Characterization of Pararealgar and Other Light-Induced Transformation Products from Realgar by Raman Microspectroscopy

Naturally occurring arsenic sulfide minerals are brightly colored and consequently have been used as artists' pigments since ancient times. Orpiment, As₂S₃, is yellow and is often found associated with realgar, As₄S₄, which is red to red-orange. When exposed to sunlight, unprotected realgar develops a surface coating of friable yellow material that, until recently, had generally been assumed to be orpiment. However, it has been determined that this material is pararealgar, a light-induced polymorph of realgar. We have identified pararealgar in a major work by the Renaissance master, Tintoretto. The accurate identification of pigments in artistic and historic works is of great importance for determining the construction, history, and future preservation of these works. We report here the characterization by Raman microspectroscopy of pararealgar and the various intermediate species involved in the light-induced transformation of realgar to pararealgar. The relative merits of Raman microspectroscopy and X-ray diffraction for the accurate and efficient characterization of these arsenic sulfide compounds are discussed.