Characterization of pigments and binders in a mural painting from the Andean church of San Andrés de Pachama (northernmost of Chile)

The Andean church of San Andrés de Pachama is located in the highland of the northernmost of Chile, near the limit with Bolivia and next to the Ruta de la Plata. This commercial route contributed in the past to the transport and commerce of various raw materials, such as silver, from the Andean mountains region to the Pacific Ocean coast and then to the European market. The walls inside the church are decorated with paintings from the end of the eighteenth century that reproduce religious motivs together with flowers, fruits, and birds. In this study, micro samples taken from one of the mural paintings have been analysed to acquire information on the artistic materials and the painting technique previous to the restoration of the paintings. Analysis by micro-Raman spectroscopy complemented with scanning electron microscopy–energy dispersive X-ray spectroscopy, attenuated total reflection infrared spectroscopy, X-ray diffraction, and high-performance liquid chromatography with diode-array detection allowed the identification of orpiment, vermilion, indigo, smalt, antlerite, hematite, carmine lake, and wood charcoal as pigments as well as gypsum as the ground layer. Lipidic and proteinaceous materials extracted from the microsamples were identified by gas chromatography coupled to mass spectrometry and indicated the use of a mixture of egg and siccative oil as binders and a secco painting technique involving animal glue as the plaster primer. Smalt and the prized cochineal lake are reported for the first time in an Andean colonial mural painting.