Calorimetric and Thermoanalytical Techniques in the Study of Ageing Phenomena and Molecular Interaction in Paintings

In the last few decades the role played by thermal analysis and calorimetry in the field of the preservation of cultural heritage has continued to increase. Advanced analytical techniques, including the full range of thermoanalytical ones, have been widely and successfully applied to questions related to cultural heritage. How damage to an artifact takes place and how these damage processes can be delayed are important questions to be addressed in cultural heritage. To this purpose to deeply know the chemical composition of the matrix and the degradation routes of the materials used is crucial. Thermal analysis and calorimetry, especially when combined with other analytical and spectroscopic techniques, offer methods for problem solving in this area and have been applied to study a wide range of different materials, from the characterization of the composition and ageing processes of paints used in paintings, to the water content of waterlogged wood samples; the effects of consolidants in wood; the action of heat, light, and pollutants on collagen-based materials such as parchments or leather books; the mechanical properties and the thermal stability of historic tapestries; and, in general, the understanding of early technologies. This chapter is focused on the use of calorimetric techniques for the characterization of organic materials used as binders by painters with the function of dispersing the pigments in the paint layers. Examples of drying oils used in oil paintings, proteinaceous material used in tempera techniques, and synthetic resins used in contemporary paintings will be considered. The main questions addressed concern the degradation reactions that take place in the binder during the curing and ageing process, the interaction occurring between the binder and the colored pigments in a paint layer, and whether the pigments modify the binder degradation pathway. The techniques here considered are especially thermoanalytical techniques such as hyphenated thermal analysisespectroscopic methods and differential scanning calorimetry, because they are sensitive to changes in ageing of materials and help identify volatile products whose emission can be directly assigned to mass loss steps. In addition, other techniques will be briefly mentioned when useful to the discussion.