Paint Components

Emerald Green Deterioration

The nineteenth-century pigment emerald green (Cu(C2H3O2)2·3Cu(AsO2)2) copperacetoarsenite, is known to degrade into arsenic oxides. Because of their solubility in water, arsenic oxides readily migrate and are therefore found throughout all the support, ground, paint and varnish layers on a painting. It is thought that first the emerald green pigment reacts with free fatty acids derived from the binding medium forming copper soaps and arsenic trioxide (a white degradation product) which dissolves in water and is mobile throughout the paint system, then further oxidizes to arsenic pentaoxide, and finally reacts with lead, calcium and other ions and is deposited in the paint system as insoluble arsenates. Physical degradation phenomena associated with emerald green include darkening, cracking and crumbling of the paint layers, and fading. It is important for the conservator to be aware that in objects where paints contain emerald green, arsenic is deposited throughout the entire object, including wood support and top varnish layers. Conservators should minimize exposure of objects containing arsenic pigments to strong light, large fluctuations in relative humidity and water-based cleaning agents.

Related Phenomena
Crumbling
Cracking
Crust
Stain
Discolouration
Fading
Darkening

Emerald Green Deterioration

Keune K., Boon J.J., Boitelle R., Shimadzu Y., Degradation of Emerald green in oil paint and its contribution to the rapid change in colour of the Descente des vaches (1834–1835) painted by Théodore Rousseau, 2013, in: Studies in Conservation Vol. 58 Nr. 3, pp 896-897

Keune K., Mass J., Mehta A., Church J., Meirer F., Analytical imaging studies of the migration of degraded orpiment, realgar, and emerald green pigments in historic paintings and related conservation issues, 2016, in: Heritage Science, vol. 4 Nr. 10,
https://link.springer.com/article/10.1186/s40494-016-0078-1