Analysis of pigments and mortars from the wall paintings of the Roman archaeological site of Las Dunas (San Pedro de Alcántara, Malaga S Spain)

Abstract

This study aims to describe the composition, texture, and arrangement of mortars and pictorial layers from a newly excavated Roman archaeological site in the province of Baetica, southern Spain. Representative fragments with well-preserved pictorial layers were analyzed using optical imaging, spectrophotometry, Scanning Electron Microscopy with Energy-Dispersive Spectroscopy (SEM-EDS) mapping, and X-ray diffraction. The results reveal similarities to other well-studied Roman archaeological sites, particularly in the use of mixtures of natural (hematite, goethite, aragonite, phyllosilicates, and quartz) and synthetic minerals (cuprorivaite) used in classical Roman pigments. However, the mortar from the investigated site shows a distinctive use of ferromagnesian silicates (olivine and pyroxenes from local ultramafic rocks) as aggregate material, making this site unique among all Roman provinces. Additionally, the common occurrence of Mg in the binder of the mortars under the form of brucite, hydromagnesite and magnesium silicate hydrates, indicates the use of a magnesian (dolomitic) lime that, according to the chemical analyses, was obtained by adding up to 15 wt% of powdered dolomitic marbles from nearby Roman quarries to the limestone. We speculate that this addition might be intended to improve the workability and mechanical resistances of the mortar in combination with the abundant ferromagnesian silicates used as aggregates, some of them showing chemically reactive rims in contact with the binder. In conclusion, this study provides further evidence of the advanced knowledge and skills achieved by Roman craftsmen who were able to adapt traditional recipes to suit local resources, obtaining materials of excellent quality and durability.