Non destructive XRD investigations on grisaille paint layerson stained glass windows

Since the end of the XIX century the study of glass corrosion and its weathering products was considered of great interest for Cultural Heritage Science. In the last decades the interest of the scientific community was mainly focused on the characterization of glass corrosion products, their formation processes, and on the development of new systems for the protection of the glass surface. An interesting and breaking research field, still nowadays almost unexplored, regards the investigations on stained glass windows alteration, especially on those painted by the grisaille method. Grisaille can be defined as a painting mixture formed by a finely powdered pigment (mainly transition metal oxides) and highly fusible glass (lead rich glass) forming a coloured layer applied on the glass surface, the adherence to the glass being guaranteed by a low temperature firing of the grisaille without melting the glass support.

Under a chemico-physical point of view, the obtained material is a system in which the crystalline phases (pigments) are embedded in an amorphous phase (low melting glass) which acts both as a dispersing medium and as an agent fixing the pigments onto the substrate (the stained glass window). The interaction of this surface with the environment (i.e. the effect of pollutant agents) promotes the formation of alteration crusts involving both grisaille paint layer and glass window surface. The aim of the present manuscript deals with the characterization of the alteration products of ancient and modern grisaille paint layers. The understanding of the complex chemical and structural nature of alteration products leads to an exhaustive comprehension of the deterioration processes and will be helpful to propose a correct project for the conservation and restoration of grisaille stained glass windows.