In-situ multi-analytical characterization of original and decay materials from unique wall mirrors in the House of Gilded Cupids, Pompeii

Abstract

Abstract The House of Gilded Cupids (Regio VI, Insula 16, 7, 38) was unquestionably one of the most important residences of Ancient Pompeii, where important archaeological artefacts such as mural paintings, mosaics, sculptures and lalariums were rediscovered. This work characterizes two wall mirrors that, together with those recovered from the House of Efebo and the Domus of Euplia, represent the only ones found in the archaeological site of Pompeii. The 2015 and 2016 expeditions of the Analytica Pompeiana Universitatis Vasconicae project performed an in situ multi-analytical study, using only portable non-destructive analytical techniques. Molecular data provided by Raman spectroscopy suggested obsidian was the reflective matrix for both mirrors. Elemental data provided by energy dispersive X-ray fluorescence and laser induced breakdown spectroscopy (LIBS) systems were concordant with Raman spectroscopic results, enabling the detection of Ca, K, Al and Na as the main elements included in the Si matrix characteristic of obsidian igneous rocks. The LIBS data confirmed the presence of obsidian hydration layers. All techniques were used to investigate the degraded white and yellow crusts of the mirrors. Gypsum (CaSO4·2H2O) and nitrocalcite (Ca(NO3)2·4H2O) were identified in the white areas and gypsum and goethite (α-FeOOH) in the yellow crusts. LIBS depth profiling on the white crusts found a layer of nitrocalcite on top of the gypsum layer. Gypsum and goethite were proposed to form after partial dissolution of the mortars patches (with high iron contents) used in modern restorations around the mirrors, followed by a reprecipitation on the surface of the mirrors. Nitrocalcite was proposed to form after the attack of atmospheric NOx on gypsum crusts. These results represent the first analytical work focused on the study of these unique mirrors and provide the knowledge needed for defining more adequate conservation treatments.