A scientific approach to the characterization of the painting materials of Fra Mattia della Robbia polychrome terracotta altarpiece

A complement of analytical techniques was used to characterize the polychromy and the painting technique adopted by Fra Mattia della Robbia to decorate the terracotta altarpiece, called Coronation of Virgin between Saints Rocco, Sebastian, Peter martyr and Antonio abbot, located in the collegiate church of S. Maria Assunta in Montecassiano (Macerata, Italy). The altarpiece dates back to the first half of the XVI century and represents an interesting example of painted terracotta produced by using two different techniques: glazed polychrome terracotta and the "cold painting" technique. The characterization of the samples’ material constituents was obtained by analysing the cross-sections and the fragments by different techniques (optical, SEM-EDS and ATR-FTIR microscopy as well as GC-MS), as the real nature of a component is often difficult to assess with one single technique. The optical microscope examination of paint cross-sections shows the presence of many layers, indicating the complexity of the paint stratigraphic morphologies. The original polychromy of della Robbia’s masterpiece is constituted of cinnabar, red lake, red lead, orpiment, red ochre, lead white, lead tin yellow, green earth, raw umber and indigo. Two different types of gilding technique have been distinguished, the first one presents a glue gilding mordant, the second one shows an oil gilding mordant composed by a mixture of red lead, red ochre, cinnabar and orpiment. The paint palette bulk GC-MS analysis allowed the characterisation of both drying oil and proteinaceous materials as binding media which have been further mapped and stratigraphically located by the use of ATR-FTIR mapping microscopy. The analytical results of the painted terracotta integrated investigations show that original technique adopted is characterized by the application of pigments in an oil-binding medium directly applied on the substrates probably treated with oil and animal glue. A large number of overpaintings above the original scheme of polychromy were found which could ascribe to almost three different interventions; the absence of modern pigments suggests they could be realized long ago.