Why Do Old Paintings Discolor

Art conservation, particularly painting restoration, is an important endeavor to preserve our cultural heritage and maintain the aesthetic value of an artistic piece. Chemical reactions occurring on a microscopic level are the origin of the macroscopic changes that we observe as ageing. 

A study of the chemical aspects of the ageing of works of art is a vast undertaking. Many of the investigations focus on the artist s selection of pigments. In partic- 

A commonly held belief is that lead-containing pigments react with hydrogen sulfide in polluted air to form the black precipitate lead sulfide. On exposure to hydrogen sulfide gas, all of these pigments will darken because of the formation of the black lead sulfide, PbS  

However, hydrogen sulfide in industrial atmospheres is rapidly oxidized to sulfur dioxide and sulfuric acid. 

Progress Report 1995-1997, FOM Institute for Atomic and Molecular Physics, Amsterdam, NL, http //www.amolf.nl/research/biomacromolecular mass spectrometry/ molart/progress report98.pdf 

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