CCI Newsletter, No. 22, November 1998

The File

by Marie-Claude Corbeil, Senior Conservation Scientist, Analytical Research Laboratory

A few years ago, scientists at CCI detected the presence of pararealgar in a painting from the Italian school [1]. Pararealgar is a yellow polymorph of arsenic (II) sulfide; it is a naturally occurring substance, but can also be formed by the action of light on orange-red realgar, another polymorph of arsenic (II) sulfide. Therefore, when pararealgar is found on an art object there is always a question as to whether the artist initially used yellow pararealgar or orange-red realgar, which was subsequently transformed into pararealgar. We recently conducted an analysis of a sculpture of Saint John (attributed to Jacques Leblond de Latour, ca. 1700). Using X-ray diffraction, phase (an intermediate product in the transformation of realgar into pararealgar) was identified in an early layer of the robe. Therefore, in this case it can be concluded that the artist originally used orange-red realgar; a light-induced transformation to pararealgar had begun but the robe was repainted with red before the reaction had gone to completion.

Cross-section of a sample from the robe of Saint John showing underlying layers of polychromy. From the bottom: a white ground layer, an orange layer which is coloured by natural red and yellow iron oxides, the orange layer containing phase , and finally a red layer coloured by vermilion.

When Isabelle Paradis, of the Centre de conservation du Québec, was treating the sculpture of Saint John, she noticed that the phase layer was lifting in several places. This is consistent with studies on the light-induced transformation of realgar by Douglass et al. [2] who noted that pararealgar (which has a greater volume than realgar) forms first as a thin layer, which then cracks and flakes off. The fact that the phase layer did not adhere is another indication that it originally contained realgar, which was altered by light.

Since the publication of our findings, pararealgar has been identified in art objects by several other scientists, confirming that its presence in our painting was not an isolated occurrence. The identification of phase in a polychrome sculpture adds to our understanding of the use and behaviour of sulfur- and arsenic-based pigments.

1. Corbeil, M.-C., and K. Helwig. "An Occurrence of Pararealgar as an Original or Altered Artists' Pigment." Studies in Conservation, Vol. 40, No. 2 (May 1995), pp. 133–138. 7.
   
2. Douglass, D.L., C. Shing, and G. Wang. "The Light-Induced Alteration of Realgar to Pararealgar." American Mineralogist, Vol. 77 (1992), pp. 1266–1274.