In situ Raman characterization of minerals and degradation processes in a variety of cultural and geological heritage sites

F Gázquez; F Rull; A Sanz-Arranz; J Medina; J M Calaforra; C de Las Heras; J A Lasheras

doi:10.1016/j.saa.2016.04.035

In situ Raman characterization of minerals and degradation processes in a variety of cultural and geological heritage sites

Spectrochim Acta A Mol Biomol Spectrosc. 2017 Feb 5:172:48-57. doi: 10.1016/j.saa.2016.04.035. Epub 2016 Apr 14.

Authors

F Gázquez¹, F Rull², A Sanz-Arranz², J Medina², J M Calaforra³, C de Las Heras⁴, J A Lasheras⁴

Affiliations

¹ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, Cambridgeshire CB2 3EQ, United Kingdom.
² Department of Condensed Matter Physics, Crystallography and Mineralogy, University of Valladolid, Paseo de Belén, 7, 47011 Valladolid, Spain.
³ Water Resources and Environmental Geology Research Group, University of Almería, Crta.Sacramento s/n, 04120, La Cañada de San Urbano, Almería, Spain.
⁴ Museo de Altamira, Ministerio de Educación, Cultura y Deportes, 39330 Santillana del Mar, Cantabria, Spain.

PMID: 27106813
DOI: 10.1016/j.saa.2016.04.035

Abstract

We test the capabilities of in situ Raman spectroscopy for non-destructive analysis of degradation processes in invaluable masterpieces, as well as for the characterization of minerals and prehistoric rock-art in caves. To this end, we have studied the mechanism of decay suffered by the 15th-century limestone sculptures that decorate the retro-choir of Burgos Cathedral (N Spain). In situ Raman probe detected hydrated sulfate and nitrate minerals on the sculptures, which are responsible for the decay of the original limestone. In addition, in situ Raman analyses were performed on unique speleothems in El Soplao Cave (Cantabria, N Spain) and in the Gruta de las Maravillas (Aracena, SW Spain). Unusual cave minerals were detected in El Soplao Cave, such as hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O), as well as ferromanganese oxides in the black biogenic speleothems recently discovered in this cavern. In the Gruta de las Maravillas, gypsum (CaSO₄·2H₂O) was identified for the first time, as part of the oldest cave materials, so providing additional evidence of hypogenic mechanisms that occurred in this cave during earlier stages of its formation. Finally, we present preliminary analyses of several cave paintings in the renowned "Polychrome Hall" of Altamira Cave (Cantabria, N. Spain). Hematite (Fe₂O₃) is the most abundant mineral phase, which provides the characteristic ochre-reddish color to the Altamira bison and deer paintings. Thus, portable Raman spectroscopy is demonstrated to be an analytical technique compatible with preserving our cultural and natural heritage, since the analysis does not require physical contact between the Raman head and the analyzed items.

Keywords: Altamira Cave; Burgos Cathedral; Cultural heritage; El Soplao Cave; Gruta de las Maravillas; In situ Raman spectroscopy; Rock-art pigments; Speleothems.