Blurring of ancient wall paintings caused by binder decay in the pigment layer

In this paper, the effect of binder decay rather than a change in the pigments on the blurring of ancient wall paintings was researched. The simulated wall paintings were prepared by brushing an aqueous solution containing gelatine and ochre grains on the surface of cylindrical compressed soil samples. Then, the dried samples were calcined at 650 °C for 2 h to obtain the simulated wall paintings with the degraded binder gelatine. Next, the calcined samples were brushed with a certain amount of acetone solution containing an ionic liquid ([BMIm]PF₆) to obtain the corresponding repaired samples. Based on the results from various characterization methods (UV-vis, FTIR, XRD, XPS, SEM, TG), the following conclusions were drawn. The degradation of the binder caused by calcination increases the surface roughness of the painting layer, resulting in enhanced scattering. In this case, because scattering decrease the light absorption by the pigments, even if unchanged pigment exists in the painting layer, its colour can become blurred. The filling of the ionic liquid into the pores caused by gelatine decay in the painting layer can decrease the scattering, and the blurred colour can be restored to some extent. As typical examples, this principle was successfully applied to restore the blurred colour of an ancient Chinese wall painting (Tang Dynasty) and a pottery (Eastern Han Dynasty).