Pulsed Thermography Applied to the Study of Cultural Heritage
In this paper, an overview of the recent applications of pulsed infrared thermography is presented. Pulsed infrared thermography, which provides stratigraphic information by analyzing the heat diffusion process within the sample after a thermal perturbation, is applied to the investigation of different kinds of cultural heritage artefacts. In particular, it is used to analyze repairs, decorative elements, and casting faults on bronzes, to detect texts hidden or damaged in ancient
... ient books/documents, and to characterize paint decorations. Moreover, the integration of pulsed infrared thermography and three-dimensional shape recording methods is proposed in order to provide a three-dimensional representation of the thermographic results. Finally, it is shown how the obtained thermographic results may be crucial from the historical and artistic points of view for understanding the modus operandi of a specific artist and/or of a workshop and for reconstructing the manufacturing process of the analyzed artefacts. heating of the sample surface, then detected by a synchronized IR camera according to the lock-in processing technique [16, 23] . With respect to pulsed IRT, a larger signal to noise ratio can be obtained in the thermographic images, but the gathering of that information originating at different depths in the sample requires several measurements performed at different frequencies instead of a single one as a function of time. In the last years, pulsed IRT has been the most applied configuration in the study of different kinds of small-size artefacts, such as library and archival materials, archaeological findings    , and works of art  . In particular, it has been proposed for the investigation of the mosaic preservation state     and for the study of sculptures [27, 32] . In 2015, IRT was used by Di Tuccio et al., to analyze the cracks of the "Ratto delle Sabine", a model mainly composed of raw clay in the outer part and of several different materials in the inner part, to evaluate the level of weathering of the statue, in particular that related to the previously mentioned cracks  . Concerning the study of books and documents, IRT has been used to study historical bindings, providing information on both the assembled structure and the various component materials. In the bookbinding analysis, the thermographic imaging allowed researchers to detect the presence of damage, to investigate the adhesion state of the different parts of the bookbinding, and consequently to evaluate their preservation state  . In the study of paintings, IRT has been applied to reveal the adhesion of the paint layer to the support, to observe the canvas or wood texture, and to detect defects    also investigated by other imaging techniques      . Different thermographic configurations have been used for the analysis of wood-based and canvas-based paintings, providing successful results, for instance, in the detection of delaminations and degraded regions and also in the study of pentimenti and underdrawings by integrating IRT with other imaging techniques [13,     . Moreover, in 2012, Daffara et al. proposed a new method called Thermal Quasi-Reflectography (TQR), which consists of recording the middle-wave infrared (MWIR) radiation reflected by the investigated sample. Such a method has been successfully applied to the study of mural paintings showing details that are not revealed by conventional infrared reflectography (IRR)  . In this review, the results obtained in the past, as well as the new ones, which were considered useful for a more complete description of the state of the art of pulsed IRT applications in the field of cultural heritage are presented. Accordingly, the results are grouped based on the three main categories: the analysis of bronze sculptures, the recovery of hidden or lost texts in ancient books and documents, and the characterization of painted decorations in archival and library supports. First, the application of pulsed IRT to the analysis of ancient bronzes, like the Capitoline She Wolf, which aimed to detect features under the polished and patinated surfaces, are reviewed    . Some geometrical characteristics of the casting faults are also investigated by means of a series of quantitative studies  . Moreover, the results obtained by IRT in the detection of hidden or invisible book contents are shown. It is described how texts found in many historical bookbindings, generally belonging to scraps of older books and reused to assemble more recent book structures, can be investigated without dismantling the bookbinding  . Different kinds of paint decorations on parchment supports are also analyzed by IRT. In particular, IRT is proposed for the investigation of structural, surface, and subsurface features in illuminations  . It is shown how IRT allows researchers to characterize the conservation state of the gildings, revealing detachments and inhomogeneities of the gold leaf, and how it can be employed in the detection of subsurface features buried beneath the paint layer, like underdrawings and pentimenti , which are commonly investigated by other techniques operating in the infrared region [52, 53] . Finally, the integration of infrared thermography and three-dimensional (3D) shape recording methods [54, 55] is discussed, presenting a 3D representation of the IRT results obtained in the case study of the Capitoline Brutus.