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Today we are going back to #Pompeii to talk about another pigment used in the mural paintings of @pompeii_sites and @mannapoli, whose color has also changed throughout time: yellow ochre.
Ochre pigments are easily obtained from iron-rich claily soils of different hues, being the yellow ochres rich in goethite (FeOOH) and the red ones, in hematite (Fe2O3).
These pigments were also cheaper than others, such as cinnabar, which could cost up to 70 sesterces per pound, while the maximum price for ochre pigments was around 8 sesterces per pound. https://twitter.com/cinnabarim/status/1257007804373762048
Even if these earth pigments could also be of local origin, some ochres were imported due to their quality, according to Pliny and Vitruvius. Yellow ochres came from the Attica region, Asia Minor and Italy, while red ochres were famous in Ponto, Egypt, Lemnos and Cappadocia.
For instance, red ochre imported from Sinope in Ponto, used for the preparatory drawing on the penultimate mortar layer of a fresco painting, gave the name of this layer, known as “sinopia”. Due to the great quality of this pigment, it could cost up to 12 sesterces per pound.
As a previous research has pointed out, our perception of the number of mural paintings with red and yellow pigments differs from that of 79 CE. Today we see 246 red walls and 57 yellow walls, while there would have been around 165 red and 138 yellow. https://www.abc.es/cultura/abci-erupcion-vesubio-erupcion-rojo-201109160000_noticia.html?ref=https%3A%2F%2Ft.co%2FFyWHSecxLd%3Famp%3D1
This color transformation occurs owing to the dehydration of goethite (FeOOH), abundant in yellow ochre pigments, and its conversion into red hematite (Fe2O3). https://twitter.com/pompeii_sites/status/924918434131070977
Through thermal ageing experiments, we have determined that the yellow into red transformation slightly starts at 225ºC and it is thoroughly completed at 300ºC. Thus, the temperature of the pyroclastic materials was enough to induce this change.
If you pay attention, it is easy to note that only the upper section of this door has been transformed into red ochre. This is due to the accumulation of materials on the roofs at the early stages of the eruption, which finally collapsed.
The debris protected the lower fraction of the wall from the impact of high-temperature eruption materials, which covered the buildings of the Vesuvian area.
In some cases, the decoration of the mural painting makes it difficult to differentiate between the original and the transformed red ochre. In this example, the socle was painted with original red ochre, while the upper part was indeed yellow ochre.
Thanks to X-ray fluorescence, an elemental analytical technique, we are able to distinguish between original and transformed red ochre, which correspond to the same molecular compound: Fe2O3. https://twitter.com/cinnabarim/status/1261697196015202304
While original red ochre shows the presence of arsenic, yellow ochre lacks of this element. Hence, transformed red ochre will not display the peaks of arsenic.
Moreover, thanks to Raman spectroscopy, a molecular analytical technique, we have identified mineral phases of volcanic origin in the ochre pigments conserved in their original bowls.
These minerals are present in very low concentrations, but are still useful to propose that the studied ochre pigments have a Vesuvian origin: langite, jarosite, atacamite, etc. They usually appear in volcanic fumaroles.
This is the end of today's #thread about the yellow ochre into red ochre transformation. If you want to learn more about other pigment transformation in @pompeii_sites, go check this #thread. https://twitter.com/cinnabarim/status/1260998135956062209
More information about our research:
https://link.springer.com/article/10.1007/s00216-017-0329-3
https://link.springer.com/article/10.1007/s00216-019-02175-5 https://onlinelibrary.wiley.com/doi/full/10.1002/jrs.5414
https://link.springer.com/article/10.1007/s00216-017-0329-3
https://link.springer.com/article/10.1007/s00216-019-02175-5 https://onlinelibrary.wiley.com/doi/full/10.1002/jrs.5414
Aquí te cuento este hilo en castellano: https://twitter.com/cinnabarim/status/1263148538470567936
