Scientific analysis versus restorer’s expertise for diagnosis prior to a restoration process: the case of Santa Maria Church (Hermo, Asturias, North of Spain)

Raman spectroscopy with portable fiber optics microprobe, FT-IR spectroscopy and ion chromatography have been applied to the study of mortars and wall paintings in the church of Santa Mar?a de Hermo (Asturias, North of Spain) before the restoration works. This scientific analysis was performed to compare with a diagnosis report based on restorer’s expertise. The church showed problems of damp as well as clear evidences of flora, fungus and mould presence. The combination of both vibrational spectroscopic techniques made it possible to determine the chromatic palette of the wall paintings composed of: CaCO3, carbon black, red ochre, yellow ochre, minium and cinnabar. In addition, some decay products have been determined, such as nitrate salts, and gypsum. From the conclusions of the scientific analysis the supposedly distemper wall paintings from the XVIIth century were confirmed as frescoes and were then dated back to the XVth century by the art historians. The detachment of the pigment grains from the wall paintings is attributed to the loss of the binding power of CaCO3 by its partial transformation into calcium nitrate due to chemical reactions with nitrates. Clear evidence of nitrate migration from the graveyard behind the northern wall of the church has been determined from the quantification of nitrate salts by ion chromatography.