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 © 2014 John Wiley & Sons, Ltd.

This paper provides a contribution to FTIR analysis of calcium sulphate‐based grounds of paintings when a mixture of compounds with different degrees of hydration is present. The study is based on the analysis with that technique, electron microscopy coupled with energy‐dispersive X‐ray spectrometry and X‐ray diffraction of both the grounds of 16th‐century Portuguese paintings and reference samples prepared with anhydrite and calcium sulphate dihydrate bound in an animal glue solution. It is shown that the interpretation of the degree of hydration of calcium sulphate ground samples using FTIR cannot be based solely on the study of the hydroxyl bands, as is usual in routine work, but that small deviations of the S–O vibration bands are an important indicator of the presence of anhydrite in the grounds.

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This work regards the methodology used on the preparation of ground layer techniques in Portuguese and Flemish workshops of the 15th and early 16th centuries.

This study presents results on a developed methodology to characterize ground layers in Portuguese workshops. In this work a set of altarpieces of the 15 th and 16 th centuries, assigned to Coimbra painting workshop was studied, overall the masters Vicente Gil (doc. Coimbra 1498-1525), Manuel Vicente (doc. Coimbra 1521-1530) and Bernardo Manuel (act. c. 1559-94), father, son and grandson, encompassing from late gothic to mannerist periods. The aim of the study is to compare ground layers, fillers and binders of Coimbra workshop, and to correlate their characteristics to understand the technical evolution of this family of painters, using complementary microscopic techniques. The cross-sections from the groups of paintings were examined by optical microscopy and the results were integrated through the analysis obtained by μ-X-ray diffraction, scanning electron microscopy with energy dispersive X-ray Spectrometry, μ-confocal Raman and occasionally with μ-Fourier transform infrared spectroscopy imaging. Ground layers are of calcium sulfate, present as gesso grosso (mainly anhydrite with small amounts of gypsum) in the first and last phases of the workshop and gesso mate (mainly gypsum with small amounts of anhydrite) in an intermediate period. Binders have protein and oleic characteristics.