Advanced optical spectroscopy techniques for the study of pigment materials

Conservation Science is a multi disciplinary field, which combines a number of scientific methods for the material characterization of art materials and their degradation products, for the definition of proper conservation and preservation protocols, for the developing of new restoration products. In particular, the characterization of the material used in an art object is fundamental not only for defining proper conservation protocols but also for the in depth study of the technology, the trade roots and the style of an epoch. Precious and irreplaceable artworks needs to be studied without compromising their integrity. For this reason,a lot of advanced analytical tools have been recently developed, with the main aim of providing a chemical description of cultural heritage materials, with a non or micro-invasive approach. This is particularly challenging, due to the fact that artwork materials are complex mixtures intrinsically heterogeneous, composed of a wide range of compounds, from organic to inorganic ones, and a wide range of size scale, which goes from the chemical identification of compounds to the mapping of trace elements, alteration or restored phases. It is from this intrinsic complexity that arises the need for multi-analytical approaches, to overcome the various limitation of individual spectroscopic methods. The application of spectroscopy techniques in the conservation science dates back to the 1970s. When an electromagnetic radiation impinges on the material, different processes take place: photons can be elastically scattered or absorbed. The exploiting of this two phenomena gives important information about the electronic and molecular structure of the sample under investigation. For this reason, the present thesis reports the results of the application of a series of advanced optical electronic and vibrational spectroscopy techniques to the study of pigment material. The research was carried out both at the Physics Department of Politecnico di Milano and at the Department of Scientific Research of the Metropolitan Museum of Art of New York. We decided to focus on the analysis of pigment materials due to their importance in conservation science. The precise identification of pigments can help in dating and authentication studies, since each pigment has its own and well-known period of use. Furthermore, it can help in understanding the painter technique and in tracing historic trade routes. The in deep knowledge of the material and of the degradation processes in act is crucial for the fine tuning of the restoration and conservation protocol, since not all the pigments react in the same way at the exposure to light or environmental conditions. Several analytical techniques for the identification of pigments have been in use for many years; they include microscopy, X-ray based techniques, UV-Vis and IR spectroscopy, laser based techniques, such as Raman and Laser-Induced breakdown and fluorescence spectroscopy, and spectrometry techniques coupled with gas or liquid chromatography. Unambiguous identification is usually obtained with a multi-analytical combination of techniques. The first chapter is focused on the detailed description of the two class of materials on which the advanced spectroscopy techniques developed and used during my doctorate have been applied: cadmium based pigments and anthraquinone dyestuffs. The physical and chemical properties together with the history and the key-role of these types of pigments will be discussed. In the second chapter, after a general introduction about the phenomenon of luminescence, the main applications of steady state and time-resolved luminescence analyses will be discussed, mainly focusing on the analysis of pigment materials. Optical electronic spectroscopy and imaging techniques have been widely applied in the Conservation Science field. In particular, luminescence spectroscopy, due to his high sensitivity, is useful in mapping non-homogeneities on the surface of a work of art even when present in low concentration. In certain cases, advanced electronic analysis can help in the detection of specific materials and in the definition of energy transfer models, as it will be shown in the next chapter. In addition to the luminescence imaging techniques, diffuse reflectance multi-spectral imaging and its application in conservation science will be presented. The devices used for the analysis reported in the next chapters are briefly presented. The third chapter reports the steady state and time-resolved luminescence analysis performed at the Physics Department of Politecnico di Milano. The in depth study of the luminescence emission of a group of commercially available cadmium based pigments gives new insight into the photophysical properties of this materials. Preliminary results on a group of historical cadmium based pigments will be presented. In the second part of the chapter, the attention will be focused on the analysis of a group of anthraquinone dyed textiles. The main aim of the work is to understand the potential and the limitations of luminescence analysis to the study of the various dye - mordant metal ion complexes, a field non yet fully investigated. The fourth chapter reports the research carried out at the Metropolitan Museum of Art of New York applying a vibrational technique, Raman spectroscopy, to the study of dyes and lake pigments. Vibrational spectroscopy techniques can provide high specific information about pigment materials, especially when used in combination with the electronic spectroscopies presented in the previous chapters. Among them, Raman spectroscopy, a molecular spectroscopy technique which measures the inelastically scattered light, allows the identification of vibrational states of molecules, providing an important analytical tool for accurate molecular finger-printing. After a brief introduction about Raman and Surface Enhanced Raman (SER) spectroscopies and their application in conservation science field for the analysis of pigments, it will be presented a newly developed SERS-based technique which combines the high sensitivity of SERS and the outstanding spatial resolution achievable through a UV-laser ablation micro-sampling (UV-LA-SERS) method. The technique permitted the isolation of signal from colorants in individual thin paint layers in the sample cross-sections taken from different type of works of art, avoiding contamination from adjacent layers. These results expand the range of analytical applications of SERS. In Conservation Science field, it is usually necessary to combine different types of techniques in order to completely understand the specific problem and analyze an intrinsic complex system, such is a works of art. For example, one useful approach consists in the combination of optical electronic spectroscopy techniques and molecular or highly specific elemental analysis. For this reason, chapter five merges in a multi-analytical approach the techniques used in the previous chapters. In this case, three different electronic imaging spectroscopies - visible reflectance, fluorescence multispectral and fluorescence lifetime imagings - are combined with a vibrational point-like analysis - micro-Raman spectroscopy. In particular, it was developed and tested a specific method for the post-processing and multivariate analysis of the multispectral imaging data. The segmentation of the image achieved with this approach allowed us to extend the chemical identification of single and selected points to the entire watercolor. The results are further combined with the steady state and time resolved luminescence imaging analysis. This approach was successful for the investigation of the palette used by the artist, in a completely non-invasive fashion. In conclusion, the application of advanced spectroscopy techniques to the study of pigment materials revealed to be crucial both for the characterization of unknown materials both for the better understanding of their photophysical properties. The future advances of this work will consider the combination of other analytical techniques such as LIBS-SERS and MS-SERS, on one hand. On the other hand, the microimaging application will be further exploited, in order to better study the impurities present in the materials.

La scienza della conservazione dei Beni Culturali costituisce un campo di ricerca multi-disciplinare, il quale combina una serie di metodi scientifici per la caratterizzazione fisico-chimica dei materiali di cui sono composte le opera d’arte e dei loro prodotti di degrado, per la definizione di protocolli di conservazione e prevenzione, per lo sviluppo di nuovi materiali per il restauro. In particolare, la caratterizzazione dei materiali presenti in un’opera d’arte è fondamentale non solo per la definizione di un oculato intervento di restauro, ma anche per lo studio approfondito della tecnologia, delle vie di scambio e dello stile di un’epoca. Gli oggetti d’arte, dato il loro valore e la loro unicità, necessitano di essere studiati senza compromettere la loro globale integrità. Per questo motivo, un elevato numero di tecniche analitiche avanzate sono state sviluppate negli ultimi anni, con lo scopo principale di fornire una descrizione chimica dei materiali dei Beni Culturali, con un approccio non o micro-invasivo. Questo aspetto è particolarmente arduo, dal momento che tali materiali sono misture complesse ed intrinsecamente eterogenee, composte di un vasto spettro di tipologie materiali, da quelli organici agli inorganici, e da un vasto spettro di concentrazioni. Proprio da questa complessità del materiale nasce la necessità di un approccio multi-analitico, in grado di superare le limitazioni delle singole tecniche spettroscopiche. Per questo motivo, la presente tesi è basta sull’ applicazione di una serie di tecniche ottiche avanzate, sia elettroniche che vibrazionali, per lo studio di pigmenti e coloranti organici. Sono stati scelti questi due materiali sulla base della loro importanza nella “conservation science”. La puntuale identificazione di un pigmento può aiutare nel datare un’opera e nell’ individuare falsi, dal momento che ogni pigmento ha un suo preciso e be noto periodo di utilizzo. In aggiunta, può aiutare nel capire meglio la tecnica di un artista e nel tracciare le reti di scambio commerciale di un’epoca. La conoscenza approfondita del materiale è cruciale nella definizione dei migliori approccio per il restauro e la conservazione dell’opera, dal momento che non tutti i materiali reagiscono allo stesso modo alla luce ed alle condizioni ambientali. Il primo capitolo è incentrato sulla descrizione dettagliate delle due classi di materiali presi in considerazione nei capitoli successivi della tesi: pigmenti moderni a base di cadmio e coloranti organici tradizionali a base di antrachinoni. Le proprietà chimico-fisiche dei due materiali verranno prese in considerazione, insieme alla loro storia. Nel secondo capitolo, dopo una introduzione generale sul fenomeno della luminescenza, le principali applicazioni delle analisi risolte in spettro ed in tempo nel campo dei Beni Culturali verranno discusse. Il terzo capitolo riporta I risultati dell’applicazione delle analisi in luminescenza sulle due classi di materiali descritti nel primo capitolo. Il quarto capitolo descrive i risultati dell’applicazione di una tecnica basata sull’analisi SERS per l’identificazione di coloranti organici rossi tradizionali in dipinti ed oggetti decorati. In particolare viene presentata una nuova tecnica per il campionamento selettivo del materiale in un’area del diametro di pochi micrometri, accoppiata all’analisi SERS del materiale campionato Il quinto capitolo unisce in un approccio multi-analitico le tecniche spettroscopiche descritte ed utilizzate nei capitoli precedenti. In particolare una serie di tecniche di imaging ottico sono state combinate con i risultati analitici ottenuti in singoli punti con la spettroscopia micro-Raman.