Chemical investigation on black pigments in the carved decoration of sixteenth century alabaster tombs from Zaragoza (Spain)

Abstract

An analytical protocol based on optical microscopy (OM), scanning electron microscopy (SEM) observation, energy-dispersive X-ray (EDX) analyses, analytical pyrolysis in the presence of hexamethyldisilazane followed by gas chromatographic/mass spectrometric analysis (Py-silylation-GC/MS) and gas chromatography/mass spectrometry (GC/MS) after alkaline hydrolysis, solvent extraction and trimethylsilylation was used to study the origin and nature of black pigments from the carved inscriptions of several panels of two alabaster tombs dated from the mid-sixteenth century. Optical microscopy and SEM observation showed the presence of an amorphous very dark-brown substance, from translucent to opaque. EDX analyses revealed that the samples were mainly made up of C and O, thus highlighting the organic nature of the material used in the inscriptions. Py-silylation-GC/MS and GC/MS analyses provided detailed molecular compositions, highlighting the presence of a wide range of compound classes including diterpenoid acids, tricyclic abietanes, mid- and long-chain monocarboxylic fatty acids, n-alkanols and nalkanes. The pyrograms, the chromatographic profiles and the presence of characteristic biomarkers indicated that a mixture of pine pitch and beeswax had been used to make the black inscriptions.