for one year; and the impact of papers and boards used in the collections as well and framing materials on direct contact to artworks and mock-ups samples are being also investigated.

A comparison of the results obtained by analytical means on the laboratory mock-ups will help to understand if the degradation source is entirely external or if the base material of the artwork could be a contributing source. The understanding of the degradation product and its origin are crucial to understand the darkening phenomena in real artworks and develop preservation and conservation strategies.

1. Smith, G.D. and R.J.H. Clark, The role of H2S in pigment blackening. Journal of Cultural Heritage, 2002. 3(2): p. 101-105.
2. Petushkova, J.P. and N.N. Lyalikova, Microbiological degradation of lead-containing pigments in mural paintings. Studies in Conservation, 1986. 31(2): p. 65-69.
3. Goltz, D., et al., Spectroscopic studies on the darkening of lead white. Applied spectroscopy, 2003. 57(11): p. 1393-1398.