In today’s #thread we are going to talk about a degradation process that endangers the red ochre pigment of the mural paintings of @pompeii_sites: its darkening.
In today’s #thread we are going to talk about a degradation process that endangers the red ochre pigment of the mural paintings of @pompeii_sites: its darkening.
In today’s #thread we are going to talk about a degradation process that endangers the red ochre pigment of the mural paintings of @pompeii_sites: its darkening.
As we have already explained in a previous #thread, both red and yellow ochre were obtained from clays and soils rich in iron oxyhydroxides, which confer the pigments their characteristic hue. https://twitter.com/cinnabarim/status/1263151382086979584
The most common deterioration process that affects this kind of pigments is the dehydration from yellow into red ochre, observable in mural paintings of @pompeii_sites and @MANNapoli. Here you have a #thread on this topic in case you are interested: https://twitter.com/cinnabarim/status/1263151377695625217
Nonetheless, this is not the only degradation pathway of ochre pigments. The darkening of red ochre has been studied in the House of Marcus Lucretius (Regio IX, 3, 5), paying special attention to the paintings of the fauces and the triclinium. https://twitter.com/cinnabarim/status/1271822071492902914
Even if the predominant hue of the fauces is of course #EgyptianBlue, the socle was painted using red ochre and appears to be blackened nowadays.

https://twitter.com/cinnabarim/status/1271822079315378176
Even if the predominant hue of the fauces is of course #EgyptianBlue, the socle was painted using red ochre and appears to be blackened nowadays. https://twitter.com/cinnabarim/status/1271822079315378176
Even if the predominant hue of the fauces is of course #EgyptianBlue, the socle was painted using red ochre and appears to be blackened nowadays. https://twitter.com/cinnabarim/status/1271822079315378176
Even if the predominant hue of the fauces is of course #EgyptianBlue, the socle was painted using red ochre and appears to be blackened nowadays. https://twitter.com/cinnabarim/status/1271822079315378176
Even if the predominant hue of the fauces is of course #EgyptianBlue, the socle was painted using red ochre and appears to be blackened nowadays. https://twitter.com/cinnabarim/status/1271822079315378176
The triclinium shows both degradation processes: yellow into red ochre dehydration and blackening of red ochre in certain areas.

https://twitter.com/cinnabarim/status/1271822098789552128
The triclinium shows both degradation processes: yellow into red ochre dehydration and blackening of red ochre in certain areas. https://twitter.com/cinnabarim/status/1271822098789552128
The triclinium shows both degradation processes: yellow into red ochre dehydration and blackening of red ochre in certain areas. https://twitter.com/cinnabarim/status/1271822098789552128
The triclinium shows both degradation processes: yellow into red ochre dehydration and blackening of red ochre in certain areas. https://twitter.com/cinnabarim/status/1271822098789552128
The triclinium shows both degradation processes: yellow into red ochre dehydration and blackening of red ochre in certain areas. https://twitter.com/cinnabarim/status/1271822098789552128
In order to understand this degradation process, an in-situ analysis campaign was performed at @pompeii_sites and certain painting microsamples were further investigated in the laboratory.
In order to understand this degradation process, an in-situ analysis campaign was performed at @pompeii_sites and certain painting microsamples were further investigated in the laboratory.
In order to understand this degradation process, an in-situ analysis campaign was performed at @pompeii_sites and certain painting microsamples were further investigated in the laboratory.
In order to understand this degradation process, an in-situ analysis campaign was performed at @pompeii_sites and certain painting microsamples were further investigated in the laboratory.
In order to understand this degradation process, an in-situ analysis campaign was performed at @pompeii_sites and certain painting microsamples were further investigated in the laboratory.
In this stratigraphy of a sample of the fauces, the following layers are distinguished thanks to their Raman bands: 1) thin black layer of magnetite (Fe3O4, 660 cm-1) and gypsum (CaSO4·2H2O, 1008 cm-1), 2-4) red pictorial layer -hematite- (Fe2O3), and 5) calcite mortar (CaCO3).
On the other hand, some whitish-red areas of the samples of the triclinium yielded Raman bands compatible with those of nonahydrate iron sulfates, such as coquimbite o paracoquimbite.
On the other hand, some whitish-red areas of the samples of the triclinium yielded Raman bands compatible with those of nonahydrate iron sulfates, such as coquimbite o paracoquimbite.
On the other hand, some whitish-red areas of the samples of the triclinium yielded Raman bands compatible with those of nonahydrate iron sulfates, such as coquimbite o paracoquimbite.
The darkening of red ochre, associated with the presence of gypsum, magnetite, and iron sulfates, is due to a SO2 attack on the painting. In order to replicate the conditions that lead to this process at @pompeii_sites, artificial ageing was accomplished using a special chamber.
Thanks to the presence of moisture on the walls, SO2 is able to dissolve and oxidise to H2SO4, which attacks the calcite of the mortar and forms gypsum. Moreover, SO2 oxidation implies the reduction of red hematite into black magnetite.
Quick Chemistry note 👩‍🔬👩‍🎨: oxidation means to lose electrons (sulfur changes from a +4 oxidation state in SO2 to +6 in H2SO4), always linked to a reduction or electron gaining by another species (iron changes from +3 in Fe2O3 to +2, +3 in Fe3O4). These are called redox reactions.
High gypsum concentrations create the perfect environment to enhance the formation of iron sulfates, such as coquimbite or paracoquimbite, detected at the triclinium.
The SO2 that attacks the paintings can come from atmospheric pollution (its concentration reached 50 ppb in the 1980s in Napoli) or volcanic phenomena, such as mofète, which implies the emission of CO2, hydrogen, and sulfur.
In fact, the excavations of Herculaneum, Portici, and Torre del Greco had to be suspended after the 1944 eruption because of this kind of poisonous volcanic emissions.

https://www.sciencedirect.com/science/article/pii/S0026265X14001763 https://twitter.com/GusmanoRino/status/1345438899208912896
Nonetheless, this is not the only red pigment that tends to darken in @pompeii_sites. My beloved red cinnabar also suffers from a blackening process: https://twitter.com/cinnabarim/status/1260998135956062209
If you want to learn more about the in-situ analysis of darkened red ochre on these paintings and how its degradation process was studied, here you have two articles on this topic:

https://pubs.rsc.org/en/content/articlehtml/2014/ay/c3ay41741g https://pubs.acs.org/doi/10.1021/ac1029192
Thermodynamic and Spectroscopic Speciation to Explain the Blackening Process of Hematite Formed by...

After many decades exposed to a polluted environment, in some areas of Marcus Lucretius House, there are clear signs that plasters and hematite pigments are suffering deterioration. In the exhaustive...

https://pubs.rsc.org/en/content/articlehtml/2014/ay/c3ay41741g
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