Characterization of gypsum and anhydrite ground layers in 15th and 16th centuries Portuguese paintings by Raman Spectroscopy and other techniques

The purpose of this work is to characterize Portuguese painting ground layers from the 15th and 16th centuries (1450-1600), focusing particularly on the Coimbra, Lisbon, and Viseu workshops. Calcium sulfate, used regularly in these layers is common in the Iberian Peninsula as a natural resource. Its use is now confirmed, as already studied in Spanish paintings, and it was prepared generally by addition of animal glue. Besides the common esthetic language to the various painting workshops at the time, chalk/calcite (calcium carbonate), gypsum (di-hydrated calcium sulfate), and anhydrite (anhydrous calcium sulfate) ground layers have specificities that allow characterizing different compounds. The cross-sections from a set of six groups of Portuguese paintings were examined by optical microscopy and micro-confocal Raman and Raman spectroscopic mapping imaged the location of the different compounds in the cross-section. Results were integrated with those obtained by micro-X-ray diffraction and scanning electron microscopy with energy dispersive X-ray spectroscopy on the same samples. In most cases, the ground layers are made of calcium sulfate in the form of the so-called gesso grosso (mainly anhydrite with small amounts of gypsum). Differently, the Viseu paintings have calcium carbonate ground layers. These results put in evidence differences between the examined groups giving important information on the various workshop practices and also on the possible future conservative intervention on the paintings. Copyright (c) 2014 John Wiley & Sons, Ltd.