SYNOPYE · SACCHARIDE MATERIALS IN PAINT SYSTEMS: NATURE, OCCURRENCE AND PHYSICOCHEMICAL EVOLUTION

Saccharide materials in paint systems: nature, occurrence and physicochemical evolution

The SYNOPYE project is the first fundamental study on saccharide materials as binders. The study was performed thorough a multidisciplinary approach, where analytical chemistry and physical chemistry, with the support of microbiology and conservation science, converge for the more complete characterisation of the same phenomena, both from the macromolecular and compositional point of view. During the project, and following the research plan provided, the activities developed allowed to fulfil the goals of the project. A first step of the project consisted in the preparation of model systems representing simplified polysaccharide paints, including proteinaceous materials and selected inorganic pigments. Part of the model systems was artificially aged to help understand the degradation processes. Thermoanalytical techniques (thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)) were used to investigate the interaction of saccharide binders with proteins and inorganic materials, and to understand how these evolve with time. The same model systems were also investigated by gas chromatography mass spectrometry (GC-MS) analytical techniques, to evaluate the modification of the sugar profile of saccharide binders occurring as a result of aging, or the presence of inorganic and proteinaceous materials. Other techniques such as gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR) and X-ray fluorescence (XRF) were used in order to complement the results obtained. Moreover, microbiology was fundamental to complement the physicochemical study by means of ATP levels determination and bacterial growth measurement. The main scientific results obtained can be summarised as follows: - The GC / MS procedure used at the University of Pisa (UniPi) was optimised. To ensure the reproducibility of the saccharide profiles obtained the procedure was further implemented, through the use of specific extracting conditions (1,2). - The influence of the analytical procedure used on the sugar profiles obtained was evaluated. The procedure developed at the UniPi was compared with a procedure developed at the Getty Conservation Institute. The results highlighted that the two methods give comparable sugar profiles, whether the samples analysed are simple raw materials, pigmented and unpigmented paint replicas, or paint samples collected from polychrome art objects (2). - A database of sugar profiles of reference materials commonly found in paint samples was compiled and published. This database highlights how many sources of saccharides can be found in a paint sample (2). - An analysis of the reference paint layers containing plant gums without pigments showed that the molecular profile and macromolecular structure of each gum was maintained after artificial ageing (3). - The composition of paint layers containing pigments showed that both qualitative and quantitative changes occur in the sugar composition due to ageing conditions and the pigment present. At a macromolecular level, the formation of new species both by hydrolysis or formation of more stable compounds by reaction of the gum with the pigments was observed right after the mixture. Changes are different depending on the gum, the pigment and the ageing stage of analysis (3). - The changes observed in the relative sugar profiles were carefully studied. Both quantitatively and qualitatively speaking, some sugars appear stable under ageing while others do not, allowing to establish a new criteria for the sugars that can be used for a reliable identification of plant gums in samples from works-of-art. The qualitative changes in the sugar profile of the samples analysed highlighted the limitations in identifying the source of the plant gum in a paint sample with the analytical models previously reported in the literature. - Reference paint layers of polysaccharide gums and proteinaceous materials (mixtures approximately 1:1) were analysed after two years of natural ageing. The data clearly show that the sugar profile of a plant gum may appear to be modified when a proteinaceous binder is present (3). - The study of water solutions containing Arabic, tragacanth and fruit tree gum showed that biological activity was occurring and the sugar profiles of the gums was changing accordingly. The biological activity on paint layers in the presence of pigments allowed to conclude that also in that case both fungi and bacteria were feeding from the polysaccharides. The analysis showed that changes (in some cases very significant) of the glycoside profile and the molecular patterns of the gums are taking place in the presence of bacteria and fungi. Prior to this investigation, the identification of polysaccharide materials in paintings lacked the necessary scientific bases (1, 4). The SYNOPYE project begins to address these gaps in knowledge demonstrating that, in order to develop a reliable model for the identification of saccharide binders in paintings, the effect of the simultaneous presence of inorganic and proteinaceous materials and environmental contamination must be taken into consideration. On this basis, a new model was developed in order to help interpret the sugar profile of a paint sample, and resulted in four different decisional schemes based on the presence/absence of arabinose, galactose, mannose, xylose and fucose (3). To evaluate its reliability, this new model was applied to the sugar profiles obtained from the analysis of a large number of samples from murals, easel paintings, manuscripts, and polychrome objects from different geographical areas and dating from the 13th century BC to the 20th century AD (3, 5, 6, 7, 8). The new model allowed a positive interpretation of the source of the saccharide material found in 52 % of the cases (compared to the 7 % obtained based on previous knowledge). Moreover, in 22 % of the cases, a suggestion of the source of the saccharide material is also given. Therefore, the results of the SYNOPYE project lay the foundations for a reliable identification of saccharide binders in ancient paintings and polychromes, which is the first step to ensure the best conservation practice when dealing with saccharide containing works of art. Some of the samples from artworks studied as part of the project are already an example of the importance of the synergy between the fundamental research developed in the project and the Conservation Science. Results from the characterisation of the painting samples have in fact already been used to support the restoration making decisions process, demonstrating the applicability of the results obtained from the project. Results from the SYNOPYE project were presented in 11 different international conferences from September 2010 to July 2012. A total of 8 papers have been already published in peer-reviewed journals in the field of Analytical Chemistry and Science for the Safeguard of Cultural Heritage. Moreover, 4 of 8 papers are published in journals with an impact factor (ISI web of knowledge) above 3. The intense dissemination done during the project highlights the interest of the scientific community in the chemistry applied to art conservation and the high-level of the results obtained. The SYNOPYE project lay foundations for the efficient safeguard of our cultural heritage and contributed to increase European competitiveness in the field. Carrying out the fundamental research to increase the knowledge necessary for the efficient preservation of our painted cultural heritage reinforces the central role that European research plays in the field, and will contribute to nourish the European request of high-level scientific support for the safeguard of cultural heritage. The current project constituted also the basis for further collaboration with other European and international researchers and institutions in the field. (1) Lluveras et al., Analytical Chemistry 82, 1 (2010) 376-386 (2) Lluveras et al., Chemistry Central journal, DOI: 10.1186/1752-153X-6-115 (3) Lluveras-Tenorio et al., Plos One, accepted for publication (4) Bonaduce et al., Journal of Chromatography A 1175 (2007) 275-282 (5) Brecoulaki et al. Journal of archaeological Science 39 (2012) 2866-2876 (6) Guttman et al., Studia Universitatis Babes-Bolyai Chemia 1 (2012) 185-194 (7) Guttman et al., Journal of Archaeological Science, accepted for publication (8) Rasmussen et al. Journal of Archaeological Science 39 (2012) 2956-2968.

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