Thermal analytical techniques applied to the study of Cultural Heritage

Chemical composition of an art paint is influenced by several factors: the painting technique used by the artist, the decay processes affecting the painting materials over time due to environmental factors, and the different materials possibly used for the protection and conservation of the object. Improved knowledge on the material composition of paint layers on heritage items will allow a better understanding of the painting technique used by the authors, will contribute to the characterization assignment and authentication of artworks, and sustainable conservation of the heritage objects. This thesis presents a research related to the characterization and physical chemical properties of organic materials used in paint layers both from natural and synthetic origin. Organic materials can be used in painted surfaces for different purposes: as adhesives, when joining together two surfaces; as binging media, if applied in order to assure cohesion in a paint layer; as consolidant if used to return the lost coherence a paint layer; as varnish or protective layer or coatings of the painted surfaces. In this thesis we focus our attention on proteinaceous materials used as binders in tempera paint layers, their interaction with coloured pigments and their ageing behavior, and on alkyd resins used in modern and contemporary art; in particular synthetic resin produced by the Winsor & Newton, Series Griffin – fast drying oil colour were considered. Two formulations of siloxanes produced by Caparol (Disboxan 450) and Phase Restauro (Hydrophase) to be used as protective coatings were studied as well. Analysis were performed by using several techniques with a particular focus on thermoanalytical ones. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), thermogravimetric and evolved gas analysis coupled with Fourier infrared spectroscopy (TG-EGA-FTIR), and Fourier infrared spectroscopy (FTIR) on the solid samples enabled the characterization of the chemical physical behavior of the paint layer components. The first chapter of the thesis introduces the different organic materials studied, natural and synthetic, in particular proteins, proteinaceous materials and alkyd resins. The second chapter presents the technical aspects of sampling giving an overview of the methods used for binding media and resins analysis (TGA, TG-FTIR, FTIR and DSC) revealing advantages and limits for each of them. Beginning from the third chapter the thesis presents the analysis of the material studied. The pure pigments (natural or synthetic) used in proteinaceous and alkyd binders, and filler are characterized by TGA and FTIR. The fourth chapter studies the proteinaceous binder (ovalbumin, casein and rabbit glue), their interaction with five different inorganic pigments (azurite, red ochre, calcium carbonate, cinnabar and minium) and the effect of artificial UV-ageing. The fifth chapter studies the alkyd resins produced by the Winsor & Newton- series Griffin- Fast drying oil colour and their interaction with ten different pigments, organic (arylamide yellow, phthalocyanine, naphtol carbamide) and inorganic (cadmium-zinc sulphide, chromium oxide and sulphoselenide, aluminium-silicate of sodium, titanium dioxide and bone black), the effect of natural ageing of 8 months and of artificial ageing with acetic acid. The sixth chapter studies the silane/siloxane coatings, in particular alkyl silane and alkyl-alkoxy siloxane, their characterization, their ageing after two years of natural outdoor ageing and their interactions with acrylic colour. These two formulations of coatings are produced by Caparol (Disboxan 450) and Phase Restauro (Hydrophase) and the selected product will be applied on the wall painting “Tuttomondo” of Keith Haring on the Church of Sant’ Antonio in Pisa.