An overview concerning the study of mediaeval wall paintings published in the last 20 years was described and a change in the use of diagnostic techniques has been emphasized. Indeed, actually they are used especially for the study of decay products with the aim of suggesting the best conservation method.

The presence of salt efflorescence is one of the main causes of deterioration of building materials. Raman spectroscopy can be used not only to identify the different compounds present in such salts but also for the semiquantification of such salt deposits based on the use of external calibration curves. Normally, this method takes time for the preparation of pellets composed of both mixtures of standard and real samples. In the present work, we propose an innovative approach for semiquantification of salts in the solid state by portable micro‐Raman spectroscopy. It is based on the use of the palme software (From Molecules to Nano‐objects: Reactivity Interactions and Spectroscopy–University Pierre and Marie Curie, Paris) that has been developed in order to obtain the quantitative information from the vibrational spectrum by using a multivariate linear regression approach. It considers the mixture spectra as a linear combination of the suspected pure material spectra. The aim of this work is to apply palme software to the study of pellets of standard samples composed of mixtures of calcium carbonate, gypsum and potassium nitrate at different concentrations in order to verify the reliability of this method by comparing the results with those obtained through the construction of calibration curves. Moreover, both methodologies have been tested on real efflorescence salts sampled in the San Esteban Church (Ribera de Valderejo, Basque Country, Spain). This research shows that the method based on the treatment of Raman spectra with palme software allows to obtain in situ semiquantitative results in a much shorter time (minutes in fact), being comparable in quality with those obtained with the external calibration curve method. Copyright © 2016 John Wiley & Sons, Ltd.

This work reports the results of an extensive study carried out on the wall paintings preserved inside the Saint Stephen's chapel in Montani (Val Venosta, Bozen, Italy), by means of μ‐Raman spectroscopy, portable X‐ray fluorescence (XRF) spectrometry, and powder X‐ray diffraction, in order to characterize materials and their alteration products that cause the blackening of paintings. In situ XRF analysis allowed identifying the areas of interest where the blackening appeared. The complementary analytical techniques allowed reconstructing a chromatic palette used by the artists that includes expensive pigments, such as lapis lazuli and cinnabar, indicating a wealthy client. Raman spectra at very low power recorded on blackened degraded samples showed the presence of a mixture of the two polymorphs of lead (IV) dioxide, plattnerite (β‐PbO2), and scrutinyte (α‐PbO2), as degradation products of lead‐based pigments. On these samples, no evidence of white lead was found, although a white lead conversion treatment had been applied on the paintings. The degradation of red lead (Pb3O4) into a mixture of plattnerite, scrutinyte, and anglesite (PbSO4) was demonstrated in some darkened samples. Furthermore, the degradation of haematite (Fe2O3) with the formation of magnetite (Fe3O4) and coquimbite (Fe2(SO4)3·9H2O) was also identified as responsible for the blackening of the paintings. The influence of original materials and environmental and anthropogenic factors such as the valley's orography and the presence of contaminating agents (SOx) is discussed to explain the decay phenomena.

The present work concerns the study of the phase transition of plattnerite [β-PbO2 lead (IV) oxide]-based samples when they are analysed by Raman spectroscopy. The laser-induced degradation process was carried out either on historical painting samples, where plattnerite was present as a degradation product of lead-based pigments, or commercial plattnerite samples as powder and pellets. The Raman spectra of plattnerite taken at low excitation power, to avoid phase transformations, are reported up to low wavenumbers, and they were characterized by the features at 159, 380, 515 and 653 cm−1 and a shoulder at 540 cm−1. The degradation of plattnerite was induced by increasing the laser power on the sample, and the formation of its secondary products red lead (Pb3O4), litharge (α-PbO) and massicot (β-PbO), when varying the laser power, is discussed. The analyses were performed in a controlled condition by coupling the Raman spectrometer to a temperature-controlled stage (Linkam THMS600- Renishaw), which allows for varying the sample temperature (from room temperature up to 600 °C) and keeping it constant inside the stage during the analysis. In this way, commercial plattnerite samples were heated by increasing the cell temperature to verify the temperature range at which the phase transitions of lead dioxide occur. In addition, thanks to the construction of temperature ramps, all the degradation pathways were shown, and other lead compounds were identified, generated by the laser power contribution. A different behaviour was found between pigments from historical painting samples and commercial samples under the effect of the laser. This information could be useful in order to recognize their nature when they are found in cultural heritage materials.

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