Identificação de corantes com interesse em arte e arqueologia

Acabou de ser disponibilizada online a seguinte tese de doutoramento que foi discutida há poucos dias:

Federica Pozzi, Development of Innovative Analytical Procedures for the Identification of Organic Colorants of Interest in Art and Archaeology, Milano, Università degli studi di Milano, 2011.

Está aqui, com acesso livre.

Resumo:

Organic colorants obtained from natural sources such as plants and insects have been widely used as textile dyes or lake pigments for paintings, sculptures and other kinds of polychrome works of art since ancient times until the second half of the 19th century, when the industrial production of synthetic dyes had begun. Chemical investigation of such materials is of great interest to art historians, restorers and art conservators. In fact, the analysis of ancient dyes can be of help in revealing what kind of substances were available in particular periods and geographical areas, providing valuable data about the historical context of a work of art, the lifestyle and the technical knowledge reached by a certain population in a given historical age, shedding light on the possible interactions between different cultures as well as the trade routes and commercial transactions which may have allowed the usage of a particular colorant far from its geographical source. Moreover, discovering nature and origin of the coloring substances employed in the production of a work of art can provide precious information regarding its original color and appearance, thus offering new insights into the artist’s choices and original intention, the techniques used and the dates ante quem and post quem the art object was produced, possibly leading to the uncovering of falsifications and forgeries. Furthermore, scientific analysis applied to the study of art materials and, specifically, of pigments and dyes, may contribute to assess suitable conservation and restoration procedures to be applied to paint defects and degraded pigments in works of art of any kind; in fact, time, environmental conditions and several other circumstances unavoidably cause damage and deterioration to art objects and artifacts, which therefore require careful conservation to be safeguarded as important elements of our cultural heritage. The identification of historical dyes is currently one of the most challenging tasks in the chemical investigation of art materials, for three main reasons. First of all, colorants in works of art and archaeological textiles are usually included in complex matrixes such as paint layers or cloth fibers, where they are present in mixture with other substances, such as binding media or mordants, and in very low concentrations due to their high tinting power. Besides, sampling of art objects is always limited to microscopic fragments, when at all allowed. An additional analytical challenge is posed by the remarkable susceptibility to deterioration of organic materials, which can undergo a number of chemical degradation processes leading to the formation of specimens with a different molecular structure in comparison to the primary organic dye. Several instrumental techniques, of both chromatographic and spectroscopic type, have been employed for the detection of colorants over the years. In recent times, the great potential of SERS has been appreciated, as the adsorption of the analyte on nanosized metal surfaces, resulting in a significant enhancement of the Raman scattering intensity and strong fluorescence quenching, leads to obtaining a specific fingerprint for many organic substances, markedly reducing the amount of sample that would be otherwise required for analysis. However, on the other hand, SERS poses a whole set of challenges, as only a small number of molecules have been studied so far and the necessity of searchable databases of reference materials is still to be fulfilled; moreover, SERS is not a separation technique, and therefore it often suffers from spectral interferences due to the presence of impurities or matrix components. In this context, the present doctoral thesis work aims to the improvement of pre-existing analytical methods as well as to the development of innovative procedures for the identification of organic dyes of artistic and archaeological interest, with special attention being devoted to SERS. The scientific results here reported are the fruit of a research activity carried out both at Università degli Studi di Milano and in the Department of Scientific Research of the Metropolitan Museum of Art (New York, USA). First of all, an effective experimental protocol for SERS analyses on silver colloids aggregated by NaClO4 was optimized and a wide spectral database of historical natural colorants was thus assembled, containing among the others the spectra of several dyes never studied before. The SERS procedure developed, in association with complementary analytical investigations of different kind, was then successfully applied to the identification of a yellow dye in ancient wool threads from the Libyan Sahara and to the detection of several colorants in Kaitag textiles, a unique embroidered textile art form from Caucasus. Work carried out at the Metropolitan Museum of Art comprises a comparative study of the most relevant SERS approaches recently introduced in art analysis: relative merits and drawbacks of HF hydrolysis and non-hydrolysis methodologies were evaluated, and a two-step procedure for the investigation of organic dyes in works of art was proposed. Results obtained from reference dyes were compared with those achieved on samples taken from a number of artworks and ancient objects, including masterpiece oil paintings, musical instruments, archaeological textiles and lake pigments. Watercolors from a historical Winsor & Newton handbook dating to 1887 were then characterized using SERS and ordinary Raman spectroscopies, and a database of original art materials was acquired, to be used for dating as well as in authentication and identification studies. Also, coupling of TLC and SERS was investigated and tested as a promising tool for the separation and identification of the main alkaloid constituents of Syrian rue dye. Finally, a comprehensive Raman study of monobromoindigo, component of the historical colorant Tyrian purple, was performed, together with a detailed assignment of the spectral lines observed by comparison with density functional theory (DFT) quantum mechanical calculations.