applied sciences Article Characterization of Ancient Marquetry Using Different Non-Destructive Testing Techniques Henrique Fernandes 1,2 , Jannik Summa 2,3,* , Julie Daudre 2, Ute Rabe 2,3, Jonas Fell 2,3 , Stefano Sfarra 4 , Gianfranco Gargiulo 5 and Hans-Georg Herrmann 2,3 1 Faculty of Computing, Federal University of Uberlandia, Uberlandia 38408-100, Brazil; [email protected] 2 Fraunhofer IZFP Institute for Nondestructive Testing, 66123 Saarbrucken, Germany; [email protected] (J.D.); [email protected] (U.R.); [email protected] (J.F.); [email protected] (H.-G.H.) 3 Chair for Lightweight Systems, Saarland University, 66123 Saarbrucken, Germany 4 Department of Industrial and Information Engineering and Economics, University of L’Aquila, I-67100 L’Aquila, Italy; [email protected] 5 Individual Company of Restoration (Gianfranco Gargiulo), I-80073 Capri, Italy; [email protected] * Correspondence: [email protected] Abstract: Non-destructive testing of objects and structures is a valuable tool, especially in cultural heritage where the preservation of the inspected sample is of vital importance. In this paper, a Citation: Fernandes, H.; Summa, J.; decorative marquetry sample is inspected with three non-destructive testing (NDT) techniques: Daudre, J.; Rabe, U.; Fell, J.; Sfarra, S.; air-coupled ultrasound, X-ray micro-tomography, and infrared thermography. Results from the three Gargiulo, G.; Herrmann, H.-G. Characterization of Ancient techniques were compared and discussed. X-ray micro-tomography presented the most detailed Marquetry Using Different results. On the other hand, infrared thermography provided interesting results with the advantage Non-Destructive Testing Techniques. of being cheap and easy in the deployment of the NDT method. Appl. Sci. 2021, 11, 7979. https:// doi.org/10.3390/app11177979 Keywords: decorative sample; cultural heritage; air-coupled ultrasound; X-ray micro-tomography; infrared thermography Academic Editors: Xavier Maldague, Valérie Kaftandjian-Doudet, Ahmad Osman, Bastien Chapuis, Gunther Steenackers, Hai Zhang 1. Introduction and Yun-Kyu An Non-destructive testing (NDT) techniques help both conservators and restorers to Received: 25 June 2021 detect incipient defects in cultural heritage objects before that the damage is visible to the Accepted: 24 August 2021 naked eye. By applying a clever and integrated approach in which each object is monitored Published: 28 August 2021 in the course of time, money can be saved and materials from the decorative layer are preserved [1]. Publisher’s Note: MDPI stays neutral Marquetries, i.e., a particular typology of cultural heritage object, have been rarely ® with regard to jurisdictional claims in monitored in the past by scientists. After an in-depth search in the Scopus database, published maps and institutional affil- papers indexed and deserving to be cited should be less than fifteen. For example, infrared iations. thermography (IRT) was used initially by Candorè et al. in [2] to detect delaminations or galleries due to woodworms. Nakamura and Naruse characterized the ornamental adhesives used in the Shosoin treasures by NDT techniques such as attenuated total reflectance Fourier transform infrared Copyright: © 2021 by the authors. spectroscopy (ATR/FTIR) and X-ray analyses. Second-derivative transformation of the Licensee MDPI, Basel, Switzerland. FTIR spectra identified the adhesives on marquetry fragments as animal glue [3]. This article is an open access article Samples reproducing marquetries with fabricated defects were inspected in [4] by distributed under the terms and means of visible imaging, ultraviolet testing, near-infrared reflectography and transmit- conditions of the Creative Commons tography, infrared thermography, holographic interferometry, digital image correlation, Attribution (CC BY) license (https:// laser speckle contrast imaging, and ultrasonic testing. Numerical simulations focusing creativecommons.org/licenses/by/ on the optimization of the provided thermal flux anticipated the experimental results. A 4.0/). Appl. Sci. 2021, 11, 7979. https://doi.org/10.3390/app11177979 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 7979 2 of 12 correlation of the individual informative content produced by each inspection procedure was explained and highlighted. In [5], holographic interferometry was tested on mock-up (marquetries in such a case) with two different light sources, in an attempt to expand the technique towards an easy-to-operate, inexpensive, and tunable approach, offering a broad spectrum and wavelength selectivity, according to the needs of the experiments. The results demonstrated the effectiveness of the proposed modified experimental scheme for defect mapping. IRT and optical coherence tomography (OCT) were used in combination to evaluate, more than one year ago, the state of conservation of the same marquetry here analyzed. Step-heating (SH) and pulsed thermography (PT) techniques allowed the implementation of a defect map refined by OCT [6]. In a view of conservation, cultural heritage objects should be inspected by routine tests because the degradation phenomena are faster than those in composite materials. This is a key message that the present research wants to leave with readers. Furthermore, air-coupled ultrasound (air-UT) [7] and X-ray micro- tomography (micro-CT) [8] complement the previous analyses [6] as, to the best of our knowledge, they have never been applied on real marquetries. Finally, two different spectral ranges (instead of one) were here studied, i.e., mid-wave infrared (MWIR) and long-wave infrared (LWIR) [9], therefore processing new thermographic data. Shrestha et al. applied numerical simulations (with the intent to gently heat the sur- face), active IRT, X-ray fluorescence (XRF) spectroscopy, and radiography on a marquetry coming from the same parietal structure (or ceiling) of the one here analyzed, by demon- strating that it is possible to provide robust information without damage the precious tessellatum layer [10]. The tessellatum describes the upper layer of a marquetry; it is composed by tesserae that can be of different nature (e.g., ivory and bone) in the so-called intarsia. It is thought that the word intarsia is derived from the Latin word interserere which means “to insert”. This explains why marquetries are formed by different pieces of wood (or other materials) with the aim to produce a figure or, as in our case, a geomet- ric/decorative surface. In the mosaics field—that is different from the marquetries fied, the main type of tessellatum that is usually cited is the opus tessellatum, i.e., a floor made of stone tesserae. The work of Garrido et al. introduced the latest state-of-the-art deep learning (DL) model for instance segmentation—mask region-convolution neural network (Mask R- CNN)—for the automatic detection and segmentation of the position and area of different surface and subsurface defects in two different artistic objects belonging to the marquetry’s family. Active IRT was used after the application of two automatic thermal image pre- processing algorithms based on thermal fundamentals to improve the contrast between defective and sound areas. It was found that the performance of the Mask R-CNN was improved by the prior application of the proposed preprocessing algorithms[11]. Two ancient marquetries containing natural defects were inspected in [12] via active IRT by using time-tested, safe, and resilient advanced signal processing algorithms applied with the aim to providing a 2D map of defects. Furthermore, dynamic thermal tomography (DTT) was used to understand the volume of the subsurface defects. A special technique for defect thermal characterization was used to validate the tomographic results; both a calibrate thermal stimulus and the use of a thermal camera were fundamental in such a case. By continuing exploring the volume of marquetries/inlays, truncated correlation photothermal coherence tomography (TC-PCT) was applied in [13] to identify subsurface features that are often invisible areas of vulnerability and damage. The TC-PCT modality proved capable of providing 3D images of specimens with high axial resolution, deep sub- surface depth profiling capability, and high signal-to-noise ration (SNR). The experimental results demonstrated the identification of various defects (natural and fabricated) up to a depth range of 2 mm. Finally, a thermographic data analysis method was proposed in [14] to overcome the shortcomings of the existing methods. The proposed method imposed both spatial Appl. Sci. 2021, 11, 7979 3 of 12 connectivity and sparsity constraints in principal component thermography (PCT). An ancient marquetry sample belonging to the same parietal (or ceiling) that “hosted” the one here inspected was selected to illustrate the feasibility of the method. Inspected Sample As said in the first two phrases of this introduction, the diagnostic protocol we are proposing demonstrates how a joint and clever application of NDTs can detect various defects of different sizes in an ancient marquetry having both an unknown origin and an unusual decorative layer. It is important to underline the fact that results from each technique (air-UT, micro-CT, and IRT) show different features. Aesthetically, the marquetry under test (found several years ago in a junk shop in the North of Italy) seems to be Middle Eastern or North African; this, from a visual inspection