Blackening of Pompeian Cinnabar Paintings:  X-ray Microspectroscopy Analysis

Abstract

Red Pompeian paintings, very famous for their deep intensity, are currently suffering from darkening. The origins of this darkening degradation are not clearly identified yet and remain a major issue for curators. In the specific case of cinnabar (HgS)-based red pigment, a photoinduced conversion into black metacinnabar is usually suspected. This work is focused on the blackening of red cinnabar paintings coated on a sparry calcite mortar. Different samples exhibiting different levels of degradation were selected upon visual observations and analyzed by synchrotron-based microanalytical techniques. Atomic and molecular compositions of the different debased regions revealed two possible degradation mechanisms. On one hand, micro X-ray fluorescence elemental maps show peculiar distributions of chlorine and sulfur. On the other hand, X-ray absorption spectroscopy performed at both Cl and S K-edges confirms the presence of characteristic degradation products: (i) Hg-Cl compounds (e.g., corderoite, calomel, and terlinguaite), which may result from the reaction with exogenous NaCl, in gray areas; (ii) gypsum, produced by the calcite sulfation, in black coatings. Metacinnabar is never detected. Finally, a cross section was analyzed to map the in-depth alteration gradient. Reduced and oxidized sulfur distributions reveal that the sulfated black coating consists of a approximately 5-mum-thick layer covering intact cinnabar.

Cinnabar alteration in archaeological wall paintings: an experimental and theoretical approach

“…Additionally, a review of the geochemical literature suggests a transformation from α--cinnabar to β--cinnabar requires the application of significant thermal energy; α--cinnabar must be heated to an excess of 381°C to cause a phase change (Dickson and Tunell 1959). 1 This stands in contrast, to anecdotal and empirical accounts that describe cinnabar alteration under ambient conditions (McCormak 1997, McCormak 2000, Radepoint 2013and Cotte et al 2006. While the theory is largely considered outmoded, it continues to be cited within the literature as a possible means of explaining the blackening of cinnabar (Béarat et al 2013).…”

“…Scholars have posited two possible pathways by which chlorine might provoke a light--induced alteration. In the first, chlorine reacts with cinnabar to form various light--sensitive mercury--chloride and mercury--sulfochloride compounds (Spring and Grout 2002, Cotte et al 2006, Cotte et al 2008, Cotte et al 2010and Radepont et al 2011. Analysis of blackened cinnabar on cultural heritage materials has identified the presence of calomel (Hg 2 Cl 2 ), mercuric chloride (HgCl 2 ) as well as Hg 3 S 2 Cl 2 polymorphs including corderoite (α--Hg 3 S 2 Cl 2 ), β--Hg 3 S 2 Cl 2 and kenhsuite (γ--Hg 3 S 2 Cl 2 ) ( Table 1).…”

“…Analysis of blackened cinnabar on cultural heritage materials has identified the presence of calomel (Hg 2 Cl 2 ), mercuric chloride (HgCl 2 ) as well as Hg 3 S 2 Cl 2 polymorphs including corderoite (α--Hg 3 S 2 Cl 2 ), β--Hg 3 S 2 Cl 2 and kenhsuite (γ--Hg 3 S 2 Cl 2 ) ( Table 1). The presence of such compounds could certainly alter the color of the surface as Cotte et al (2006) and Radepont et al (2011) suggest; colomel and corderoite in particular are grayish in color. To our knowledge, however, there is no known mercury chloride--containing compound that would account for the black color observed.…”

“…The Roman architect Vitruvius wrote in the first century AD: "When the lumps of ore are dry, they are crushed in iron mortars and repeatedly washed and heated until impurities are gone and the colors come" (Vitruvius 1960). Cotte et al (2006) has suggests that Punic wax, a protective coating applied to fresco surfaces in antiquity might have served as the source of chlorine. Pliny the Elder wrote in his work Nauralis Historia: "Punica wax is made this way: yellow beeswax is often exposed to air for a long time, then it is boiled in water from high sea, to which added nitre.…”

“…Latin author Vitruvius -writing about Greek and Roman art and architecture during this same period -mentions in his book De Architectura, the application of a wax coating over areas painted with cinnabar to prevent it from darkening (Vitruvius 1960: VII:9). In modern times, cultural heritage professionals at the Roman site of Pompeii, Italy and at other archaeological sites with wall painting pigmented with cinnabar have reported the same phenomenon; red paint layers darken significantly after excavation (Gettens et al 1986, Cotte et al 2006 , egg--tempera and oil--based paint films in fine art paintings (Feller 1967, Gettens et al 1986, Daniels 1987, Spring and Grout 2002, Keune and Boon 2005and Radepont 2013 and Byzantine wall paintings (Pique et al 2007;Kakoulli and Fischer 2009;Sotiropoulou et al 2008, Radepont et al 2011 well as the few studies on archaeological wall paintings (Cotte et al 2006, Cotte and Susini 2009, Radepont et al 2011, Radepont 2013, this paper attempts to further clarify the mechanism(s) of cinnabar alteration in fresco applications, specifically the role of light and chlorine ions, through laboratory--based experiments and thermodynamic modeling. The aim is to understand better the transformations involved as a means to aid conservators in determining what steps may be taken during or immediately following excavation of wall paintings pigmented with cinnabar to slow down, mitigate or avoid the further darkening of the paint layer.…”