Deterioration of White Tempera Mock-Ups Paints Under a SO2-Rich Atmosphere

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Abstract
Historical tempera paints exposed to pollutant gases suffer chemical and mineralogical deterio-ration which manifest through physical changes. Knowledge about these changes is fundamental to develop strategies for preventive conservation of wall paintings. In this research, binary tem-pera mock-ups composed of calcite, gypsum or lead white mixed with a proteinaceous binder (i.e., egg yolk or rabbit glue) were exposed to an ageing test by SO2-rich atmosphere exposure to learn about the degradation mechanisms and forms related to the pigment-binder interaction. Reference (unaltered) and aged mock-ups were studied from a physical point of view, characterizing the morphological changes by stereomicroscope and profilometry, colour variations by spectropho-tometry, gloss changes, and reflectance changes by hyperspectral camera. Also, mineralogical and chemical changes were studied by means of x-ray diffraction, Fourier transform infrared spec-troscopy and scanning electron microscopy with energy-dispersive x-ray spectroscopy. Egg yolk-based paints showed higher chromatic changes than their counterparts made of rabbit glue binder. Also, sulphate and sulphite salts precipitated on the surface of the aged paints re-gardless of their binder, influencing the painting reflectance which subsequently increased. Egg yolk-based mock-ups exhibited roughness increases while the rabbit glue-based paints showed roughness reduction, with exception of lead white-based paints. Therefore, an important influence of the type of binder and the interaction between the binder and the pigment on the durability of tempera paints in atmospheres rich in SO2 was confirmed.

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