Multi-technique characterisation of medieval mastic encrustation sculptures

Abstract

Mastic incrustation sculptures were used to decorate different types of objects by filling the marble incisions with an impasto containing materials such as crushed pottery, glass or rock fragments, possibly in combination with organic substances, and was employed both in Byzantine and Occidental areas. From the 11th century onwards these sculptures are applied in important churches such as San Marco in Venice (Italy), but, despite their widespread use and conservation problems, the composition of these amalgams has never been thoroughly studied.Here we present the results of the examination of the mastic encrustations present on the medieval marble panels and precious cathedra in the San Nicola church of Bari (Italy). The matrix was analysed with Fourier transform infrared spectroscopy and pyrolysis gas chromatography-mass spectrometry. Organic binders were found and, in particular, the occurrence of heated Pinaceae resin, as well as of beeswax was assessed. The inclusions, including different types of rock, carbon black and opaque glass fragments, were characterised with optical microscopy, scanning electron microscopy, and powder X-ray diffraction. Micro-Raman spectroscopy was carried out on both inclusions and matrix.