Detection of unexpected copper sulfate decay compounds on late Gothic mural paintings: Assessing the threat of environmental impact

Abstract

A multianalytical study was proposed for the study of late-gothic wall paintings preserved inside the Romanic church of Saint Ementerio and Saint Celedonio of Goikolexea (Basque Country, Spain), erected in the 15th century and then renewed in the 17th. Raman spectroscopy coupled with energy-dispersive X-ray fluorescence was used in the in situ analysis in order to identify the nature of the original compounds and those used in the restoration works. The mortar composition and some original pigments (haematite, carbon black, cinnabar, goethite) were characterized. However, the identification of the original pictorial layer was difficult by the presence of chromatic retouching and mainly by the presence of the protective acrylic resin layer applied in modern times. Thus, the information obtained by the portable technique was used to select the sampling areas to perform laboratory Raman measurements especially in the green areas in which different shades appeared, because no antique green pigments were identified during the in situ analyses. Although the church was restored a couple of decades ago, the presence of salt crystallization was noted during the field campaign. Thus, some samples of salt efflorescences were collected to complete the analyses in the laboratory. Sulphate (calcium sulphate dihydrate CaSO4·2H2O and thenardite Na2SO4·10H2O) and nitrate compounds, (nitratine NaNO3 and nitromagnesite Mg(NO3)2·6H2O) were identified. Even, an extensive biodeterioration phenomenon was detected on the samples in the form of calcium oxalates whewellite and weddellite. All these decay products are strictly related to the characteristics of the surrounding natural environment and they represent the main factors that endanger the integrity of the masonry and consequently the substrate of the wall paintings. In particular, the presence of humidity of the walls and of the organic matter around the church seems to be the main factors of the formation of decay products. In addition, the hydrated sulphate copper connellite, generally identified as a product of corrosion of archaeological pieces in copper or bronze, was identified on the wall painting samples as a degradation product of malachite, the only original green pigment found in the mural paintings.