Cinnabar, hematite and gypsum presence in mural paintings in Teotihuacan, Mexico

In the color palette of mural paintings in the pre-Hispanic city of Teotihuacan, Mexico, the raw materials used to produce the diverse tonalities of red applied to the decoration of their main buildings has been associated with hematite, iron oxides and, to a lesser extent, with cinnabar. The latter has been described as a mineral related to cult and was reserved exclusively for the upper social hierarchies (ruling, religious and military classes). Previous studies have identified cinnabar in loose fragments of mural paintings dated as belonging to the late stages of the stylistic and technological development of the city (Xolalpan phase, 350-550 CE). Nevertheless, it had never been detected directly on standing mural paintings of earlier periods, until now. This research exposes the detection of these valuable minerals in mural designs of early structures of the architectural complex of Quetzalpapalotl, by means of a non-destructive portable X-ray fluorescence technique. The results were verified through the application of other analytical techniques such as scanning electron microscopy with energy dispersive spectroscopy and Raman spectroscopy. The analyses not only confirmed the existence of cinnabar and hematite minerals in mural paintings, but also identified the presence of gypsum as part of the composition of the stuccos. Although gypsum has been found in grounds of the Mayan area, it has never been reported before for the stucco technology in Teotihuacan. The outcomes state an earlier technological development than formerly proposed for mural painting production in Teotihuacan.