Study of a tabernacle with a remarkable architectural structure: In situ examination using Raman spectroscopy

In order to characterize ornamental stones and gemstones from the monstrance ‘Tabernáculo de la Colegiata de S. Pedro’ in Lerma, Burgos, a Raman spectroscopy in situ non‐destructive study has been performed on these materials. The Raman spectra obtained correspond to cornalline‐like chalcedony, nephrite jade, quartz veinlets, agates with moganite and jasper, together with goethite showing several degrees of alteration. Various types of marble and lapis lazuli were identified. The lapis lazuli samples show different Raman spectra depending on the blue–white chromatic gradient, due to the variations in the sulfur concentration. Raman spectroscopy allows for the correlation of the presence of radical molecular ions S₂⁻, S₃⁻ and SO₄²⁻, with the colour centres and colouration of lapis lazuli. Two varieties of marble are present, one of brown tones with greyish‐white incrustations and another with alternating white and black veinlets with yellow inclusions. In the former, white masses of microcrystalline calcite appear together with quartz inclusions, around which brown halos of goethite develop as an alteration product. In the latter, they show calcite in the white veinlets and calcite with graphite in the black ones. The mineralogical analysis of ornamentals stones on art objects allows determining their degree of conservation and the extension of the deterioration, in order to establish strategies for the cleaning and restoration. These results are an important factor to date the object and for shedding light about its authorship. Furthermore, it could eventually help to establish connections between the tabernacle‐monstrance and other tabernacles in Castilla y León. Copyright © 2013 John Wiley & Sons, Ltd.