Investigation of organic additives in Italian Renaissance devotion stucco reliefs from French collections

Abstract A series of Italian Renaissance stucco artworks from Museum of Fine Arts of Strasbourg and Louvre Museum in Paris were investigated with the aim of revealing and characterizing possible organic additives. As the stucco and its additives are little known but, according to the literature, could be complex and contain many different matters, the methodology was first tuned on the laboratory models prepared with gypsum or gypsum/lime plaster with various quantities of organic matter: animal glue or gum Arabic. The methodology consisted of observation of the surfaces of sampled fragments by optical and electronic microscopy, preliminary investigation of the presence of organic compounds by infrared spectroscopy, then extraction of the organic matters from the samples, preparation, and analysis by gas chromatography. The adopted sample preparation scheme allows screening the substances from various classes of organic products from one sample. The optimized analytical approach was applied to the samples from historical objects, showing the presence of proteins, oils, wax and sugars in reliefs. The interpretation of obtained results is not straightforward because of the low response of these substances in the samples, their possible mixture, or unknown origin. In some cases, the protein matter response varies with the deepness of sampling in the matter of stucco, which could be connected with its layered structure or particular surface treatment.