The role of zinc white pigment on the degradation of shellac resin in artworks

The stability and degradation behaviour of natural resins have been investigated by many authors for their important role in conservation chemistry but resin compositions and related degradation issues are not completely understood. In particular, shellac and its interaction with conservation materials during the ageing processes is still almost uninvestigated. In this work, some results of an extensive investigation on stability under thermal and photo-oxidative ageing of natural materials employed in the field of artistic production and artworks conservation are presented, and in particular, the ageing behaviour of shellac in presence of zinc white (ZnO) is described. This work underlines the importance of natural resin/pigment interactions from the point of view of the conservation of paintings since their decaying paths may be dramatically affected. During the photo-oxidative ageing of zinc white and shellac mixtures, the formation of zinc oxalate has been also detected. The formation of oxalates and the different degradation products from natural resins in presence of pigments are of particular interest considering the way the different degradation processes (double bond formation, ester formation, hydrolysis and polymerization) and the new chemical species (oxalates, metal soaps) can affect the removability of layers and, more generally, the cleaning processes, modifying in many cases the boundary between painting and varnish layers, and the aesthetical aspects of the work surfaces.