Speaker
Description
Non-invasive and portable approaches are fundamental for the analysis of cultural heritage, especially in the case of extremely fragile items. This is particularly true for Herculaneum papyri, which represent one of the most extraordinary archaeological discoveries of all times, being the only library directly survived from antiquity and providing insights into ancient book production, Greek philosophy and classical literature. These precious manuscripts survived from the eruption of Mount Vesuvius in 79 AD thank to the carbonisation process that occurred in Herculaneum. After their discovery in 1751, most of the scrolls were mechanically unrolled using a specifically designed machine. This system allowed the rolls to be opened but produced fragments with complex multilayered morphologies with texts buried within the layers. Moreover, owing to carbonisation, the papyrus substrate has darkened or blackened, making it difficult or impossible to read the text, which is also written in black, with the naked eye. Therefore, it is mandatory the use of specific non-invasive techniques for enhancing textual legibility. Studies on the recovery of texts in ancient manuscripts show that techniques such as NIR reflectography (NIRR) or hyperspectral imaging (HSI) achieve best results in terms of the overall readability but show also limitations in terms of identifying hidden details (NIRR) and image definition (HSI). This contribution presents the application of Pulsed Thermography (PT) for the analysis of these peculiar objects with the following goals: (i) readability enhancement of the text on the superficial layer; (ii) identification of textual elements buried among multiple layers; (iii) characterisation of the papyrus substrate. PT is a non-invasive photo-thermal technique that consists of recording the infrared (IR) radiation emitted by the sample following the heating induced by the absorption of short pulses of visible (VIS) light emitted by flash lamps. In the case of optically semi-transparent materials, such as the papyri under investigation, two mechanisms can be identified for the contrast generation of thermal images. On one hand, when the VIS light reaches the papyrus surface, the inked parts are instantaneously heated by the light absorption to a greater extent in comparison with the papyrus substrate, thus resulting in a corresponding larger local IR emission. In this case, the highest contrast between the text and the substrate is achieved just after the light pulse. On the other, if text is buried inside multiple layers and if a portion of excitation VIS light reaches the text, the latter is heated but its consequent direct IR emission is totally absorbed by the carbonized layers above the text. An IR contrast can be then detected by the camera at certain time delay from the light pulse, when the heat generated in the buried text reaches the sample surface because of its diffusion. These mechanisms allowed the inspection of both surface textual features and the complex stratigraphy of unrolled Herculaneum papyri. The experimental setup for these analyses consisted in two flashes for the light stimulation and a thermal camera for data acquisition (MWIR spectral range 3.5–5 μm, FPA 640 × 512 pixels). A close-up ring was also used for closer inspections when the sample was difficult to handle during the measurements.
