From Mock-ups to Artworks: Development and Application of Specific Analytical Methodologies

Abstract

The main scope of this doctoral thesis was the evaluation of micro- and non-destructive techniques applied to artwork characterization. The need to study capabilities and limitations of these techniques arises from their increased use in the field of Cultural Heritage, without sufficient previous evaluation. Oil paintings, photographs and a large metal alloy object were investigated as representatives of three important artwork categories. Chapter 1 is an overview of the material properties of the artwork types investigated. Chapter 2 presents the study of the detection limits and the evaluation of the reproducibility of SEM-EDS and micro-FTIR in the application to paint layer investigation. Despite the good quality of the obtained spectra and the high sensitivity of these techniques, the low amount of material and the distribution of the different compounds across the sample made it difficult to obtain representative results. These problems could partially be overcome by analysing large areas rather than spots for SEM-EDS and performing replicates on different points of the specimen for micro-FTIR. Moreover, the capacities and limitations of midIR-FORS when employed to the examination of oil paintings were evaluated. It has been proved that the detection limits for this kind of set-up are especially high, although the major areas investigated permit to achieve better reproducibility and more representative information with respect to micro-FTIR. The physical feature/roughness of the analysed surface is a critical factor that may cause distortions in the spectra and be responsible for a decreased signal-to-noise ratio. It was achieved the creation of pigment mapping of oil paintings by means of midIR-FORS and multivariate methods. In a comprehensive study, the optimal data pre-processing methods as well as a protocol for the application of PCA and PLS-DA for FORS spectra modelling have been proposed. Furthermore, these methods were successfully applied to investigate and prove the influence of the surface roughness on the variability of the FORS spectra. Chapter 3 is focused on the assessment of the palette that was used by Pablo Picasso during the first years of his artistic career. The non-destructive punctual analysis techniques allowed achieving mappings of the elements/compounds detected across the entire surface of six portraits and it was possible to obtain a wide information about all the different regions of the artworks. In this study, it has been confirmed that an important limitation of this kind of techniques is the lack of information about the in depth composition and materials distribution, which has been partially overcome with the simultaneous and accurate evaluation of all available data complemented with NIR reflectography and XR radiography. In Chapter 4 the establishment of a procedure for faded silver image recovery in old photographs using XRF spectroscopy is proposed. The optimization of the method for sulphur detection in a first step, and the possibility to investigate two emblematic cases of silver images with this method allowed the demonstration of a relation between sulphur and silver, which is a key to the identification of high density areas that have lost the silver component over time. Chapter 5 present the study of the optimization and application of an analytical method for the characterization of ancient bronze objects. The unique opportunity to follow the analysis of the Donatello’s San Lorenzo Pulpit from the sampling to the ICP determinations allowed the investigation of all the possible sources of data variability. Thus, it was possible to compare the different parts of the structure and establish relations between the compositions of different samples that will be used by the conservator to complement the stylistic and historic study of the artwork.

El objetivo principal de la tesis es la evaluación de técnicas micro- y no destructivas para la caracterización material de obras de Patrimonio. Pinturas al óleo, fotografías antiguas y un objeto metálicos de gran formato se analizaron como representantes de tres importantes categorías artísticas. En el Capítulo 1 se presenta una introducción sobre los aspectos materiales y estructurales de las obras investigadas. El Capítulo 2 está dedicado a la determinación del límite de detección y reproducibilidad de Microscopia Electrónica de Rastreo (Scanning electron microscopy with Energy Dispersive X-Ray analysis, SEM-EDS), Espectroscopia de infrarrojo en transmisión (Fourier transform infrared spectroscopy, micro- FTIR) y en modalidad remota una sonda de fibras ópticas (mid infrared fiberoptics reflectance spectroscopy midIR-FORS). En el caso específico del midIR-FORS se ha demostrado la capacidad de métodos multivariantes para la modelización de los espectros obtenidos con tal técnica, y la creación de mapas de distribución de pigmentos a lo largo de la superficie de pinturas al óleo. El Capítulo 3 se centra en el estudio de la paleta usada por Pablo Ruiz Picasso durante sus primeros años de producción artística mediante el análisis no-destructivo de seis retratos conservados en el Museo Picasso de Barcelona. En el Capítulo 4 se propone un método analítico optimizado para la detección de azufre en fotografías antiguas (silver print image) mediante Fluorescencia de Rayos-X (X-Ray Fluorescence XRF) y un procedimiento para la recuperación de imágenes que manifiesten fenómenos de descoloración. Finalmente en el Capítulo 5 se presenta el estudio de la optimización de un procedimiento analítico para el análisis de muestras metálicas y aplicación a muestras reales extraídas del Pulpito de San Lorenzo de Florencia. La investigación de estas tres categorías artísticas ha demostrado la importancia de optimizar procedimientos de análisis específicos para incrementar el conocimiento de las obras teniendo en cuenta su heterogeneidad, características físico-químicas y respetando su integridad física.