Non-invasive spectroscopic techniques and photographic methods for the study of the so called “Saint Thomas” bible from the Ardenti library in Viterbo (Italy)

The so called “Saint Thomas” Bible is a precious medieval illuminated manuscript dated back to the third quarter of the 13th century, at present stored in the Ardenti Library at Viterbo (Italy). The significance of this rare illuminated manuscript not only as codex but also as religious relic, has led to the need of studying the execution techniques and the constitutive materials especially of the two illuminated initial letters. The impossibility of carrying the Bible in a laboratory and of sampling has led to the choice of non-invasive methodologies of analysis. In particular the illuminated letters have been analysed through XRF spectroscopy, reflectance spectrophotometry, infrared reflectography, false colour infrared and UV fluorescence digital photography, video microscope acquisitions. The application of these non-invasive techniques and the analysis of the results allowed to identify the pigments used for the illuminated letters and to make hypothesis about the execution technique compared with the most important medieval treatise prescriptions on illumination. Moreover the analysis highlighted the presence of possible non-original interventions on the Bible.3n