Effects of ageing on different binders for retouching and on some binder–pigment combinations used for restoration of wall paintings

In restoration of colour layers, the selection of the most appropriate retouching binder is a very important step that may have a crucial impact on materials durability. As different weather conditions can have versatile influence on stability of colour layers, we determined the effect of ageing on carefully selected samples of binders (Tylose, Klucel, ammonium caseinate, gum arabicum, fish and skin glues and some other synthetic binders) as well as on several binder–pigment combinations (the pigments in combinations being cinnabar, green earth and smalt). The samples were subjected to accelerated ageing tests in climatic chambers. In these tests the temperature and the relative humidity were daily oscillating between − 20 °C and 50 °C and 50% to 90%, respectively, for a period of one month. Then the samples were exposed to UV and visible light generated by a metal halide lamp for a month. The differences in microstructure before and after ageing were determined by optical and scanning electron microscopy, while the ageing of the organic structures in binders was investigated by Fourier transform infrared (FTIR) microscopy.