In today’s thread, we will be explaining the darkening of the vivid red cinnabar, a pigment employed in the most lavish mural paintings at @pompeii_sites. 
This example, shared by @PaulHar77393852, is surely among the most well-known. Let’s recreate ourselves with this poisonous yet fabulous red hue.
As you know, one of the most important mercury (and cinnabar) ore is located in Almadén (the Roman Sisapo). According to Pliny the Elder, it was exploited by the "socii sisaponensis" and only a certain quantity was allowed to be extracted per year (up to 100 net pounds).
After that, the mines were locked until the following year with a key that was guarded by the governor of the province. The prices reached by this fine red pigment needed to be limited by law to 70 sesterces per pound.
As you may infer from these circumstances, the pigment manufacture was controlled as well and only executed in Rome, in workshops located between the Temples of Flora and Quirinus, according to Vitruvius.
As we already mentioned in this #thread, the pigment manufacture consisted in grinding, washing and heating the mineral multiple times in order to obtain the purest red hue. They did not synthesise artificial HgS, but purified the natural ore.
The “dominus” of the house to be decorated signed a contract with the “redemptor”, the businessman in charge of the completion of the works, who was responsible of providing the materials, except from some costly pigments (our cinnabar), directly delivered by the “dominus”.
However, neither Vitruvius nor Pliny encouraged the use of this pigment, which proved the wealthy status of the “dominus”. Vitruvius even criticised the application of costly pigments, since this could mask the lack of talent of the painters (subtilitas artificis).
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“Who in former times used [cinnabar] otherwise than as a medicine? In the present age, however, walls are everywhere covered with it. To this may be added the use of chrysocolla, purple, and azure decorations, which, without the aid of real art, produce a splendid effect.”
Moreover, Vitruvius gave the example of secretary called Faberius, who wanted to decorate the walls of the peristyle of his domus on the Aventina with cinnabar red, resulting in an ugly colouration after thirty days due to the permanent exposure to light.
If we head now to the archaeological excavations at #Pompeii during the 19th century and the reports by the archaeologist Wolfgang Helbig, the cinnabar of certain mural paintings was red and vivid when excavated but darkened once exposed to light.
This late republican mural painting excavated at the Domus dei Bucranes (Ostia) appears to present the same problem if we compare the conservation state in 2005 and 2006.
The early attempts to explain this degradation were based on mineralogical studies, which have proved the occurence of another crystaline HgS phase (beta-HgS), metastable at room temperature and obtained heating red cinnabar above 350 ºC.
Whereas cinnabar presents a deep red hue, metacinnabar is grayish-black. Fortunately, both can be distinguished via X-ray diffraction. During the early 20th century, scientists used to attribute the darkening to a cinnabar into metacinnabar conversion through light exposure.
However, there exist scarce examples of metacinnabar detection in mural paintings which show darkened cinnabar, so this conclusion was sometimes obtained without being able to compare the X-ray diffractograms of the sample and the minerals.
Luckily, one of those examples are the mural paintings of the Sucevița Monastery (Rumania), where metacinnabar was identified in the samples taken at places directly exposed to solar light.
Even if the pyroclastic materials of the 79 CE eruption are known to have achieved a temperature above 350-380 ºC, no metacinnabar has been detected yet in #Pompeii.
Thanks (again) to geologists, it was established that cinnabar ores with a chlorine weight concentration higher than 0.25 % were photosensitive. In other words, they were prone to darken upon light exposure.
In addition, even if certain cinnabar ores (such as Almadén) were not intrinsically photosensitive, they could become photosensitive after being exposed to chlorine. The geographical location of #Pompeii could favour the accumulation of chlorides, coming from the marine aerosol.
After this breakthrough, certain cinnabar-chloride degradation products have been detected in darkened cinnabar areas in mural and easel paintings: calomel (Hg2Cl2), corderoite (α-Hg3S2Cl2) or kehnsuite (γ-Hg3S2Cl2). However, none of these minerals is intrinsically black.
Moreover, other influences need to be considered as well when we talk about mural paintings, since the examples at #Pompeii are generally composed by lime-based mortars and aggregates of different size.
Hence, when gypsum (CaSO4·2H2O) is detected in fresco paintings, it is known to be a degradation product of a sulphation process that occurs due to environmental SO2. Even if gypsum is white, it can accumulate organic pollutants, thus leading to a blackish colour (black crust).
In a research work in which darkened cinnabar samples from the Villa Sora (Torre del Greco), near #Pompeii, have been analysed, both calomel, corderoite and gypsum were identified in degraded areas.
On the other hand, there are studies concerning tempera painting mock-ups which have identified metallic mercury as degradation product in darkened areas. Nonetheless, both metallic mercury and metacinnabar are difficult to detect in-situ using portable spectroscopic techniques.
Fortunately, innovative techniques have recently allowed the detection of metacinnabar in tempera painting (14th century), without the need of sampling. This is one of the first identifications of metacinnabar in darkened cinnabar, next to the Sucevița Monastery.
To sum up, it seems clear that there are several open paths to explain the darkening of this precious pigment and further research is still needed to clarify the role of each one in this process.
Aquí te cuento este hilo en castellano:
