Direct evidence for metallic mercury causing photo-induced darkening of red cinnabar tempera paints

The photo-induced darkening of red cinnabar (α-HgS), also known as vermilion, significantly alters the visual appearance of artworks. This phenomenon has been observed in numerous masterpieces, causing considerable concern among art conservators. Previous research has often linked this darkening to the formation of metallic mercury (Hg0) in the presence of chlorides, a process suggested to involve a redox reaction where chlorides act as either a catalyst or intermediate product. This process would occur via photo-induced electron transfer and relies on the semiconductor properties of cinnabar. However, the energy of photo-excited electrons in cinnabar might not be sufficient to directly reduce Hg2+ to Hg0 in the absence of chlorides. Previous research, based on electrochemical experiments and analysis of alteration products, has proposed that chlorides are necessary for the reduction of Hg2+ to Hg0. Several chloride sources have been proposed, ranging from impurities in the pigment to external contaminants such as sea spray, protective coatings, varnishes, and even cleaning products. Despite the variety of potential chloride sources, there have been cases of cinnabar blackening where chloride-containing impurities or alteration products could not be detected, suggesting the possibility of an alternative mechanism. This study aims to investigate whether cinnabar blackening can occur without the presence of chlorides and to elucidate the chemical reactions involved. To this end, the researchers used both natural cinnabar pigment and cinnabar-based tempera paint mock-ups, subjecting them to accelerated ultraviolet (UV) aging under conditions of high relative humidity to simulate a highly oxidative environment.

Numerous studies have explored the photo-induced darkening of cinnabar, focusing on the analysis of alteration products and electrochemical experiments to understand the chemical changes. The semiconductor properties of cinnabar, particularly its n-type behavior and band gap energy of 2.0 eV, have been considered crucial. Light with wavelengths less than 620 nm can activate cinnabar, while wavelengths greater than 620 nm are reflected, resulting in the pigment’s characteristic red color. Different pathways for blackening have been proposed, including the formation of metacinnabar (β-HgS). The formation of metallic mercury (Hg0) is widely accepted as a primary cause of darkening, however, direct evidence of this formation on altered cinnabar paints has been lacking. The crucial role of chlorides has been recognized, either as a catalyst for cinnabar’s redox reaction or as intermediate products that are photochemically reduced to metallic mercury. However, research has shown that cinnabar darkening can occur in the absence of halides, albeit at a slower rate. Therefore, the need for an alternative explanation of cinnabar darkening that considers the semiconductor properties of the pigment and the interaction with the binder becomes evident. The proposed alternative pathway should clarify the darkening reaction in the absence of chlorides.

The study used natural Chinese cinnabar pigment and prepared cinnabar-based tempera paint mock-ups using fresh egg yolk as a binder. These samples were subjected to UV aging at room temperature and high relative humidity (80 ± 3%) for two months. The UV source was a mercury lamp emitting primarily at 254 nm (UV-C radiation). Multiple analytical techniques were employed to analyze the samples, both before and after UV exposure. These methods included: X-ray diffraction (XRD) to identify crystalline phases, Field emission scanning electron microscopy-energy dispersive X-ray spectroscopy (FESEM-EDS) for textural and compositional analysis, micro X-ray fluorescence (µ-XRF) for elemental mapping, X-ray photoelectron spectroscopy (XPS) to determine elemental oxidation states, optical microscopy for color and surface changes, and spectrophotometry to quantify color differences. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) was also used to study changes in the organic binder composition. The researchers were particularly careful to verify the absence of chlorides in their samples using multiple methods to ensure that chloride contamination did not impact the results.

UV exposure led to significant darkening and changes in both the cinnabar pigment and the tempera paint. In the pure cinnabar pigment, XRD analysis revealed the formation of mercury sulfate hydrate (HgSO4·H2O) and basic mercury(II) sulfate (schuetteite, Hg3(SO4)O2) after four weeks of UV aging, accompanied by a decrease in cinnabar Bragg peak intensity. FESEM observations confirmed surface changes, including crack formation and the appearance of mercury sulfate crystals. Nanometer-sized droplets, believed to be metallic mercury (Hg0), were observed on the pigment surface, although their composition could not be definitively determined due to their small size. XPS revealed significant changes in the oxidation states of Hg and S, indicating the formation of sulfates. In the tempera paint mock-ups, UV aging also caused darkening, significant binder degradation and a substantial loss of organic material, but less mercury sulfate was detected than in the pure pigment samples. FESEM revealed nano- and micrometer-sized mercury droplets on the paint surface, confirming the presence of Hg0. XPS showed the presence of sulfates, primarily in the outer surface layer, which was consistent with XRD data showing no detectable mercury sulfate phases within the paint. ATR-FTIR revealed that cinnabar accelerated the photo-induced degradation of the egg yolk binder. The oxidation of lipids and proteins, specifically the reduction of double bonds and the formation of aldehyde functional groups, was evident. The presence of cinnabar increased the rate of this degradation compared to the pure egg yolk, suggesting a competitive depletion of photo-generated holes by the organic components.

The results support a new pathway for cinnabar darkening that does not require chlorides. Photo-induced oxidation of cinnabar leads to the formation of mercury sulfates. These sulfates are then reduced to metallic mercury (Hg0) through photo-induced electron transfer. The organic binder plays a crucial role, not only as a protective layer reducing the UV radiation reaching the pigment, but also as a competitor for the photogenerated holes, thus slowing down the formation of mercury sulfates. However, the organic binder also provides sites for Hg0 sorption, ultimately responsible for the observed darkening. The formation of schuetteite in the pure pigment acted as a sink for Hg2+, limiting Hg0 formation. The absence of schuetteite in the paint, coupled with the presence of Hg0 and the degraded binder, suggests that the organic material provides sorption sites which allow for the accumulation of metallic mercury and hence darkening of the paint. The study concludes that the presence of metallic mercury droplets on the UV-exposed tempera paint surface, obtained using different methods, directly supports the widely accepted hypothesis that metallic mercury is responsible for cinnabar darkening.

This study provides direct evidence for the formation of metallic mercury (Hg0) as the cause of cinnabar darkening in tempera paints. The process is shown to occur through photo-induced electron transfer, leading to the oxidation of cinnabar to mercury sulfates and their subsequent reduction to Hg0. The organic binder plays a key role in this process by competing for photogenerated holes and providing sorption sites for Hg0. Future research could focus on investigating the precise nature of the Hg0-organic binder interactions and how this affects the rate of darkening under different environmental conditions.

The study used accelerated UV aging, which may not perfectly replicate natural aging processes. The high relative humidity used during the experiment might have influenced the reaction rates and the formation of certain alteration products. Finally, the study focused on a specific type of cinnabar and egg yolk tempera paint; the results might not be directly generalizable to all types of cinnabar-based paints.