Roman mural painting from the Domus de Salvius in Cartagena. Credit: Nanosanchez, Public domain, via Wikimedia Commons.
The mixture of paint pigments found in a Cartagena domus has revealed the technical expertise of first-century craftsmen.
Roman painters commissioned at the end of the 1st Century to decorate the walls of the Domus of Salvius in present-day Cartagena could hardly have imagined that their technical expertise would still attract attention twenty centuries later.
Analysis of wall paintings from one of the house’s rooms – among the best preserved in ancient Carthago Nova – shows that these craftsmen possessed a sophisticated understanding of the materials used to produce pigments, as well as the effects achieved through combining them.
In particular, researchers identified an advanced ‘recipe’ that enabled them to reduce costs while ensuring the durability of the paint.
This method relied on a mixture of pigments, including one of the most valued minerals of the time: costly cinnabar, often referred to as ‘red gold’.
This conclusion is the result of a multidisciplinary study conducted by researchers from several departments at the University of Córdoba (UCO).
Through a range of analytical techniques, the remains discovered in the domus have revealed a unique combination of pigments never before documented in Hispania, with only one known parallel in Ephesus, Turkey.
Researcher Daniel Cosano Hidalgo and José Rafael Ruiz Arrebola analyse wall paintings from the Domus of Salvius. Credit: University of Cordoba. Public domain.
Multidisciplinary research
The groups’ multidisciplinary research is in line with previous work on topics such as the world’s oldest wine and aromas that perfumed the Roman Empire.
In this case, analyses made it possible to determine the composition of the mortars used in the house through X-ray diffraction, as well as to identify pigment residues using Raman spectroscopy; a technique that detects chemical compounds based on how they interact with light.
The results support a previously proposed theory: that the Domus of Salvius belonged to a wealthy family capable of affording expensive construction and decorative materials.
However, the analysis of the pigments also led the team to propose a complementary hypothesis; one that is not related to the purchasing power of the domus’ inhabitants but rather to the technical skill of the craftsmen.
Techniques for preserving colour
Calcium carbonate for the white pigment, charcoal for the black, goethite for the yellow, and glauconite for the green with traces of Egyptian blue; the first synthetic pigment and a status symbol.
For the red pigment, a mixture of cinnabar and iron oxide, for which there are also documented precedents.
Researchers Daniel Cosano Hidalgo and José Rafael Ruiz Arrebola explained: “Iron oxide was a cheap material that was commonly used in workshops to create reddish tones. Cinnabar was more costly and had to be supplied by the client.”
They state it was common practice to mix these two elements to reduce costs without losing the chromatic intensity of the cinnabar, which thus lasted longer.
However, what was truly striking and innovative was not the mixture itself, but the way in which it had been applied to the walls of the Domus of Salvius.
Upon analysing the sample using scanning electron microscopy in the SCAI laboratories, the researchers discovered that the mixture that created the mural’s intense red colour had not been applied directly to the wall.
Instead, the surface had first been ‘primed’ with a layer of yellow goethite. This was no coincidence.
The study authors, who believe that the craftsmen applied the layer of goethite to protect the mixture of lime and iron oxide, possibly allowing it to act as a stabiliser, said: “Cinnabar tends to blacken when exposed to light, moisture, and caustic environments.”
In this way, the craftsmen ensured that the costly cinnabar not only went further but also retained its appearance for longer.
The use of this technique indicates a high level of expertise on the part of the craftsmen, who would have studied the materials, the effects resulting from combining them, and the application of various techniques.
The researchers suggest the existence of recipe books and workshops where this knowledge was developed and shared, not only in Cartago Nova but also beyond the borders of Hispania.
In this way, archaeometric analysis and co-operation between fields of knowledge that are, at first glance, very different, such as chemistry and archaeology, allow us to study the remains of antiquity from new perspectives and learn more about the past, by comparing the information obtained from classical sources, such as Vitruvius or Pliny the Elder, with the archaeological reality.
The study is published in Heritage Science.
