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Review

Wall Mosaics: A Review of On-Site Non-Invasive Methods, Application Challenges and New Frontiers for Their Study and Preservation

by
Antonina Chaban
1,*,
Rita Deiana
2 and
Vivi Tornari
3
1
National Institute of Optics, Italian National Research Council, 50125 Florence, Italy
2
Department of Cultural Heritage, University of Padova, Piazza Capitaniato 7, 35139 Padova, Italy
3
Institute of Electronic structure and Laser, Foundation for Research and Technology Hellas, N. Plastira 100, 70013 Voutes, Crete, Greece
*
Author to whom correspondence should be addressed.
J. Imaging 2020, 6(10), 108; https://doi.org/10.3390/jimaging6100108
Submission received: 10 August 2020 / Revised: 23 September 2020 / Accepted: 24 September 2020 / Published: 12 October 2020
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Abstract

:
This review concerns the challenges and perspectives of on-site non-invasive measurements applied to wall mosaics. Wall mosaics, during the centuries, decorated numerous buildings, nowadays being part of world cultural heritage. The preservation and maintenance of these valuable decorations are undoubtedly directly dependent on identifying possible problems that could affect their hidden structure. On-site non-invasive methods, using different contact or no-contact technologies, can offer support in this specific field of application. The choice of the appropriate technique or combination of different techniques depends, in general, on the depth of investigation, the resolution, the possibility to have direct contact with the surfaces or, on the contrary, limited accessibility of the wall mosaics due to their location (e.g., vaults), as well as deterioration problems, (e.g., voids, detachments, or humidity effects). This review paper provides a brief overview of selected recent studies regarding non-invasive methods applied to the analysis of wall mosaics. This review, discussing the assessment of advantages and limitations for each method here considered, also considers possible future developments of imaging techniques in this specific context for cultural heritage applications.

1. Introduction

The use of wall mosaics as surface decoration in historical buildings (e.g., houses, palaces, churches etc.) can be traced back to the ancient Orient using wall coverings made by clay cones, different both in size and thickness from the traditional wall mosaics [1]. The extensive use of the most known small tesserae, as a decoration of walls, columns, pillars, ceilings, and vaults, is documented over the centuries, particularly in Greek, Roman, and Byzantine periods [1,2,3,4,5,6,7,8,9,10], in numerous examples of buildings, which are part of the world cultural heritage. In general, the mosaics located on walls, domes, and vaults are more difficult to study (due to their position, geometric and surface features etc.).
The possibility to quickly and non-invasively identify any anomaly that may compromise the integrity of these decorations, as well as the ability to recognize previous restorations, replacements, and other changes that have occurred over time, in a preventive, non-invasive manner, is crucial in the context of the interventions for their conservation and protection. This review is focused on the main methods currently used and now available for the non-invasive analysis and monitoring of ancient wall mosaics. In general, these decorations below the tesserae present simple preparation layers, compared to the floor mosaics. Moving from the surface towards the wall support, we have the tesserae, generally made of glass, stone, nacre, or ceramic, then the first layer of lime mortar and aggregates of different types (sand, pozzolana, marble dust, fragments of straw etc.) and another layer of bedding mortar, with or without aggregates [3,4]. Differences in the composition, number, and thickness of layers allow for identifying contributions by different mosaic workers, or, more generally, the period of its realization. Among the various problems that may affect the conservation of this type of decoration, there are the alterations of the surface of the tesserae, the presence of voids and inhomogeneities in the underlying layers of mortar. These can lead to the detachment and consequent fall and loss of tesserae. Other problems are related to humidity (e.g., moisture infiltration) that can also generate detachments and efflorescences. In the last decade, many advances have been made in the applications of on-site non-invasive measurements to the analysis of cultural heritage, supporting restoration works and studies aimed at understanding the transformations that may have affected the historical structures over the centuries [11,12,13,14,15,16]. In order to detect or to predict damage, efficient diagnosis and monitoring tools are essential. In particular, the possibility offered by on-site non-invasive measurements to preliminarily analyze the decorations as wall mosaics, identifying any problems and driving the conservation and restoration interventions, opens up new horizons in the field of heritage protection. This ability offered today by some specific methods in the identification of detachments (e.g., using IR thermography) or areas altered/compromised by previous restoration interventions (e.g., presence of different mortars or concrete, metal reinforcements, e.g., using ground-penetrating radar), and the detection of the presence and extent of any water leaks and infiltration (e.g., using electrical resistivity tomography, ground-penetrating radar, IR thermography), therefore, represents the critical issue in the new concept of cultural heritage preservation.

2. Wall Mosaic Characteristics and Main Diagnostic Challenges

Although each study case is different, the three fundamental challenges in wall mosaic diagnostics are represented by their complex, layered structure, presence of inhomogeneities (e.g., nails), and location (limited accessibility).

2.1. Structural Layering

The ancient wall mosaics, in general, are layered structures (Figure 1a), composed in most cases by three main levels: (1) a superficial decoration layer constituted by tesserae; (2) two or more different mortar layers; (3) wall structure made by bricks o stones [1,2,3,4,5,6,7,8,9]. While stone tesserae were mostly used in floor mosaics, glass tesserae dominate in wall mosaics. The most well known and preserved wall mosaics are of the ancient Roman, Greek, and Byzantine medieval periods.
The Byzantine wall mosaics were mostly realized in several stages directly on the wall, covering it by three different thickness layers of lime mortar. The first two layers were commonly made of lime, bricks, and cut straw, while the third contained only lime and marble powder [9].

2.2. Presence of Inhomogeneities

Cut straw was sometimes added to improve the binding characteristics and to control the migration of moisture between pores [3,4,9]. To enhance the cohesion between the layers, the first and the second mortar layers presented a rough surface, with deep bumps or ripples made by the tip of the trowel [3,4,9], as shown in Figure 1b,c, creating an irregular subsurface. To ensure the mortar layer (e.g., on vaults), strong steel flat-headed nails were often placed protruding the wall, as shown in Figure 1a. The final (bedding) layer of mortar could contain additives as linseed oil and adhesive substances to ensure the slowing down of mortar setting while often not regularly placing the tesserae. The tesserae were predominantly made of glass-paste, containing metal oxides, or a gold leaf, extensively used for gilded backgrounds, halos, and details of religious scenes in the churches [1,2,3,4,5,6,7,8,9]. Ancient wall mosaics have been often restored and/or reshuffled over the centuries without documentation of these interventions.

2.3. Limited Accessibility

Highly valuable historical and archaeological sites are characterized by limited accessibility due to opening hours to visitors or site logistical conditions. Besides, wall mosaics are often located in high, hardly accessible, or inaccessible parts of buildings (domes, vaults etc.). The limitations are represented by the need to use scaffoldings (if available), by the form of the structural support, and by the impossibility to respect the necessary geometry of the acquisitions (distance, access on both sides of the wall etc.).

3. Some Examples of Applications of Non-Invasive Methods to Study Wall Mosaics

From a theoretical point of view, there is a wide range of non-invasive methods and related instruments applied in engineering and cultural heritage studies.
This review considers only non-invasive methods useful to the study of wall mosaics and focuses on the developments and experimental setup proposed by the international scientific community interested in this topic. The most documented literature for in-situ non-invasive methods to study wall mosaics regards ground penetrating radar (GPR) and infrared thermography (IRT) and sporadic example of the use of electrical resistivity tomography (ERT), acoustic methods (e.g., sonic and ultrasonic tests), microwave reflectometry (MWR), and fiber optics microscopy (FOM). More recent literature also reports promising results of laboratory tests by different methods, e.g., electro-optic holography, also called electronic speckle pattern interferometry (ESPI), as well as recent interesting complementary approaches, using digital holographic speckle pattern interferometry and stimulated infrared thermography (DHSPI-SIRT), holographic subsurface radar (HSR), and scanning laser doppler vibrometry (SLDV). The main advantages, limitations, and potentials of each method are discussed below.

3.1. Ground Penetrating Radar (GPR)

The use of high-frequency GPR antennas (2 GHz) increased in the last two decades in the field of cultural heritage inspections [11,12,13,14,15,16,17,18,19,20,21,22], with some examples also including wall mosaics structure analysis. Based on the propagation of electromagnetic waves, geophysical methods, applied mainly in environmental and archaeological studies, gained popularity also in the cultural heritage field of investigation thanks to its capability for the fast and easy identification of voids and moisture content variations, both related to the contrast of dielectric permittivity. These aspects are crucial in the localization of in-homogeneities, presence of voids, and moisture content mapping and time-lapse monitoring of these phenomena in mosaic wall analysis. On the other hand, despite the fast and easy acquisition of the data, the principal limitation in using this powerful non-invasive method of investigation is represented by the needed contact between the antenna and the investigated surface that often preclude its application. For a more comprehensive analysis of physical principles, data acquisition, and processing of the GPR method, we suggest consulting the specific literature [23,24].

3.2. Infrared Thermography (IRT)

The infrared thermography is an on-site no-contact method widely applied to cultural heritage studies [25,26,27,28,29,30,31,32], with documented applications on wall mosaics analysis. The principle of infrared thermography consists of measuring, without contact, all thermal variations detectable on the surface by the infrared radiation. In general, the thermal variations could indicate different materials, including inhomogeneities, under the decoration layer. Although this measurement refers only to the first few centimeters below the decoration, it represents a perfect combination with the GPR method to identify shallow voids (detachments) or humidity, mainly where the contact with the surface is not admitted. Active infrared thermography, or also called stimulated infrared thermography, foresees to induce a minimal thermal excitation artificially to the surface of the investigated mosaic decoration, in order to increase characteristic thermal differences [25]. The use of passive thermography is preferred when the method is applied to cultural heritage objects; nevertheless, it does not always provide efficient results. The accuracy of temperature measurements is limited when the inspected surface consists of various types of materials with different emissivity values and if there are no naturally occurring thermal contrasts, in particular when examining vault/dome mosaics at a long distance.
Natural thermal contrasts are stronger on externally exposed walls and weaker or not detectable on the internal walls of a building. The advantage of active (or stimulated) infrared thermography is its potential to increase the informativity of a thermographic inspection in the absence of natural thermal contrasts, especially for the study of shallow depth detachments and voids. However, the informativity can still be limited due to insufficient maximum thermal excitation (usually, not higher than ΔΤ = 3.5 °C) in low thermal conductivity material in the superficial layer of the wall.
The most significant advantage of infrared thermography is its non-contact, remote control, and full-field investigation technique. The instruments are generally easily portable. The operator can visualize the results, i.e., thermal images, in real-time. Further thermal data processing and analysis for real case studies of wall mosaics are often necessary. For more details about the theory and application of this method, we suggest to see the specific literature [33,34,35].

3.3. Complementary On-Site Non-Invasive Methods

The two above-mentioned established techniques can be complemented by other structural diagnostic methods, which are not systematically used in real case studies yet. They can be divided into two groups: (1) on-site non-invasive techniques with documented applications on real wall mosaics, which include seismic methods (sonic and ultrasonic testing) and electrical resistivity tomography (ERT), and (2) non-invasive methods, tested in laboratory conditions but to the authors’ awareness never applied on real case studies yet, including electro-optic holography (ESPI), digital holographic speckle pattern interferometry (DHSPI) applied in combination with stimulated infrared thermography (DHSPI-SIRT), holographic subsurface radar (HSR), scanning laser doppler vibrometry (SLDV).
The principle of acoustic methods lies in the measurement of sonic/ultrasonic wave velocity propagation through the material. The interpretation of the results is based on the analysis of wave propagation velocity, which differs for different kinds of medium and is altered by the presence of cracks, voids, moisture areas etc. [36]. It is suitable for the evaluation of subsurface and deep wall structure. However, it cannot be regularly applied to highly precious surfaces. The main limitations of seismic methods include their manual contact operation and possible invasivity for deteriorated areas, and insufficient resolution for shallow depth levels.
The principle of electrical resistivity tomography lies in the characterization of the subsurface materials in terms of their electrical properties [37,38]. It can create a 3D representation of electrical resistivity distribution inside the examined objects, e.g., walls underneath a mosaic decoration. It provides the possibility to localize humidity (and then the provenience of the water infiltration and rise) or the presence of cracks and voids, as well as with the GPR method. The principal limitation on ERT applicability on wall decorations is its invasivity. Although the use of medical electrodes is possible, commonly to guarantee the better signal to noise ratio, the use of micro-electrodes (nails) is preferred. This invasivity then effectively limits the use of the ERT method in the mosaic wall analysis. Some studies were carried out in order to overcome this limitation, in particular, by limiting the number of mini electrodes. This method, in its current operation mode, still cannot be regularly applied to real wall mosaic decorations.
Innovative optical techniques are of particular interest in the structural evaluation of cultural heritage objects. Interferometric methods were tested during the last decades in laboratory conditions for non-contact full-field wall mosaic subsurface structural evaluation. The first documented application on a mosaic wall model refers to the method called electro-optic holography, or electronic speckle pattern interferometry (ESPI). It is the optical technique that enables interferometric measurements of surface displacements upon induced stress [39,40] and can be considered as a video equivalent of holographic interferometry [41,42].
A highly promising method is digital holographic speckle pattern interferometry (DHSPI). It is a non-invasive full-field, remote control, and non-contact optical technique based on the combination of holographic interferometry (HI) and electronic speckle pattern interferometry (ESPI). The holographic principle of DHSPI consists of recording phase variations of two mutually coherent laser beams at the single wavelength (532 nm), which give rise to interference fringes recording out-of-plane micro-displacement of the surface, with the resolution of 266 nm. The data acquisition is carried out through interferometric comparison of the two states of the surfaces under examination: the reference state and the excited state. For a more comprehensive study of the DHSPI operating principle and various applications, dedicated literature sources are available [41,42,43,44,45,46,47,48]. Digital holographic speckle pattern interferometry (DHSPI) coupled with stimulated infrared thermography (SIRT) proved to be a powerful non-invasive, non-contact, full-field, and remote-control subsurface diagnostics system, already applied to a variety of movable and immovable works of art [42,43,44,45,46,47,48,49,50]. When DHSPI is applied in combination with SIRT, thermal excitation is used for simultaneous acquisition by the two techniques. The differences in surface micro-deformations are induced by differences in the thermal expansion behavior of the materials and elements in the object’s subsurface structure. This technique can be applied to temporal monitoring of structural changes, based on environmental and climatic changes, conservation treatments, natural or stimulated aging, etc. To the authors’ awareness, it has not been applied yet to wall mosaics in-situ. The significant advantages of DHSPI and DHSPI-SIRT include its applicability and informativity, independently from the form and regularity of the surface (it can be applied to non-planar surfaces), even at long distances (up to 15 m). The system provides real-time images visualization, and raw results were already easy to analyze and interpret.
The method called scanning laser doppler vibrometry (SLDV) is used to perform non-contact vibration measurements of a surface [51,52,53]. The surface of an object under investigation is slightly vibrated by mechanical and acoustical actuators, while a laser doppler vibrometer scans the surface velocity and produces velocity amplitude and phase 2D or 3D maps. In the presence of a defect, i.e., a detachment or void, the velocity in the affected area is higher than in the neighboring areas. The Scanning LDV measures surface velocities point by point using interferometric techniques and a couple of galvanometric driven mirrors steering the laser beam. The main advantage of the system for the structural evaluation of wall mosaics lies in its remote control and non-contact operation, high sensibility, and resolution.
The main limitation of ESPI, DHSPI, and SLDV consists of their high sensitivity to surrounding vibrations (movement of operators and other people, other technical and mechanical vibrations).
Holographic subsurface radar (HSR) is an innovative type of a surface penetrating radar, which proved some successful results in structural evaluation on planar surfaces in engineering and built heritage field [54,55,56,57,58]. It operates with continuous-wave, at several close frequencies around 2 GHz, 4 GHz, or 7 GHz, depending on the model. Both the reflected signal and reference signal have the same frequency, but the reflected signal has a phase shift depending on the distance (depth) to the reflector. The holographic principle refers to image formation, and the method can be referred to as microwave holography [50]. The detectability of the target (a defect or an inhomogeneity) within the subsurface is based on contrasts in dielectric permittivity. This technique proved its efficiency in the immediate detection of dielectric contrasts in the subsurface structural evaluation of built heritage, including metallic and plastic elements, moisture infiltrations. The method allows for the estimation of the location depth of the target, as well as the size and geometry of revealed features if dielectric permittivity values are known. The HSR instrument, called Rascan, is lightweight and easily portable. However, the acquisition procedure is manual, requires advanced accuracy and the obtained results depend strongly on the precision of acquisition lines. They can be performed with a minimum possible offset (0.5 cm and an intermediate layer (PVC film or a transparent sheet) is suggested to protect the mosaic surfaces. Repeated acquisitions in two perpendicular directions are recommended. The main applicability limitation is that it encounters difficulties when evaluating irregular or non-planar (for example, curved) surfaces, and currently, the developers of the technique are working on the further developments of the scanning system [59]. For a more comprehensive treatment of the subject, the reader can refer to reliable bibliographic sources [54,55,56,57,58,59].
The results obtained on real case studies and laboratory simulated models by established and innovative methods, presented in scientific literature, are presented below.

3.4. In-Situ Results

Ground-penetrating radar (GPR), infrared thermography (IRT), and fiber optics microscopy (FOM) were applied in combination with wall mosaics in Hagia Sophia (Istanbul, Turkey). The church of Hagia Sophia is decorated with wall mosaics dating back to the fourth century [60]. The church suffered many interventions, damages, earthquakes. After its first conversion to a mosque upon the fall of Constantinople (1453), the original mosaic decoration was plastered over. In 1935, it became a museum, with another conversion into a mosque in 2020. The research issues of the interior mosaics in Hagia Sophia, approached by different expert groups in the last years, vary from revealing hidden layers of covered (plastered) mosaics to the evaluation of complex surface and subsurface structural conditions. The GPR was efficient in revealing the presence of covered mosaic layers (Figure 2). The IRT was not capable of detecting covered mosaic layers; however, it provided complementary information on differences in emissivity properties of different materials (Figure 3). The fiber optics microscope (FOM) was used to assess the conservation state of the decoration layer and bedding mortar, providing detailed complementary information to the assessment of aging and decay of materials due to porosity, cracking, to correlate the revealed water intrusions and the identification of the possible level of moisture in the subsurface beneath the mosaics [61,62,63,64]. The studies conducted since 2000 also included evaluations of compatibility of consolidation interventions [65] and led to a series of advancing experimental laboratory studies [66,67].
Another interesting case study is based on the use of Ground Penetrating Radar (GPR) in combination with seismic techniques. They were applied in-situ on wall mosaics at the Katholikon of Dafni Monastery (Athens, Greece) in 2003. It is an XI century World Heritage monument, famous for its mosaics [68]. After an earthquake in 1999, the monument was heavily damaged, and the subsurface structural investigation is part of a big conservation-restoration campaign. The research issues were the following: to study the internal masonry structure and of the mosaics substrates, to assess the performance of conservation-restoration treatments. In some areas, these techniques were applied before and after hydraulic grouts injections for filling cracks and voids in the shallow depth subsurface and structural support of wall mosaics. High-frequency ground-penetrating radar was successfully applied to the mapping of more than 50 mosaics in order to identify critical areas with delaminations, changes of substrata mortar (i.e., recent repairs), or buried heterogeneities. With the use of seismic tomography technique, as well as by manual sonic investigation by conservators, maps revealing the internal mechanical properties of the masonry walls were produced.
Complementary use of non-invasive techniques showed usefulness in the detection of subsurface structural problems and the efficiency assessment of a grouting intervention [69]. The results based on the assessment sonic transmission velocities of grouted and not grouted areas of a wall are shown in Figure 4.
An innovative and challenging in-situ investigation of wall mosaics was carried out using electrical resistivity tomography (ERT) [70]. The method was applied to a precious wall mosaic in the fountain room of the Zisa Palace, Palermo (Italy), in 2009. A 3D map of electrical resistivity distribution inside the wall was realized. The lower resistivity zone was interpreted as the moisture infiltration in the middle of the wall (Figure 5). It corresponds to the location of the water-pipe of the fountain. The method proved its effectiveness in the detection of the problem and showed clear readability and interpretation of the results. This method is still scarcely applied in the field of cultural heritage, due to the invasivity of mini electrodes. As shown by the first experimental results, if the problem of invasivity can be solved, this method shows strong potential for subsurface and deep wall structure evaluation.

3.5. Laboratory Simulation Studies

Electronic speckle pattern interferometry (ESPI) was applied to a custom-built mosaic model with known defects [71]. The decoration of the mosaic model was realized using glass paste tesserae; therefore, it was representative of a wall mosaic. The ESPI system was capable of revealing the two known defects (detachments at different depth levels), showing limitations due to partial transparency of glass tesserae, characteristic for wall mosaics, and high sensitivity of the system to external vibrations. In scientific literature, it is the first documented application of ESPI to a wall mosaic in 1998. The reader is invited to consult the relative sources for a more comprehensive treatment of the subject [39,40,41,42,71].
More laboratory studies are dedicated to the combination of holographic interferometry and infrared thermography in wall mosaic diagnostics. Laboratory tests were performed on plastered mosaics and revealing hidden layers and differences by means of holographic interferometry and transient thermography in 2012 [72]. The search of the most appropriate methods in the detection of plastered mosaics is important for the cases like Hagia Sophia mosaics, the reader can refer to the literature sources cited above [61,62,63,64,65]. Laboratory experimentation confirmed the efficiency and complementarity of the two techniques. The transient thermography provided results concerning the identification of the different subsurface tesserae areas of the plastered panel, differentiating between stone, metal leaf, and glass paste. Holographic interferometry identified the precise demarcation lines of these areas, which were not possible to perceive for all the hidden areas by the thermographic survey.
Complementary approach by DHSPI-SIRT and HSR was studied in 2018 on a custom-built mosaic model with known defects and subsurface mortar inhomogeneities, covered by glass paste, golden leaf, and stone tesserae [73]. The DHSPI system was capable of detecting the stitches between mortar portions with slightly variable composition, as well as the presence of voids, detachments, and cracks underneath the mosaic decoration layer (Figure 6). The differences in the mortar (application mode, slight changes in the proportions of ingredients) were observed on an interferogram starting from very low thermal excitations (less than 1 °C). All the applied excitation values were below Δt = 3.5 °C, within the limits of allowed indoor museum temperature fluctuations. The SIRT results acquired simultaneously to DHSPI, did not reveal internal mortar characteristics but were useful to locate and to confirm the presence of detachments and, through subsequent data analysis, the presence of hidden metallic elements (nails, a mounting plate) and wooden sticks. Different studies on advanced potentials of active infrared thermography for revealing hidden layers and deterioration problems, e.g., through different acquisition and data processing modes [67] or the automatic detection of inhomogeneities [74], are available too.
Laboratory tests by DHSPI-SIRT were complemented by the HSR method [73]. This technique was primarily applied on wall mosaic panels in the church of Santa Maria dell’Ammiraglio (Palermo, Italy) [75], though it did not provide useful results. The signal was reflected from the irregular mosaic surface, made of gold leaf tesserae, without the possibility to provide information about the underlying layers. The absence of informative in-situ results led to a series of laboratory experiments. The subsequent tests confirmed the absence of HSR signal penetration underneath the gold leaf background but proved its capability to detect metallic elements underneath glass paste and stone tesserae, even at irregular surfaces [73]. The HSR laboratory results are shown in Figure 7.
A complementary approach by DHSPI-SIRT and HSR allowed for the detection of mortar subsurface structure (by DHSPI), detachments (by DHSPI-SRT), and buried metallic elements (by HSR) [73]. Each of these methods showed certain advantages and limitations, and the proposed combination can be a starting point and a reference study for future frontiers in the non-invasive techniques development study. A possible workflow for non-invasive mosaic subsurface structural evaluation, based on laboratory results, was proposed using DHSPI-SIRT and HSR (see Figure 8).
Laboratory tests in 2003 include applications of another promising technique scanning laser doppler vibrometry (SLDV) to series of art objects, including a mock-up of a newly made mosaic with an area of detached glass paste tesserae [76]. With acoustic excitation, before and after gluing, the detached tesserae on the mosaic model showed the promising results of the technique (Figure 9). This revealed the difference in vibration amplitude before and after gluing the tesserae. The examined mosaic model was held in front of the SLDV, and further studies are to be carried out to verify this condition at different angles.

4. Conclusions and Future Directions

The review of scientific publications dedicated to the complex non-invasive study of wall mosaics showed that a variety of methods is available, which are already or potentially applicable in-situ, at their current development stage or, hopefully, through further improvements, in the next future. The applicability of main on-site non-invasive techniques to the complex evaluation of wall mosaics is illustrated in Table 1. Based on the results published by the international scientific community, the available non-invasive techniques are assessed in terms of their applicability and informativity for a mosaic wall decoration. Table 2 summarizes the advantages and limitations of each considered technique.
The most discussed research questions in wall mosaics study are conservation problems like moisture rise, cracks, detachments, assessment of conservation treatments, and historical issues, e.g., the presence of hidden valuable mosaic layers. The most informative in-situ acquisitions, described in scientific literature, include the well-known IRT and GPR. Laboratory studies show the growth of interest in the development of new portable non-contact full-field methods, where a strong potentiality is shown by DHSPI, DHSPI-SIRT, SLDV, as well as by HSR (with certain limitations) etc.. The literature review has shown a strong interest in structural deterioration and a specific issue of plastered (intentionally covered) wall mosaics. On the other hand, studies dedicated to reshuffled/restored mosaics, with the identification of presumably original and transferred/restored areas, in authors’ awareness, are not present in the scientific literature. Further studies have to be carried out also on gold leaf backgrounds, which are very common in Byzantine wall mosaics and where the application of surface penetrating radar and infrared thermography techniques is limited (the radiation is reflected from the metallic surface without the possibility to obtain information about the underlying layers).
Through the overview and comparison of documented in-situ applications, it can be confirmed that research questions and the conservation problems differ from mosaic to a mosaic. A unique, universally applicable method to wall mosaic decoration, providing information at all the depth levels, does not exist. Every method is characterized by certain advantages and limitations by a possible penetration depth, resolution parameters, and acquisition requirements. Therefore, the only universal rule is that a complementary use of at least two or more informative scientific methods is essential (Figure 10).
The most critical parameters in choosing a method for in-situ wall mosaic diagnostics are their non-destructivity, portability, efficiency in answering the research question, and (possibly) non-contact operation. If these requirements are satisfied, the method, characterized by a fast, easy, and full-field acquisition, should be preferred. Further aspects to be considered include time management, access to the site and acquisition conditions, team expertise, available instrumentation, and costs.
Further frontiers of the development of these and new methods are represented by increasing the informativity, feasibility, and timing, while decreasing invasivity, contact operation, and costs. There is a strong demand for further development of full-field visualization and non-contact methods, providing a comprehensive structural evaluation approach at all the depth levels. This non-invasive and non-destructive structural evaluation, in an ideal case, will be performed in automated monitoring mode. This will support the achievement of the fundamental goal in the field of cultural heritage diagnostics: timely and efficient detection, prevention of deterioration, limiting loss, microsampling, or any other impact and keeping alterations to a minimum, as an exception, or finally to zero.

Author Contributions

Conceptualization, R.D. and V.T.; methodology, R.D., V.T., and A.C.; validation, R.D. and V.T.; resources, A.C.; writing—original draft preparation, A.C.; writing—review and editing, V.T.; supervision of the Ph.D. thesis bibliographic and experimental study, R.D.; supervision of the Ph.D. research period abroad, V.T.; project administration, A.C.; funding acquisition, A.C. and R.D. All authors have read and agreed to the published version of the manuscript.

Funding

The bibliographic study was performed within the Ph.D. program, supported by the University of Padova in the form of a Ph.D. studentship.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Waterman, A.E. A History of Mosaics; Hacker Art Books: New York, NY, USA, 1968. [Google Scholar]
  2. King, S. Mosaic Techniques and Traditions: Projects and Designs from around the World; Stirling Publishing Co. Inc.: New York, NY, USA, 2003. [Google Scholar]
  3. Fabbri, B. Science and Conservation for Museum Collections; Nardini: Firenze, Italy, 2012. [Google Scholar]
  4. Masini, N.; Soldovieri, F. Integrated non-invasive sensing techniques and geophysical methods for the study and conservation of architectural, archaeological and artistic heritage. J. Geophys. Eng. 2011, 8, 083E01. [Google Scholar] [CrossRef]
  5. Lerma, C.; Gil Benso, E.; Mas, Á.; Vercher, J.; Torner, M. Non-destructive techniques methodologies for the detection of ancient structures under heritage buildings. Int. J. Archit. Herit. 2019. [Google Scholar] [CrossRef]
  6. Fais, S.; Casula, G.; Cuccuru, F.; Ligas, P.; Bianchi, G. An innovative methodology for the non-destructive diagnosis of architectural elements of ancient historical buildings. Sci. Rep. 2018, 8, 4334. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  7. Sala, R.; Tamba, R.; Garcia, E. Application of geophysical methods to cultural heritage. Elements 2012, 12, 19–25. [Google Scholar] [CrossRef]
  8. Cozzolino, M.; Di Giovanni, E.; Mauriello, P.; Piro, S.; Zamuner, D. Book Geophysical Methods for Cultural Heritage; Springer International Publishing Switzerland AG: Cham, Switzerland, 2018. [Google Scholar]
  9. Cosentino, P.; Capizzi, P.; Fiandaca, G.; Martorana, R.; Messina, P. Advances in microgeophysics for engineering and cultural heritage. J. Earth Sci. 2009, 20, 626–639. [Google Scholar] [CrossRef]
  10. Fiorentini Roncuzzi, I.; Fiorentini, E. Mosaic: Materials, Techniques and History; MWeV: Ravenna, Italy, 2002. [Google Scholar]
  11. Farneti, M. Glossario Tecnico-Storico del Mosaico: Con Una Breve Storia del Mosaico; Longo: Ravenna, Italy, 1993. [Google Scholar]
  12. Henig, M.; Ithaca, N.Y. A Handbook of Roman Art: A Comprehensive Survey of All the Arts of the Roman World; Cornell University Press: Ithaca, NY, USA, 1983. [Google Scholar]
  13. Demus, O. Byzantine Mosaic Decoration: Aspects of Monumental Art in Byzantium; Routledge & Kegan Paul: London, UK, 1948. [Google Scholar]
  14. Cilento, A. The Mosaics of Norman Sicily; Magnus Edizioni: Reggio Emilia, Italy, 1950. [Google Scholar]
  15. Demus, O. The Mosaics of San Marco in Venice; University of Chicago Press: Chicago, IL, USA, 1984; pp. 358, 495. [Google Scholar]
  16. Mouriki, D.; Burgi, R. The Mosaics of the New Chios Monastery (Ditomo), Byzantine Monuments; Commercial Bank of Greece publisher: Athens, Greece, 1985. (In Greek) [Google Scholar]
  17. Martinho, E.; Dionísio, A. Main geophysical techniques used for non-destructive evaluation in cultural built heritage: A review. J. Geophys. Eng. 2014, 11, 1–15. [Google Scholar] [CrossRef]
  18. Santos-Assunçao, S.; Perez-Gracia, P.; Caselles, O.; Clapes, J.; Salinas, V. Assessment of complex masonry structures with gpr compared to other non-destructive testing studies. Remote Sens. 2014, 6, 8220–8237. [Google Scholar] [CrossRef] [Green Version]
  19. Cosentino, P.L.; Capizzi, P.; Martorana, R.; Messina, P.; Schiavone, S. From geophysics to microgeophysics for engineering and cultural heritage. Int. J. Geophys. 2011, 2011, 428412. [Google Scholar] [CrossRef] [Green Version]
  20. Ranalli, D.; Scozzafava, M.; Tallini, M. Ground penetrating radar investigations for the restoration of historic buildings: The case study of the Collemaggio Basilica (L’Aquila, Italy). J. Cult. Herit. 2004, 5, 91–99. [Google Scholar] [CrossRef]
  21. Barone, P.M.; Di Matteo, A.; Graziano, F.; Mattei, E.; Pettinelli, E. GPR application to the structural control of historical buildings: Two case studies in Rome, Italy. Near Surf. Geophys. 2006, 407–413. [Google Scholar] [CrossRef]
  22. D’Aranno, P.J.V.; De Donno, G.; Marsella, M.; Orlando, L.; Renzi, B.; Salviani, S.; Santarelli, M.L.; Scifoni, S.; Sonnessa, A.; Verri, F.; et al. High-resolution geomatic and geophysical techniques integrated with chemical analyses for the characterization of a Roman wall. J. Cult. Herit. 2015, 141–150. [Google Scholar] [CrossRef]
  23. Daniels, D.J. Ground Penetrating Radar, 2nd ed.; IEE: London, UK, 2004; ISBN 0-86341-360-9. [Google Scholar]
  24. Reynolds, J.M. An Introduction to Applied and Environmental Geophysics; Wiley-Blackwell: Hoboken, NJ, USA, 2011. [Google Scholar]
  25. Candoré, J.C.; Bodnar, J.L.; Detalle, V.; Grossel, P. Non-destructive testing of works of art by stimulated infrared thermography. Eur. Phys. J. Appl. Phys. 2012, 57, 21002. [Google Scholar] [CrossRef] [Green Version]
  26. Mercuri, F.; Orazi, N.; Paoloni, S.; Cicero, C.; Zammit, U. Pulsed thermography applied to the study of cultural heritage. Appl. Sci. 2017, 7, 1010. [Google Scholar] [CrossRef]
  27. Vallet, J.-M.; Detalle, V.; De Luca, L.; Bodnar, I.-L.; Guillon, O.; Trichereau, B.; Mouhoubi, K.; Martin-Beaumont, N.; Syvilay, D.; Giovannacci, D.; et al. Development of a NDT toolbox dedicated to the conservation of wall paintings: Application to the frescoes chapel in the Charterhouse of Villeneuve-lez-Avignon (France). Digit. Herit. 2013, 67–74. [Google Scholar] [CrossRef]
  28. Jo, Y.H. Study on Applicability of Passive Infrared Thermography Analysis for Blistering Detection of Stone Cultural Heritage. The korean society of conservation science for cultural properties. J. Conserv. Sci. 2013, 29, 55–67. [Google Scholar] [CrossRef] [Green Version]
  29. Paoletti, D.; Ambrosini, D.; Sfarra, S.; Bisegna, F. Preventive thermographic diagnosis of historical buildings for consolidation. J. Cult. Herit. 2013, 14, 116–121. [Google Scholar] [CrossRef]
  30. Sfarra, S.; Ibarra-Castanedo, C.; Theodorakeas, P.; Avdelidis, N.P.; Perilli, S.; Zhang, H.; Nardi, I.; Koui, M.; Maldague, X.P. Evaluation of the state of conservation of mosaics: Simulations and thermographic signal processing. Int. J. Therm. Sci. 2017, 117, 287–315. [Google Scholar] [CrossRef]
  31. Sfarra, S.; Ibarra-Castanedo, C.; Tortora, M.; Arrizza, L.; Cerichelli, G.; Nardi, I.; Maldague, X.P. Diagnostics of wall paintings: A smart and reliable approach. J. Cult. Herit. 2016, 18. [Google Scholar] [CrossRef]
  32. Titman, D.J. Applications of thermography in non-destructive testing of structures. NDT&E Int. 2001, 34, 149–154. [Google Scholar] [CrossRef]
  33. Maldague, X.P. Theory and Practice of Infrared Technology for Nondestructive Testing; Wiley-Interscience: Hoboken, NJ, USA, 2001; ISBN 978-0-471-18190-3. [Google Scholar]
  34. Grinzato, E. IR thermography applied to the cultural heritage conservation. J. Appl. Sci. 2017, 7, 1010. [Google Scholar]
  35. Mercuri, F.; Zammit, U.; Orazi, N.; Paoloni, S.; Marinelli, M.; Scudieri, F. Active infrared thermography applied to the investigation of art and historic artefacts. J. Therm. Anal. Calorim. 2011, 104, 475. [Google Scholar] [CrossRef]
  36. Côte, P.; Abraham, O. Seismic Tomography in Civil Engineering; NDT-CE: Berlin, Germany, 1995; pp. 459–466. [Google Scholar]
  37. Athanasiou, E.; Tsourlos, P.; Tsokas, G.; Papazachos, C.; Vargemezis, G. Nondestructive DC resistivity surveying using flat base electrodes. Near Surf. Geophys. 2009, 5, 263–272. [Google Scholar] [CrossRef] [Green Version]
  38. Cosentino, P.L.; Capizzi, P.; Fiandaca, G.; Martorana, R.; Messina, P.; Amoroz, R. Integrated full 3D geoelectrical and GPR Tomographies in the ambulatory of the Roman “Villa del Casale” (Piazza Armerina). In Proceedings of the 14th European Meeting of Environmental and Engineering Geophysics, Krakow, Poland, 15–17 September 2008. [Google Scholar] [CrossRef]
  39. Paoletti, D.; Schirripa Spagnolo, G. Interferometric methods for artwork diagnostics. Prog. Optics 1996, XXXV, 197–255. [Google Scholar] [CrossRef]
  40. Lokberg, O.J. Electronic speckle pattern interferometry. Phys. Technol. 1980, 11, 16–22. [Google Scholar] [CrossRef]
  41. Vest, C.M. Holographic Interferometry; John Wiley and Sons, Inc.: Hoboken, NJ, USA, 1979. [Google Scholar]
  42. Jones, R.; Wykes, C. Holographic and Speckle Interferometry, 2nd ed.; Cambridge Press: Cambridge, UK, 1989. [Google Scholar]
  43. Kosma, K.; Andrianakis, M.; Hatzigiannakis, K.; Tornari, V. Digital holographic interferometry for cultural heritage structural diagnostics: A coherent and a low-coherence optical set-up for the study of a marquetry sample. Strain 2018, 54. [Google Scholar] [CrossRef]
  44. Tornari, V. Optical and digital holographic interferometry applied in art conservation structural diagnosis. E-Preserv. Sci. 2006, 3, 51–57. [Google Scholar]
  45. Tornari, V. On development of portable digital holographic speckle pattern interferometry system for remote-access monitoring and documentation in art conservation. Strain 2019, 201. [Google Scholar] [CrossRef]
  46. Tornari, V.; Tsiranidou, E.; Bernikola, E. Interference fringe-patterns association to defect-types in artwork conservation: An experiment and research validation review. Appl. Phys. A 2012, 106, 397–410. [Google Scholar] [CrossRef]
  47. Tornari, V.; Bernikola, E.; Nevin, A.; Kouloumpi, E.; Doulgeridis, M.; Fotakis, C. Fully non contact holography-based inspection on dimensionally responsive artwork materials. Sensors 2008, 8, 8401–8422. [Google Scholar] [CrossRef] [Green Version]
  48. Tornari, V.; Bernikola, E.; Tsigarida, N.; Andrianakis, M.; Hatzigiannakis, K.; Leissner, J. Preventive deformation measurements on cultural heritage materials based on non-contact surface response of model samples. Stud. Conserv. 2015, 60, S143–S158. [Google Scholar] [CrossRef] [Green Version]
  49. Tornari, V.; Andrianakis, M.; Hatzigiannakis, K.; Kosma, K.; Detalle, V.; Bourguignon, E.; Giovannacci, D.; Brissaud, D. Complimentarity of digital holographic speckle pattern interferometry and simulated infrared thermography for cultural heritage structural diagnostic research. Int. J. Eng. Res. Sci. (IJOER) 2016, 2, 129–141. [Google Scholar]
  50. Chaban, A.; Tornari, V.; Andrianakis, M.; Deiana, R. Comparison of induced thermal change to climate chamber simulated environmental change in mosaic model by digital holographic speckle pattern interferometry (DHSPI). SPIE 110580I 2019. [Google Scholar] [CrossRef]
  51. Tornari, V.; Bonarou, A.; Castellini, P.; Esposito, E.; Osten, W.; Kalms, M.; Smyrnakis, N.; Stasinopulos, S. Laser based systems for the structural diagnostic of artworks: An application to XVII century Byzantine icons. In Proceedings of the Laser Techniques and Systems in Art Conservation, Munich, Germany, 22 October 2001; Volume 4402. [Google Scholar]
  52. Drain, L.E. The Laser Doppler Technique; Lincoln: John Wiley & Sons: Hoboken, NJ, USA, 1980. [Google Scholar]
  53. Castellini, P.; Esposito, E.; Paone, N.; Tomasini, E.P. Non-invasive measurements of damage of frescoes paintings and icon by laser scanning vibrometer: Experimental results on artificial samples and real works of art. Measurement 2000, 28, 33–45. [Google Scholar] [CrossRef]
  54. Razevig, V.V.; Ivashov, S.I.; Vasiliev, I.A.; Zhuravlev, A.V.; Bechtel, T.; Capineri, L.; Falorni, P. RASCAN holographic radars as means for non-destructive testing of buildings and edificial structures. In Proceedings of the Structural Faults and Repair-2010, Edinburgh, Scotland, UK, 15–17 June 2010. [Google Scholar]
  55. Ivashov, S.I.; Makarenkov, V.I.; Masterkov, A.V.; Razevig, V.V.; Sablin, V.N.; Sheyko, A.P.; Tchapourski, V.V.; Vasiliev, I.A. Concrete floor inspection with help of subsurface radar. In Proceedings of the 6th Meeting Environmental and Engineering Geophysics, Bochum, Germany, 3–7 September 2000. [Google Scholar]
  56. Ivashov, S.; Razevig, V.; Sheyko, A.; Vasilyev, I.; Zhuravlev, A.; Bechtel, T. Holographic subsurface radar technique and its applications. In Proceedings of the 12th International Conference on Ground-Penetrating Radar, GPR 2008, Birmingham, UK, 16–19 June 2008. [Google Scholar]
  57. Capineri, L.; Falorni, P.; Borgioli, G.; Bulletti, A.; Valentini, S.; Ivashov, S.; Zhuravlev, A.; Razevig, V.; Vasiliev, I.; Paradiso, M.; et al. Application of the RASCAN holographic radar to cultural heritage inspections. archaeol. Prospect 2009, 16, 218–230. [Google Scholar] [CrossRef]
  58. Capineri, L.; Falorni, P.; Ivashov, S.; Zhuravlev, A.; Vasiliev, I.; Razevig, V.; Bechtel, T.; Stankiewicz, G. Combined holographic subsurface radar and infrared thermography for diagnosis of the conditions of historical structures and artworks. Eur. Geosci. Union Gen. Assem. 2009, 11, EGU2009-5343-2. [Google Scholar] [CrossRef] [Green Version]
  59. Razevig, V.V.; Zhuravlev, A.V.; Bugaev, A.S.; Chizh, M.A.; Ivashov, S.I. Imaging under irregular surface using microwave holography. In Progress in Electromagnetics Research Symposium—Fall (PIERS-FALL); IEEE: Piscataway, NJ, USA; pp. 172–177. [CrossRef]
  60. Mainstone, R.J. Hagia Sophia. Architecture, Structure and Liturgy of Justinian’s Great Church; Thames & Hudson: London, UK, 1997. [Google Scholar]
  61. Moropoulou, A.; Avdelidis, N.P.; Delegou, E.T.; Aggelakopoulou, E.; Karoglou, M.; Haralampopoulos, G.; Griniezakis, S.; Koui, M.; Karmis, P.; Aggelopoulos, A.; et al. NDT Investigation on Hagia Sophia Mosaics, in Research Work; NTUA: Athens, Greece, 2000. [Google Scholar]
  62. Moropoulou, A.; Bakolas, A.; Karoglou, M.; Delegou, E.T.; Labropoulos, K.C.; Katsiotis, N.S. Diagnostics and protection of hagia sophia mosaics. J. Cult. Herit. 2013, 14S, 133–139. [Google Scholar] [CrossRef]
  63. Moropoulou, A.; Labropoulos, K.C.; Katsiotis, N.S. Assessing the preservation state and revealing plastered mosaics in hagia sophia using ground penetrating radar. J. Mater. Sci. Eng. 2012, 2, 183–189. [Google Scholar]
  64. Moropoulou, A.; Avdelidis, N.P.; Aggelakopoulou, E.; Griniezakis, S.; Koui, M.; Aggelopoulos, A.; Karmis, P.; Uzunoglou, N.K. Examination of plastered mosaic surfaces using NDT techniques. Insight Non-Destr. Test. Cond. Monit. 2001, 43, 241–243. [Google Scholar]
  65. Moropoulou, A.; Avdelidis, N.P.; Delegou, E.T.; Aggelakopoulou, E.; Karoglou, M.; Haralampopoulos, G.; Griniezakis, S.; Koui, M.; Karmis, P.; Aggelopoulos, A.; et al. Investigation for the compatibility of conservation interventions on Hagia Sophia mosaics using NDT techniques. J. Eur. Study Group Phys. Chem. Biol. Math. Tech. Appl. Archaeol. Athens 2000, 59, 103–120. [Google Scholar]
  66. Theodorakeas, P.; Cheilakou, E.; Ftikou, E.; Koui, M. Passive and active infrared thermography: An overview of applications for the inspection of mosaic structures. 33rd UIT Ital. Union Fluid Dyn. Heat Transf. Conf. 2015, 655. [Google Scholar] [CrossRef]
  67. Theodorakeas, P.; Koui, M. Quantitative infrared thermography for the investigation of plastered mosaics: Comparison of experimental results and numerical modelling. In Proceedings of the 9th International Symposium on the Conservation of Monuments in the Mediterranean Basin, Ankara, Turkey, 3–5 June 2014; pp. 431–445. [Google Scholar]
  68. Delinikolas, N.; Miltiadou-Fezans, A.; Chorafa, E.; Zaroyianni, E. Restoration Study of the Katholikon of Dafni Monastery, Phase A-Architectural and Historical Survey; Hellenic Ministry of Culture: Athens, Greece, 2003.
  69. Côte, P.; Dérobert, X.; Miltiadou-Fezans, A.; Delinikolas, N.; Durand, O.; Alexandre, J.; Kalagri, A.; Savvidou, M.; Chryssopoulos, D.; Anamaterou, L.; et al. Application of non-destructive techniques at the katholikon of dafni monastery for mapping the mosaics substrata and grouting monitoring. In Structural Analysis of Historic Construction; D’Ayala, D., Fodde, E., Eds.; Taylor and Francis: London, UK, 2007; pp. 1149–1156. [Google Scholar]
  70. Fiandaca, F.; Martorana, R.; Messina, P.; Cosentino, P. 3D ERT for the study of an ancient wall covered by precious mosaics. In Proceedings of the Near Surface 15th European Meeting of Environmental and Engineering Geophysics, Dublin, Ireland, 7–9 September 2009. [Google Scholar] [CrossRef]
  71. Schirripa, S.G.; Ambrosini, D.; Paoletti, D. Optical methods for mosaic diagnostics. J. Opt. 1998, 29, 394–400. [Google Scholar]
  72. Theodorakeas, P.; Ibarra-Castanedo, C.; Sfarra, S.; Avdelidis, N.P.; Koui, M.; Maldague, X.P.; Paoletti, D.; Ambrosini, D. NDT inspection of plastered mosaics by means of transient thermography and holographic interferometry. NDT E Int. 2012, 47, 150–156. [Google Scholar] [CrossRef]
  73. Chaban, A.; Tornari, V.; Deiana, R.; Andrianakis, M.; Giovannacci, D.; Detalle, V. A combined non-invasive approach to the study of a mosaic model: First laboratory experimental results. J. Imaging 2019, 5, 58. [Google Scholar] [CrossRef] [Green Version]
  74. Garrido, I.; Lagüela, S.; Sfarra, S.; Zhang, H.; Maldague, X.P. Automatic detection and delimitation of internal moisture in mosaics from thermographic sequences. Exp. Tests Proc. 2019, 27, 7. [Google Scholar] [CrossRef] [Green Version]
  75. Chaban, A. Non-Invasive Methods for the Study of Wall Decorations in Art History and Archaeology. Ph.D. Thesis, University of Padua, Padua, Italy, 2019. [Google Scholar]
  76. Castellini, P.; Esposito, E.; Marchetti, B.; Paone, N.; Tomasini, E.P. New applications of Scanning Laser Doppler Vibrometry (SLDV) to non-destructive diagnostics of artworks: Mosaics, ceramics, inlaid wood and easel painting. J. Cult. Herit. 2003, 4, 321s–329s. [Google Scholar] [CrossRef]
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Figure 1. Structural layering of Byzantine wall mosaics: (a) section, IV—wall with protruding nails, III—1st plaster layer; II—2nd plaster layer; I—3rd plaster layer and tesserae, after [3]; (b) 2nd substrate layer, consisting of lime plaster, sand and straw and (c) 1st substrate layer, consisting of pure lime and straw (on display at the museum of Hosios Lukas, Greece).
Figure 1. Structural layering of Byzantine wall mosaics: (a) section, IV—wall with protruding nails, III—1st plaster layer; II—2nd plaster layer; I—3rd plaster layer and tesserae, after [3]; (b) 2nd substrate layer, consisting of lime plaster, sand and straw and (c) 1st substrate layer, consisting of pure lime and straw (on display at the museum of Hosios Lukas, Greece).
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Figure 2. Ground-penetrating radar (GPR) results on wall mosaics in Hagia Sophia (Istanbul, Turkey): (a) examined area of a plastered surface; (b) GPR scans, the highlighted ripples reveal the presence of plastered mosaics. After Moropolou et al. [62] (reprinted with copyright permission).
Figure 2. Ground-penetrating radar (GPR) results on wall mosaics in Hagia Sophia (Istanbul, Turkey): (a) examined area of a plastered surface; (b) GPR scans, the highlighted ripples reveal the presence of plastered mosaics. After Moropolou et al. [62] (reprinted with copyright permission).
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Figure 3. Infrared thermography (IRT) results at the plaster (top)–mosaic (underneath) interface in Hagia Sophia (Istanbul, Turkey): (a) visible image and the examined area; (b) thermogram showing the temperature difference of a mean value of 1.1 °C between the plastered surface and the mosaic. After Moropolou et al. [62] (reprinted with copyright permission).
Figure 3. Infrared thermography (IRT) results at the plaster (top)–mosaic (underneath) interface in Hagia Sophia (Istanbul, Turkey): (a) visible image and the examined area; (b) thermogram showing the temperature difference of a mean value of 1.1 °C between the plastered surface and the mosaic. After Moropolou et al. [62] (reprinted with copyright permission).
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Figure 4. Results of acoustic techniques applied in-situ to the assessment of efficiency of grouting, Katholikon of Dafni Monastery, Athens, Greece. Sonic transmission velocities across a similar wall grouted (red, higher velocity in repaired areas) or not yet grouted (purple, lower velocity in the presence of cracks and voids), showing the assessment capabilities of the technique. After Côte & X. Dérobert et al. [69] (reprinted with copyright permission).
Figure 4. Results of acoustic techniques applied in-situ to the assessment of efficiency of grouting, Katholikon of Dafni Monastery, Athens, Greece. Sonic transmission velocities across a similar wall grouted (red, higher velocity in repaired areas) or not yet grouted (purple, lower velocity in the presence of cracks and voids), showing the assessment capabilities of the technique. After Côte & X. Dérobert et al. [69] (reprinted with copyright permission).
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Figure 5. Electrical resistivity tomography (ERT) results in precious wall mosaics, fountain room of the Zisa Palace, Palermo, Italy. 3D representation of the inversion model, from a front point of view (left picture) and rear (right picture). Plane A—external wall surface. Plane B—investigation depth. Plane C—internal wall mosaic surface. Green and blue colors indicate the areas of low resistivity corresponding to the detected presence of moisture. After Fiandaca et al. [70] (reprinted with copyright permission).
Figure 5. Electrical resistivity tomography (ERT) results in precious wall mosaics, fountain room of the Zisa Palace, Palermo, Italy. 3D representation of the inversion model, from a front point of view (left picture) and rear (right picture). Plane A—external wall surface. Plane B—investigation depth. Plane C—internal wall mosaic surface. Green and blue colors indicate the areas of low resistivity corresponding to the detected presence of moisture. After Fiandaca et al. [70] (reprinted with copyright permission).
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Figure 6. Digital holographic speckle pattern interferometry (DHSPI) results in a custom-built mosaic model. Efficient recognition of mortar inhomogeneity, stitches, and cracks: (a) sample preparation in progress, detail of mortar application with a defect; (b) Interferogram at 170 s after Δt = 3.1 °C thermal excitation on a completed mosaic model, overlap with the photographic documentation of the sample preparation (25% transparency mode). After Chaban et al. [73].
Figure 6. Digital holographic speckle pattern interferometry (DHSPI) results in a custom-built mosaic model. Efficient recognition of mortar inhomogeneity, stitches, and cracks: (a) sample preparation in progress, detail of mortar application with a defect; (b) Interferogram at 170 s after Δt = 3.1 °C thermal excitation on a completed mosaic model, overlap with the photographic documentation of the sample preparation (25% transparency mode). After Chaban et al. [73].
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Figure 7. Holographic subsurface radar (HSR) results on a custom-built mosaic model: (a) Visible image showing the location of defects in the subsurface structure; (b) Rascan image at 6.7–6.8 GHz, where the red arrows indicate the location of the detected embedded metallic elements; (c) Rascan image with the location of defects (contour in red), stone tesserae (blue) and golden leaf tesserae (yellow). After Chaban et al. [73].
Figure 7. Holographic subsurface radar (HSR) results on a custom-built mosaic model: (a) Visible image showing the location of defects in the subsurface structure; (b) Rascan image at 6.7–6.8 GHz, where the red arrows indicate the location of the detected embedded metallic elements; (c) Rascan image with the location of defects (contour in red), stone tesserae (blue) and golden leaf tesserae (yellow). After Chaban et al. [73].
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Figure 8. A proposed complementary DHSPI-SIRT and HSR approach for a complex subsurface structural evaluation of wall mosaics, based on the first laboratory experimental results. FF—full field; DHSPI—Digital Holographic Speckle Pattern Interferometry; SIRT—Stimulated Infrared Thermography; HSR—Holographic Subsurface Radar. After Chaban et al. [73].
Figure 8. A proposed complementary DHSPI-SIRT and HSR approach for a complex subsurface structural evaluation of wall mosaics, based on the first laboratory experimental results. FF—full field; DHSPI—Digital Holographic Speckle Pattern Interferometry; SIRT—Stimulated Infrared Thermography; HSR—Holographic Subsurface Radar. After Chaban et al. [73].
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Figure 9. Scanning laser doppler vibrometry (SLDV) results in a custom-built mosaic model. Vibration maps of sample mosaic at 1525 Hz (a) a grayscale image representing the vibrational amplitude before restoration, the areas of detached tesserae show higher amplitude, corresponding to lighter shades of grey; (b) contour view before restoration; (c) amplitude after restoration, showing the more homogeneous distribution of amplitude values along the scanned surface. After Castellini et al. [76] (reprinted with copyright permission).
Figure 9. Scanning laser doppler vibrometry (SLDV) results in a custom-built mosaic model. Vibration maps of sample mosaic at 1525 Hz (a) a grayscale image representing the vibrational amplitude before restoration, the areas of detached tesserae show higher amplitude, corresponding to lighter shades of grey; (b) contour view before restoration; (c) amplitude after restoration, showing the more homogeneous distribution of amplitude values along the scanned surface. After Castellini et al. [76] (reprinted with copyright permission).
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Figure 10. Complementarity of different techniques to a complex (depth-resolved) non-invasive study of wall mosaics, where: GPR—Ground Penetrating Radar, DHSPI—Digital Holographic Speckle Pattern Interferometry, FOM—Fiber Optics Microscopy, MI—Multispectral Imaging, IRT—Infrared Thermography, SIRT—Stimulated Infrared Thermography, SLDV—Scanning Laser Doppler Vibrometry; HSR—Holographic Subsurface Radar. 1 non-contact spot analysis; 2 method that requires physical contact to the surface, spot/line acquisition; 3 non-contact full-field.
Figure 10. Complementarity of different techniques to a complex (depth-resolved) non-invasive study of wall mosaics, where: GPR—Ground Penetrating Radar, DHSPI—Digital Holographic Speckle Pattern Interferometry, FOM—Fiber Optics Microscopy, MI—Multispectral Imaging, IRT—Infrared Thermography, SIRT—Stimulated Infrared Thermography, SLDV—Scanning Laser Doppler Vibrometry; HSR—Holographic Subsurface Radar. 1 non-contact spot analysis; 2 method that requires physical contact to the surface, spot/line acquisition; 3 non-contact full-field.
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Table
Table 1. Applicability of non-invasive techniques on wall mosaics.
Table 1. Applicability of non-invasive techniques on wall mosaics.
MethodsCharacteristicsApplicabilityResolutionCase Studies, Lab Tests on Wall Mosaics * (References)
PortableNon-ContactFull-FieldIrregular, Inhomogeneous SurfacesGold Leaf TesseraeShallow DepthDeep Structure
GPR+−−+/−−+/−+[61,62,63,64,65,69]
IRT/SIRT++++−++/−[61,62,64,65,66,67,72,73,74,75]
DHSPI/DHSPI-SIRT+++(+)(+)(+)o[73,75]
HSR+−−(−)(−)(+)o[73,75]
ERT+−−++++/−[70]
acoustic+−−+o++[69]
SLDV++−(+)o(+)o[76]
other (MI/FOM)+++/−++−−[61,62,75]
“()”: only laboratory simulation results available; “+”: applicable and informative; “−“: not applicable and/or not informative; “+/−“: applicable with limitations; “o”: no documented case studies reported in literature. * only references dealing with applications on wall mosaics are reported in this table.
Table
Table 2. Advantages and limitations of portable NDT for precious wall mosaics.
Table 2. Advantages and limitations of portable NDT for precious wall mosaics.
MethodsAchievements and AdvantagesLimitations and Issues to Solve
GPROptimal technique for revealing deep wall structural defects, possibility of quantitative analysis (depth target estimation)Contact operation, line acquisition, low resolution in shallow subsurface, signal attenuation (by moisture, salts, highly conductive materials etc.)
IRT/SIRTNon-contact and full-field acquisition, instant visualization, easy interpretation, can be coupled to other techniques for complementary informationLow informativity if thermal gradient is not present, influence of thermal emissivity values on the mosaic surface
DHSPI/DHSPI-SIRTNon-contact full-field acquisition, instant visualization, high resolution, detection of defects within the subsurface (voids, cracks), inhomogeneities and differences in mortar structure, can be coupled to other techniques for complementary informationHigh sensitivity to surrounding vibrations, sensitivity only in the direction of the optical axis of the system; bibliographic sources on in-situ tests are still missing
HSRHigh efficiency in the detection of metallic elements and humidity; possibility to estimate the depth, reconstruct the exact horizontal dimensions and shape of the detected targetsContact operation, line acquisition, not applicable on gold leaf tesserae; results high influenced by the accuracy in the acquisition; further laboratory and in-situ tests are still missing
ERTSubsurface and deep wall structural evaluation, possibility of 2D/3D representation of resistivity values, efficiency in detectability of moisture areasNot applicable on precious wall mosaics due to invasivity, bibliographic sources on further laboratory and in-situ tests are still missing
Acoustic (sonic/ultrasonic)Subsurface and deep wall structural evaluation, possibility of 2D/3D representation of sonic/ultrasonic velocity values, efficiency in assessment of grouting Contact operation, spot acquisition, not applicable on deteriorated surfaces, access to both sides of the wall for in-depth visualization
SLDVNon-contact operation, efficient in detectability of shallow subsurface detachmentsSpot acquisition, bibliographic sources on further laboratory and in-situ tests are still missing
other (MI/FOM)Non-contact and full-field, high resolutionLow penetration (only surface analysis)

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MDPI and ACS Style

Chaban, A.; Deiana, R.; Tornari, V. Wall Mosaics: A Review of On-Site Non-Invasive Methods, Application Challenges and New Frontiers for Their Study and Preservation. J. Imaging 2020, 6, 108. https://doi.org/10.3390/jimaging6100108

AMA Style

Chaban A, Deiana R, Tornari V. Wall Mosaics: A Review of On-Site Non-Invasive Methods, Application Challenges and New Frontiers for Their Study and Preservation. Journal of Imaging. 2020; 6(10):108. https://doi.org/10.3390/jimaging6100108

Chicago/Turabian Style

Chaban, Antonina, Rita Deiana, and Vivi Tornari. 2020. "Wall Mosaics: A Review of On-Site Non-Invasive Methods, Application Challenges and New Frontiers for Their Study and Preservation" Journal of Imaging 6, no. 10: 108. https://doi.org/10.3390/jimaging6100108

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