Virtuous colours for Mary. Identification of lapis lazuli, smalt and cochineal in the Andean colonial image of Our Lady of Copacabana (Bolivia).

The image of Our Lady of Copacabana, a gilded polychrome sculpture carved in maguey wood in 1583, is one of the most important devotions in the Americas. In former research, we have identified the use of gypsum, Armenian bole, cerussite and atacamite in its polychromy. In this study, a red sample taken from the Virgin's tunic and a blue sample extracted from the cloak have been analysed with the aim to identify both pigments and offer insights into the painting technique. Analysis by micro-Raman spectroscopy complemented with scanning electron microscopy-energy dispersive spectroscopy and high-performance liquid chromatography allowed the identification of carmine lake in the red sample. Analysis by micro-Raman spectroscopy of the surface of the blue sample and its cross section showed the presence of smalt-the blue-glass pigment-over a cerussite layer, bathed by a very thin ultramarine layer-from a probable native origin-following a pictorial tradition that would last even until the eighteenth century. This is the first time that lapis lazuli has been scientifically identified in a Spanish American colonial painted layer.This article is part of the themed issue 'Raman spectroscopy in art and archaeology'.

Triplet quantum yields in light-scattering powder samples measured by laser-induced optoacoustic spectroscopy (LIOAS).

In recent years, different methods and techniques have been applied to study the primary photophysical processes occurring in dye-loaded light-scattering powdered samples. In spite of this, there are still no reliable methods for the determination of triplet quantum yields for this kind of systems. Laser-induced optoacoustic spectroscopy (LIOAS) has been extensively used for the determination of triplet quantum yields of dyes in solution. In a previous work, LIOAS was applied to the measurement of absolute emission quantum yields of highly fluorescent powdered samples. Excellent agreement was found with values obtained from reflectance data. In this work, we apply the same technique for the determination of triplet quantum yields of Rose Bengal and Erythrosine B adsorbed on microcrystalline cellulose. In contrast to water and other solvents, internal conversion cannot be neglected in the cellulose environment. The triplet quantum yield for both dyes is around 0.55 and does not change with dye concentration.

Validation of fluorescence quantum yields for light-scattering powdered samples by laser-induced optoacoustic spectroscopy.

Determination of quantum yields for various processes (such as fluorescence and triplet formation) in dye-loaded light-scattering powdered samples is an open issue. Here, we report the testing of laser-induced optoacoustic spectroscopy (LIOAS) for the determination of fluorescence quantum yields of Rhodamine 101 and Rhodamine 6G adsorbed at various loadings on microgranular cellulose powder. The results of the LIOAS experiments are consistent with those from the method developed in one of our laboratories based on the measurement of apparent reflectance using an integrating sphere [Mirenda, M.; Lagorio, M. G.; San Roman, E. Langmuir 2004, 20, 3690-3697], which allows the simultaneous calculation of fluorescence quantum yield and reflectance devoid of fluorescence artifacts. Criteria to quantify overall errors and detect outlying values are developed. The theory underlying the application of LIOAS to light-scattering samples and experimental details are presented.