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Article and Sharing Best Practices on the Sustainable Development of Small Underground Settlements

Laura Genovese 1,†, Roberta Varriale 2,†, Loredana Luvidi 3,*,† and Fabio Fratini 4,†

1 CNR—Institute for the Conservation and the Valorization of Cultural Heritage, 20125 Milan, Italy; [email protected] 2 CNR—Institute of Studies on Mediterranean Societies, 80134 Naples, Italy; [email protected] 3 CNR—Institute for the Conservation and the Valorization of Cultural Heritage, 00015 Monterotondo St., Italy 4 CNR—Institute for the Conservation and the Valorization of Cultural Heritage, 50019 Sesto Fiorentino, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-06-90672887 † These authors contributed equally to this work.

 Received: 28 December 2018; Accepted: 5 March 2019; Published: 8 March 2019 

Abstract: Both Southern Italy and Central China feature historic rural settlements characterized by underground with residential and service functions. Many of these areas are currently tackling economic, social and environmental problems, resulting in unemployment, disengagement, depopulation, marginalization or loss of cultural and biological diversity. Both in and in China, policies for rural development address three core areas of intervention: agricultural competitiveness, environmental protection and the promotion of rural amenities through strengthening and diversifying the economic base of rural communities. The challenge is to create innovative pathways for regeneration based on raising awareness to inspire local rural communities to develop alternative actions to reduce poverty while preserving the unique aspects of their local environment and culture. In this view, cultural heritage can be a catalyst for the sustainable growth of the rural community. Through a series of projects on a national and international scale, the authors have addressed some of these problems by exchanging best practices in conservation, sustainable use and the enhancement of the underground heritage.

Keywords: underground settlements; earthen architecture; conservation; regeneration; enhancement

1. Introduction The Italian and Chinese rural villages studied are the result of successful and sustainable balancing attempts between maximization of environmental opportunities and minimization of natural conflicts and social interactions in habitat. Both locations represent ‘troglodyte or negative building culture’ which is characterized by the systematic application of building techniques based on quarrying and removal rather than the vertical use of shaped rocks, or various mixtures. It is a worldwide modus aedificandi which was developed during prehistoric times and is still in use today: this long lasting building approach is one of the most typical building techniques in plateaus located between latitude 34◦ and 40◦ North (Figure1). In this particular climatic zone, surprisingly, underground sections of historical excavated settlements are very similar to each other. Based on the study of historical artifacts excavated below the ground level, it appears that local building know-how was reconsidered and new underground building skills were adopted. Thus, underground architecture of worldwide troglodyte settlements has much more in common than their corresponding above-ground

Heritage 2019, 2, 53; doi:10.3390/heritage2010053 www.mdpi.com/journal/heritage Heritage 2019, 3 FOR PEER REVIEW 2

natural forces and produces significant artifacts with strong identity and distinctive features responding to local culture. The underground architecture was also recently investigated within VerSus (Vernacular HeritageKnowledge2019, 2, 53 for Sustainable Architecture), a European project developed in the framework of the2 of 13 Culture 2007–2013 program, which had partners CRAterre-Ecole Nationale Supérieure d’Architecture de Grenoble (France), Escola Superior Gallaecia (Portugal), DIDA (Department of building styles. This is the case because the underground world interprets raw materials with the Architecture, University of Florence-Italy), DDICAAR (Department of , adoptionEnvironment of essential and shapesArchitecture, in response University to natural of forces Cagliari-Italy), and produces Escuela significant Técnica artifacts Superior with strongde identityArquitectura and distinctive of Polytechnic features University responding of Valencia, to local Spain culture. [1].

FigureFigure 1. 1.Map Map of of the the case case studies: studies: inin evidence,evidence, the ty typicalpical climatic climatic zone zone of of the the excavated excavated villages villages (Southern(Southern Italy Italy and and plateau in in China China areare shownshown in red).

TheSince, underground by definition, architecture the term Underground was also recently Built Heritage investigated (UBH) refers within to VerSushistorical (Vernacular artifacts Knowledgeexcavated forin underground Sustainable Architecture), areas that become a European significant project expressions developed of inlocal the cultural framework heritage, of the Culturetroglodyte 2007–2013 villages program, are perfect which case had studies partners since CRAterre-Ecolethey house the strongest Nationale concentration Supérieure d’Architectureof historical deartifacts Grenoble excavated (France), to manage Escola Superiorurban functions Gallaecia [2]. The (Portugal), Loess Plateau DIDA (Department villages of (36°52 Architecture,′59.99″ UniversityN, 108°42 of′59.99 Florence-Italy),″ E) and the excavated DDICAAR villag (Departmentes in Southern of CivilItaly (40°40 Engineering,′11.39″ N, Environment 16°35′50.03″ E) and Architecture,represent two University of the most of significant Cagliari-Italy), underground Escuela settlements Técnica Superior in the world. de Arquitectura A comparative of Polytechnic analysis Universitycan provide of Valencia, new insight Spain by [testing1]. the theories related to the connections between cultural identity andSince, climatic by zone, definition, as the as termverifying Underground the differences Built and Heritage possible (UBH) future actions refers toaimed historical towards artifacts the enhancement and valorization of Western and Eastern excavated villages (Figure 2a,b). excavated in underground areas that become significant expressions of local cultural heritage, This paper is based on some research by the authors on a national and international scale [3]. troglodyte villages are perfect case studies since they house the strongest concentration of historical More specifically, on one hand, it is a National Research Council of Italy (CNR) interdepartmental artifacts excavated to manage urban functions [2]. The Loess Plateau yaodong villages (36◦52059.9900 N, project that began in 2015 coordinated by the CNR-Institute of Studies on Mediterranean Societies 108◦42059.9900 E) and the excavated villages in Southern Italy (40◦40011.3900 N, 16◦35050.0300 E) represent (ISSM) that concerns the analysis of underground settlements in Southern Italy, aiming to define twogood of the practices most significant for their recovery, underground refunctionalizatio settlementsn in and the enhancement world. A comparative [4]. On the analysis other hand, can provideit is a newbilateral insight project by testing that began the theories in 2016, related coordinated to the by connections the CNR and between Institute cultural for the identity Conservation and climatic and zone,the asValorization well as verifying of Cultural the differences Heritage (ICVBC) and possible together future with actions the aimedChinese towards Academy the of enhancement Cultural andHeritage valorization (CACH) of Western[5] and is and related Eastern to the excavated sharing of villages good practices (Figure2 betweena,b). Italy and China for the conservationThis paper of is monuments based on some and site researchs in earthen by the material. authors Finally, on a national the paper and collects international the results scale of a [ 3]. Morecollaboration specifically, that on was one recently hand, born it is aof National the authors Research with the Council World Heritage of Italy (CNR)Institute interdepartmental of Training and projectResearch- that began and in 2015Pacific coordinated (WHITRAP) by under the CNR-Institute the auspices of StudiesUNESCO on to Mediterranean survey, analyze Societies and (ISSM) that concerns the analysis of underground settlements in Southern Italy, aiming to define good practices for their recovery, refunctionalization and enhancement [4]. On the other hand, it is a bilateral project that began in 2016, coordinated by the CNR and Institute for the Conservation and the Valorization of Cultural Heritage (ICVBC) together with the Chinese Academy of Cultural Heritage (CACH) [5] and is related to the sharing of good practices between Italy and China for the conservation of monuments and sites in earthen material. Finally, the paper collects the results of Heritage 2019, 2, 53 3 of 13

a collaborationHeritage 2019, 3 FOR that PEER was REVIEW recently born of the authors with the World Heritage Institute of Training3 and Research-Asia and Pacific (WHITRAP) under the auspices of UNESCO to survey, analyze and disseminatedisseminate the the knowledge knowledge on on small small historical historical settlements, settlements, including including the the Chinese Chinese yaodong, yaodong, and and raise awarenessraise awareness about the about importance the importance of their conservationof their conservation within their within historical their andhistorical natural and landscape. natural landscape.

(a) (b)

FigureFigure 2. (2.a )( Materaa) in Southernin Southern Italy, Italy, view view from from the easternthe eastern side side of the of gorgethe gorge and (andb) Lijiashan (b) Lijiashan village onvillage the Loess on the Plateau, Loess ChinaPlateau, (by China F.F., (by 2017). F.F., 2017).

BasedBased on on the the hypothesis hypothesis that that both both underground underground settlements settlements resulted resulted from from the use the of use the of negative the buildingnegative culture building to cope culture with to or cope solve with common or solve environmental common environmental and social problems, and social the problems, paper compares the thepaper historical compares uses the and historical the contemporary uses and the valorization contemporary processes valorization carried proces out inses Italy carried and out China in Italy while proposingand China mutual while exchangesproposing mutual between exchanges eastern and between western eastern approaches. and western approaches.

2. Southern2. Southern Italy Italy and and Loess Loess Plateau Plateau UndergroundUnderground Settlements, Closer Closer than than Believed Believed MateraMatera is is a a relevant relevant example example inin SouthernSouthern Italy of an architectural architectural and and landscape landscape ensemble ensemble resultingresulting from from a a process process of of urban urban growthgrowth inin symbiosissymbiosis with with nature nature (Figure (Figure 2a).2a). As As a amatter matter of offact, fact, thethe development development of of the the town town was was byby nono meansmeans accidental,accidental, however however rather rather it itwas was the the outcome outcome of of human ingenuity and a carefully thought out process. The old town of Matera () that human ingenuity and a carefully thought out process. The old town of Matera (Sassi di Matera) that was developed entirely on the western side of the homonymous canyon, locally referred to as was developed entirely on the western side of the homonymous canyon, locally referred to as gravina (gorge), deeply erodes the Murgia, the carbonate platform that constitutes part of the (gorge), deeply erodes the Murgia, the carbonate platform that constitutes part of the province of Matera province of Matera and a wide part of the Puglia . Its precise geographical location was and a wide part of the Puglia region. Its precise geographical location was determined in view of the determined in view of the different lithological characteristics of the two sides of the valley. The differentstratigraphic lithological sequence characteristics presents two of thegeological two sides formations: of the valley. at the The bottom, stratigraphic we find sequence the presents twolimestone geological which formations: is of Cretaceous at the bottom, age and we isfind a very the compact Altamura and limestone hard stone, which making is of Cretaceousit difficult to age andexcavate is a very and compact work with, and hardwhile stone, the top making is characterized it difficult by to excavategravina Calcarenite, and work with,a “calcareous while the tufa” top is characterizedwhich is of byUppergravina Pliocene-PleistoceneCalcarenite, a “calcareous age and is tufa” soft and which easy is ofto Upperexcavate Pliocene-Pleistocene while ensuring high age andstability is soft and[6,7]. easy On the to excavatewestern side while of ensuring the gorge, high the stabilitycalcarenite [6, 7crops]. On out the with western a greater side ofthickness the gorge, thethan calcarenite on the eastern crops outside, with which a greater is why thickness on that side than of onthe the valley, eastern side, were which being is dug why as on early that as side ofancient the valley, times, caves often were obliquely. being dug In this as earlyway, asduring ancient the times,winter often season obliquely. when the In sun’s this way,rays are during more the winteroblique, season sunlight when and the heat sun’s could rays penetrate are more oblique,from the sunlighttop to the and bottom heat of could the walls, penetrate while from in summer the top to thewhen bottom the ofsun’s the rays walls, are while almost in perpendicular summer when to the the sun’s ground, rays only are the almost entrance perpendicular of the hypogea to the would ground, onlybe theaffected, entrance leaving of the the hypogea dwelling wouldfresh and be affected,humid. leaving the dwelling fresh and humid. InIn the theSassi Sassiof of Matera, Matera, there there are are many many churches churches and and hermitages, hermitages, many many of of which which have have beautiful beautiful wall paintingswall paintings that tell that the storytell the and story religiosity and religiosity of a simple of lifea simple rich in life faith rich [8]. in Most faith of [8]. these Most churches of these date backchurches to the Highdate back Middle to the Ages High to theMiddle period Ages when to the the period region when was disputed the region between was disputed the Byzantine between and Longobardthe Byzantine dominions and Longobard and an important dominions phenomenon and an important was affirmed phenomenon in the Christian was societyaffirmed of in the the time: monasticism.Christian society Unfortunately, of the time: these paintingsmonasticism. have Unfo conservationrtunately, problemsthese paintings due to waterhave infiltrationsconservation and problems due to water infiltrations and biological growths. These conservation issues have been biological growths. These conservation issues have been addressed by Italian scholars [9], however addressed by Italian scholars [9], however these are problems that are common to other sites on the these are problems that are common to other sites on the planet, problems that have been addressed in planet, problems that have been addressed in international conferences [10,11]. The Sassi are the result of a collective architecture that has developed over the centuries, starting with the houses

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Heritageinternational 2019, 3 FOR conferences PEER REVIEW [10,11 ]. The Sassi are the result of a collective architecture that has developed4 over the centuries, starting with the cave houses and progressively evolving with the addition of andexternal progressively rooms utilizing evolving the same with excavation the addition material of external and, finally, rooms with utilizing the diffusion the same of entirely excavation built materialhouses [12 and,13, ].finally, with the diffusion of entirely built houses [12,13]. For centuries, the Sassi have been an example of anan eco-sustainableeco-sustainable urbanurban agglomeration:agglomeration: the system of water collection tunnels in in fact fact served served as as a a conveyor conveyor of of th thisis precious precious element element in in the the tanks tanks,, however al alsoso as a “collector”“collector” of rainwater during heavy rains, therefore limiting the of the slopes to a minimum. in itself reduced the consumption of to a minimum minimum;; the resource was exploited to the maximum to include hanging gardens on the roofsroofs of thethe cavecave houses.houses. In the dry season, moreover, during the night,night, the excavated cavities served the purpose of aspirators of atmospheric humidityhumidity thatthat condensescondenses inin thethe terminal terminal tank tank of of the the hypogea, hypogea, which which is is always always full, full even, even if ifit it is is not not connected connected with with external external channels. channels. The Chinese Chinese case case study study instead instead consists consists of a system of a system of small of rural small settlements rural settlements scattered throughout scattered througcentralhout China central on the China Loess Plateauon the (FigureLoess 2 Plateaub). This (areaFigure extends 2b). forThis 640,000 area extends km 2, covering for 640,000 most parts km2, coveringof the provinces most parts of of the provinces and of andShaanxi it comprises and Shanxi parts and of it , comprises parts and of Gansu, . Ningxia Theand plateauInner Mongolia. was formed The byplateau the deposition was formed of veryby the fine deposition particles of very soilblown fine particles in by windstorms of soil blown over in bythousands windstorms of years. over Asthousands a result, of the years. ground As herea result, is very the fineground and loamyhere is andvery is fine highly and fertile, loamy easyand tois highlyfarm and fertile, dig [ 14easy,15 ].to Despite farm and the dig high [14,15 erodibility,]. Despite the Plateau’sthe high erodibility, have the the Plateau’ characteristics terrains of beinghave pseudothe characteristic coherent, i.e.,of being they pseudo can be excavated, coherent, i.e. and, they the cavities can be excavated are more or, and less the self-sustaining. cavities are more Thanks or lessto this self property,-sustaining. the Loess Thanks Plateau to this has property, been historically the Loess inhabited, Plateau forming has been one ofhistorically the earliest inhabited cradles of, formingChinese civilization.one of the earliest cradles of Chinese civilization. The yaodong ( 窰洞)) areare aa typetype ofof undergroundunderground spacespace withwith aa variedvaried function,function, constituting the minimal unit in which innumerable rural villages are organized in the Plateau’sPlateau’s area. Without this specificspecific type of ground ground,, there would be no : thanks thanks to to this this gro groundund quality, quality, yaodongs can be carved out of the wrinkled landscape and be insulated from cold winters andand hot summers. The most common type of yaodong is dug on the faces and slopes of the plateau and present presentss terraces for crops. They They are are located in in on the edges of the Loess Plateau, while on the inner areas where where there there are are no no hillsides hillsides and and ravines, ravines, peasants peasants du dugg a square a square pit pitin the in ground the ground and andthen thendug thedug dwellings the dwellings horizontally horizontally on the on the four four walls walls to to form form a acentral, central, sunken sunken courtyard courtyard or or pit pit dwelling (siheyuansiheyuan). The e entrancentrance to these pit houses is usually through a ramp or an underground corridor.corridor. Typically, aa yaodongyaodong has has a a long long vaulted vaulted room room with with a semicircular a semicircular entrance entrance closed closed by walls, by walls, built withbuilt with fired fired bricks, bricks, earth earth bricks, bricks, stones stones and and wood, wood, depending depending on theon the specific specific site site conditions, conditions and, and is iscovered covered with with a woodena wooden door door or or a a quilt. quilt. This This arch-shapedarch-shaped structurestructure alsoalso allowsallows thethe sun to further penetrate inside the cave in winter, thereforetherefore makingmaking fullfull useuse ofof solarsolar radiation.radiation. A third third type type of of dwelling dwelling is the is the hoophoop yaodongyaodong (gūyáo (guy)¯ oráo independent) or independent yaodong yaodong.. This kind This of kindcave houseof cave is house generally is generally built in built those in areas those where areas where bedrockthat is exposed that is exposed on top onof the top hill of the makes hill excavationmakes excavation somewhat somewhat difficult difficult and motivatesand motivates the the construction of of houses houses wholly wholly above above ground ground (Figure(Figure3 3).). Otherwise,Otherwise, ifif thesethese holdhold onon thethe slopeslope ofof thethe hill,hill, theythey areare onlyonly partiallypartially excavatedexcavated andand areare inspired by the typical underground dwellings’dwellings’ shape.shape. Usually, the roof is made of stone flakes covered with earth to ensure thermal insulation of the interiors. Alternatively, the underground yaodong houses have a thick layer of earth on top (about 3 to 5 m deep) which acts as an effective insulation coverage and humidity modulator (Figure4). Above the yaodongs, there are often little chimneys and a constructed in the earth. These two features, together with a small ventilation window that is kept open in the façade, represent the breathing system of a yaodong. They work on the properties of hot air rising to achieve natural air circulation inside and to drive out humidity. The inner space is usually utilized in more than one way: the main living room, kitchen and sometimes bedroom (Figure5a). The brazier for cooking also serves as a heating element for the environment and for the bed (kang) attached to it. The toilet is a stand-alone unit located outside of the cave, making it rather inconvenient on cold or rainy days.

Figure 3. Laoniuwan village in Shanxi Province (China). A general view of hoops yaodongs, (by F.F. and L.L., 2017).

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and progressively evolving with the addition of external rooms utilizing the same excavation material and, finally, with the diffusion of entirely built houses [12,13]. For centuries, the Sassi have been an example of an eco-sustainable urban agglomeration: the system of water collection tunnels in fact served as a conveyor of this precious element in the tanks, however also as a “collector” of rainwater during heavy rains, therefore limiting the erosion of the slopes to a minimum. Digging in itself reduced the consumption of soil to a minimum; the resource was exploited to the maximum to include hanging gardens on the roofs of the cave houses. In the dry season, moreover, during the night, the excavated cavities served the purpose of aspirators of atmospheric humidity that condenses in the terminal tank of the hypogea, which is always full, even if it is not connected with external channels. The Chinese case study instead consists of a system of small rural settlements scattered throughout central China on the Loess Plateau (Figure 2b). This area extends for 640,000 km2, covering most parts of the provinces of Shaanxi and Shanxi and it comprises parts of Gansu, Ningxia and Inner Mongolia. The plateau was formed by the deposition of very fine particles of soil blown in by windstorms over thousands of years. As a result, the ground here is very fine and loamy and is highly fertile, easy to farm and dig [14,15]. Despite the high erodibility, the Plateau’s terrains have the characteristic of being pseudo coherent, i.e., they can be excavated, and the cavities are more or less self-sustaining. Thanks to this property, the Loess Plateau has been historically inhabited, forming one of the earliest cradles of Chinese civilization. The yaodong (窰洞) are a type of underground space with a varied function, constituting the minimal unit in which innumerable rural villages are organized in the Plateau’s area. Without this specific type of ground, there would be no yaodongs: thanks to this ground quality, yaodongs can be carved out of the wrinkled landscape and be insulated from cold winters and hot summers. The most common type of yaodong is dug on the faces and slopes of the plateau and presents terraces for crops. They are located in regions on the edges of the Loess Plateau, while on the inner areas where there are no hillsides and ravines, peasants dug a square pit in the ground and then dug the dwellings horizontally on the four walls to form a central, sunken courtyard or pit dwelling (siheyuan). The entrance to these pit houses is usually through a ramp or an underground corridor. Typically, a yaodong has a long vaulted room with a semicircular entrance closed by walls, built with fired bricks, earth bricks, stones and wood, depending on the specific site conditions, and is covered with a wooden door or a quilt. This arch-shaped structure also allows the sun to further penetrate inside the cave in winter, therefore making full use of solar radiation. A third type of dwelling is the hoop yaodong (gūyáo) or independent yaodong. This kind of cave house is generally built in those areas where bedrock that is exposed on top of the hill makes excavation somewhat difficult and motivates the construction of houses wholly above ground Heritage(Figure2019 3)., 2, Otherwise, 53 if these hold on the slope of the hill, they are only partially excavated and are5 of 13 inspired by the typical underground dwellings’ shape.

Heritage 2019, 3 FOR PEER REVIEW 5 Heritage 2019, 3 FOR PEER REVIEW 5 Usually, the roof is made of stone flakes covered with earth to ensure thermal insulation of the interiors.Usually, Alternatively, the roof is the made underground of stone flakes yaodong covered ho useswith have earth a to thick ensure layer thermal of earth insulation on top (about of the 3 tointeriors. 5 m deep) Alternatively, which acts the as underground an effective yaodonginsulation ho coverageuses have and a thick humidity layer of modulator earth on top (Figure (about 4).3 to 5 m deep) which acts as an effective insulation coverage and humidity modulator (Figure 4). Above the yaodongs, there are often little chimneys and a tunnel constructed in the earth. These two Above the yaodongs, there are often little chimneys and a tunnel constructed in the earth. These two features, together with a small ventilation window that is kept open in the façade, represent the features, together with a small ventilation window that is kept open in the façade, represent the breathing system of a yaodong. They work on the properties of hot air rising to achieve natural air breathing system of a yaodong. They work on the properties of hot air rising to achieve natural air circulation inside and to drive out humidity. The inner space is usually utilized in more than one circulation inside and to drive out humidity. The inner space is usually utilized in more than one way: the main living room, kitchen and sometimes bedroom (Figure 5a). The brazier for cooking also way: the main living room, kitchen and sometimes bedroom (Figure 5a). The brazier for cooking also serves as a heating element for the environment and for the adobe brick bed (kang) attached to it. The serves as a heating element for the environment and for the adobe brick bed (kang) attached to it. The toilet isFigure a stand-alone 3. Laoniuwan unit village located in outsideShanxi Province of the cave,(China). making A general it rather view of inconvenient hoops yaodongs, on (by cold F.F. or rainy toiletFigure is a stand-alone 3. Laoniuwan unit village located in Shanxi outside Province of the cave, (China). making A general it rather view inconvenient of hoops yaodongs, on cold (by or F.F. rainy days. and L.L., 2017). days.and L.L., 2017).

FigureFigure 4. 4.4. Underground UndergroundUnderground village villagevillage around aroundaround the thethe Wang Wang Family Family Compound CompoundCompound inShanxi inin ShanxiShanxi Province ProvinceProvince (China). (China).(China). New caveNewNew dwellingscave cave dwellings dwellings have been havehave built beenbeen in builtbuilt yaodong in yaodong style annexed style annexed to the traditional toto thethe traditionaltraditional ones that ones ones have that that been have have partially been been abandonedpartiallypartially abandoned abandoned or adapted oror to adaptedadapted the needs toto the of modernneeds of lifemodern (by F.F. life and (by(byL.L., F.F.F.F. and 2017).and L.L.,L.L., 2017). 2017).

((aa)) ((bb))

FigureFigure 5. 5. ( a(a)) Qikou Qikou towntown in in Shanxi Shanxi Province Province (China), the interiorinterior ofof aa traditionaltraditional yaodong yaodong adapted adapted to to modernmodern lifelifelife (by (by(by F.F. F.F. andF.F. L.L.,andand 2017);L.L.,L.L., (b2017);) Matera (b) in Matera Southern inin Italy, SouthernSouthern Re-interpretation Italy,Italy, Re-interpretationRe-interpretation of accommodation ofof inaccommodationaccommodation excavated rooms in in excavatedexcavated in a luxury roomsrooms hotel inin (by aa R.V.,luxury 2018). hotel (by R.V.,R.V., 2018).2018).

OftenOften multiple multiplemultiple dwellings dwellingsdwellings are areare built builtbuilt adjacent adjacent to orto on oror top ononof toptop one ofof another oneone anotheranother and are andand connected areare connectedconnected together totogethertogether form a to multi-tieredto form form aa multi-tieredmulti-tiered village, often village,village, for aoften single for clan a single or an clanclan extended oror anan family.extendedextended Nevertheless, family.family. Nevertheless, Nevertheless, dwellings dwellingsdwellings are are hidden hidden inin naturalnatural alcovesalcoves that are not easily visible. Thus,Thus, itit isis said said that that “no “no house house was was seenseen inin thethe wholewhole villagevillage andand nono peoplepeople were seen, but onlyonly trees”.trees”. InIn fact,fact, thethe architecturearchitecture of of a a yaodongyaodong village village isis notnot aboutabout addingadding to, however is rather aboutabout subtractingsubtracting fromfrom the the earth. earth. Indeed, Indeed, allall the the materials materials required required forfor buildingbuilding a yaodong can be found inin thethe locallocal area.area.

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The construction of a traditional yaodong merely produces natural waste for the environment. Everything is natural and produced locally, making these houses one of the most eco-friendly in HeritageChina, 2019and, 2 ,they 53 constitute the very expression of local geo-climatic conditions and traditional6 of 13 customs. There are greatly significant ways in which yaodong villages can integrate with nature with minimal impact. are hiddenThe daily in natural activities alcoves at thata yaodong are not easilyinclude visible. everyday Thus, itmaintenance is said that “nosuch house as washardening seen in the wholeyaodong’s village earth and after no peoplerainfall, were ensuri seen,ng butproper only air trees”. circulation In fact, in the summe architecturer, firing of a akang yaodong in winter village to keepis not the about earth adding dry and to, stable, however scraping is rather off about from subtracting500 mm to 1000 from mm the of earth. eroded Indeed, soil every all the 10 materials or more yearsrequired and for so buildingon. The longer a yaodong a yaodong can be is found inhabited, in the the local harder area. and more stable the earth becomes. AlthoughThe construction during ofancient a traditional times, many yaodong of these merely villages produces prospered natural thanks waste to for the the fertility environment. of the land,Everything centuries isnatural of and produced and over-grazing, locally, making exacerbated these houses by China’s one of population the most eco-friendly increase, have in resultedChina, and in degenerated they constitute ecosystems, the very expression of and local poor geo-climatic local economies conditions with the and consequence traditional ofcustoms. the abandonment There are of greatly the village. significant Just as ways in Southe in whichrn Italy yaodong (Figure villages 6a), around can integratethe middle with of the nature last withcentury, minimal these impact.vernacular buildings were often abandoned because they were considered unhealthy and, Theabove daily all, activitiesregarded atas a hopelessly yaodong include outdated everyday compared maintenance to the living such standards as hardening of a “modern” the yaodong’s city dweller.earth after rainfall, ensuring proper air circulation in summer, firing a kang in winter to keep the earth dry andIn recent stable, years, scraping many off of from the residents 500 mm to have 1000 move mm ofd to eroded big cities soil in every search 10 orof morework yearsand nowadays, and so on. Theonly longera few avillagers yaodong remain is inhabited, in the the Plateau’s harder andsettl moreements. stable It has the earthbeen estimated becomes. that 90 thousand villagesAlthough disappear during annually. ancient times, many of these villages prospered thanks to the fertility of the land, centuriesSome of rehabilitation deforestation andprojects over-grazing, that have exacerbated been carried by China’s out over population the years increase, were have ineffective. resulted Recently,in degenerated there ecosystems,have been a desertification few cases where and poorsurvival local was economies possible with by attracting the consequence tourism of and the convertingabandonment some of theof the village. traditional Just as inhouses Southern into Italyaffordable (Figure 6accommodationa), around the middle for tourists of the last(Figure century, 6b). Nevertheless,these vernacular many buildings problems were still often restrict abandoned the su becausestainable they development were considered of this unhealthy resource and,and abovecould possiblyall, regarded compromise as hopelessly the material outdated and compared immaterial to theheritage living of standards the site. of a “modern” city dweller.

(a) (b)

Figure 6. ((aa)) Ginosa abandoned abandoned settlement settlement in in Southern Southern Italy Italy Plateau Plateau (by (by R.V., R.V., 2018); (b) Lijiashan Village in Shanxi Province (China):(China): renovation workwork inin traditionaltraditional houseshouses (by(by F.F.F.F. and and L.L., L.L., 2017). 2017).

ConsideringIn recent years, that many earth of architecture the residents is have particular movedly tosubject big cities to decay in search processes, of work both and systems nowadays, of undergroundonly a few villagers settlements remain must in the be Plateau’s monitored settlements. to ensure Ittheir has beenconservation estimated over that time. 90 thousand For this villagesreason, theydisappear require annually. constant maintenance not only from a safety point of view, however also to guarantee their Someactual rehabilitation functionality, projects a prerequisite that have to ensure been carried that this out architecture over the years continues were ineffective. to be inhabited Recently, and built.there have been a few cases where survival was possible by attracting tourism and converting some of theIn traditional particular, houses the factors into affordable of earth structure accommodation decay are: for touristsinternal factors (Figure due6b). to Nevertheless, structural defects many problemsand external still factors restrict such the sustainableas the action development of water and ofthis the resourcetype of anduse. couldEarthpossibly material compromise is particularly the sensitivematerial andto the immaterial action of water heritage because of the water site. tends to make earth plastic again with consequent loss Considering that earth architecture is particularly subject to decay processes, both systems of underground settlements must be monitored to ensure their conservation over time. For this reason, Heritage 2019, 2, 53 7 of 13 they require constant maintenance not only from a safety point of view, however also to guarantee their actual functionality, a prerequisite to ensure that this architecture continues to be inhabited and built. In particular, the factors of earth structure decay are: internal factors due to structural defects andHeritageexternal 2019, 3 FOR factors PEERsuch REVIEW as the action of water and the type of use. Earth material is particularly7 sensitive to the action of water because water tends to make earth plastic again with consequent loss of the mechanical characteristics of resistance and rigidity. Water acts both as an erosive agent (rain) and through capillary action, therefore damp rises with the consequent disaggregation phenomena. The decay phenomena of the earth architecture includesincludes superficialsuperficial decay of physical and biological nature, loss of material, strong decay of the materialmaterial through deep cracking or partial fall of the masonries (Figure7 7).). InIn orderorder toto developdevelop adequateadequate measuresmeasures ofof interventionintervention thatthat areare non-damagingnon-damaging and preventive in the event of new decay phenomena, it is important to identify the actual causes of the processesprocesses andand notnot merely merely intervene intervene on on a singlea single decay decay aspect aspect [16 [16–20].–20]. Since Since the the factors factors of decay of decay are essentiallyare essentially due due to water to water action action and structuraland structural problems, problems, a preventive a preventive conservation conservation will have will tohave act into theseact in twothese directions. two directions. With With regard regard to water, to water, all the all solutions the solutions designed designed to protect to protect buildings buildings from from the rainthe rain should should avoid avoid capillary capillary action action and infiltrations. and infiltrations. With regard With toregard structural to structural problems, problems, it is necessary it is tonecessary implement to implement strategies strategies that limit thethat consequenceslimit the consequences of poor connections of poor connections within the within walls, the roof walls, and foundations.roof and foundations. In the case In of the the case Loess of undergroundthe Loess underground dwellings, dwellings, the main problems the main are problems due to the are stability due to ofthe the stability slopes of where the slopes the houses where are the dug houses and are tothe dug growth and to ofthe roots growth and of vegetation. roots and Also,vegetation. in this Also, case, preventivein this case, conservation preventive conservation must ensure themust regulation ensure the ofwater, regulation avoiding of water, runoff avoiding on the cliffs, runoff erosion on the at thecliffs, base erosion of the at cliffs the base and infiltrationof the cliffs above and the cliffs. above the cliffs.

Figure 7. Lijiashan Village in Shanxi Province (China). Mud bricks bricks made made of of the the earth earth of of Loess Loess (by (by F.F. F.F. and L.L., 2017).

3. The Theoretical Approach: From Historic Functions to RE-USE for Italian and Chinese Underground Settlements Underground Settlements As reviewed in the previous paragraph, the Southern Italy underground settlements and Loess As reviewed in the previous paragraph, the Southern Italy underground settlements and Loess Plateau yaodong system seem to have much in common with reference to their morphological aspects. Plateau yaodong system seem to have much in common with reference to their morphological The application of the newborn functional classification (introduced by one of the authors for the first aspects. The application of the newborn functional classification (introduced by one of the authors time at the 2018 International Conference on Urban History (EAUH) conference and not published for the first time at the 2018 International Conference on Urban History (EAUH) conference and not yet [21]) to both systems strengthens this hypothesis (Figure8). Both in Southern Italy and in Loess published yet [21]) to both systems strengthens this hypothesis (Figure 8). Both in Southern Italy and Plateau, in fact, underground space has historically faced environmental conflicts and has managed in Loess Plateau, in fact, underground space has historically faced environmental conflicts and has several social interactions, giving shape to excavated structures and artifacts that have become among managed several social interactions, giving shape to excavated structures and artifacts that have the most significant expressions of local cultural heritage. become among the most significant expressions of local cultural heritage.

Heritage 2019, 2, 53 8 of 13 Heritage 2019, 3 FOR PEER REVIEW 8

(a) (b)

FigureFigure 8.8. UndergroundUnderground Built Built Heritage Heritage (UBH) (UBH) charts charts in in Italian Italian ( a(a)) and and Chinese Chinese ( b(b)) excavated excavated villagesvillages (by(by R.V.).R.V.).

InIn bothboth systems, systems, the the most most significant significant elements elements classified classified under under UBH UBH are represented are represented by the classesby the “waterclasses management” “water management” and “living and space” “living with space” reference with reference to the management to the management of environmental of environmental conflicts, andconflicts, by “religion”, and by “knowledge”,“religion”, “knowledge”, “defense”, “communication” “defense”, “communication” and “economy” and with “economy” reference towith the managementreference to the of comparable management social of conflicts.comparable Spaces social excavated conflicts. to Spaces manage excavated “food” have to manage been excavated “food” ashave an answerbeen excavated to environmental as an answer and to social environm problemsental in and both social countries. problems in both countries. TheThe undergroundunderground waswas historicallyhistorically usedused toto collectcollect andand managemanage water,water, givinggiving shapeshape toto significantsignificant elementselements ofof locallocal UBH:UBH: inin ChinaChina channelschannels andand poolspools havehave alwaysalways beenbeen excavatedexcavated toto optimizeoptimize thethe distribution,distribution, mostly during the dry seasons, while in Italy, aa complexcomplex systemsystem ofof publicpublic (cisterns,(cisterns, publicpublic fountains)fountains) andand privateprivate waterwater facilitiesfacilities (pools)(pools) completedcompleted thethe urbanurban scenario.scenario. TheThe KarstKarst habitathabitat andand thethe locationlocation alongalong communicationcommunication pathwayspathways motivatedmotivated negativenegative edificationedification ofof BuddhaBuddha cavecave settlementssettlements andand rupestrianrupestrian monasteriesmonasteries inin ChinaChina andand aa complexcomplex ofof overover 150150 excavatedexcavated churcheschurches inin SouthernSouthern Italy.Italy. InIn additionaddition toto beingbeing placesplaces ofof prayer,prayer, theythey alsoalso servedserved asas commercialcommercial gateways,gateways, socialsocial meetingmeeting pointspoints andand asas educationaleducational centers;centers; thosethose placesplaces stronglystrongly influencedinfluenced thethe spreadingspreading andand thethe consolidationconsolidation ofof common common languages languages for for cultural cultural exchange. exchange. The undergroundThe underground was excavatedwas excavated to create to escapecreate escape tunnels tunnels in China in and China detention and detention prisons prisons in Italy. in In Italy. both cases,In both they cases, have they been have strategic been facilitiesstrategic forfacilities fortified for villagesfortified orvillages castles. or Wallcastles. paintings Wall paintings in underground in underground caves were caves used were to used represent to represent social, politicalsocial, political and religious and religious values values in both in cases both andcases such and cavessuch caves have alwayshave always supported supported local, local, rural rural and urbanand urban economies. economies. Historical Historical tuff caves,tuff caves, stables, stables, farming farming facilities, facilities, warehouses warehouses for localfor local products, products, oil andoil and vine vine transformation transformation factories factories in in Italy Italy and and historical historical steel, steel, iron iron and and coal coal mines mines in in China,China, reflectreflect thethe stagesstages ofof thethe relativerelative economieseconomies throughoutthroughout history.history. Last,Last, howeverhowever notnot least,least, bothboth inin ChinaChina andand Italy,Italy, thethe undergroundunderground hashas beenbeen thethe placeplace forfor reconversionreconversion ofof urbanurban waste in agriculture and today, theirtheir geologicalgeological layers layers are are open open books books on historic on historic stratifications, stratification allowings, allowing the monitoring the monitoring of geological of riskgeological and soil risk degradation. and soil Yaodongdegradation. in China Yaodong and underground in China and settlements underground in Italy settlements represent thein mostItaly significantrepresent the use most of the significant underground use of as the a livingundergroun space.d In as both a living systems, space. public In both and systems, private public functions and wereprivate historically functions managed were historically in excavated managed spaces, givingin excavated shape to significantspaces, giving elements shape of UBH.to significant elementsAlthough of UBH. the application of the UBH classification to both historical systems confirms the hypothesisAlthough that the artificial application excavated of the structures UBH classification can be considered to both historical as direct systems expressions confirms of thethe underground,hypothesis that the artificial analysis ofexcavated the processes structures put in placecan be in Italyconsidered and China as fordirect the valorizationexpressions ofof such the areasunderground, reveals substantial the analysis differences of the processes between theput twoin place countries. in Italy The and application China for of the the valorization newborn scale of ofsuch RE-USE areas actionsreveals forsubstantial UBH to thedifferences Italian and between Chinese the underground two countries. settlements The application is the instrument of the newborn used toscale compare of RE-USE the different actions approaches for UBH of to their the management,Italian and sustainableChinese underground conservation, settlements regeneration is andthe touristicinstrument use. used Thenewly to compare introduced the evaluation different scale approaches includes fourof differenttheir management, levels of actions sustainable carried outconservation, to address regeneration the valorization and of tour UBH.istic The use. first The three newly refer introduced to innovative evaluation approaches scale toincludes the various four usesdifferent of historical levels of spaces; actions the carried fourth out level to refersaddress to thethe edificationvalorization of of new UBH. excavated The first buildings three refer using to historicalinnovative skills: approaches Re-inventing, to the Re-introduction, various uses of Re-interpretation historical spaces; and Re-building.the fourth level refers to the edification of new excavated buildings using historical skills: Re-inventing, Re-introduction, Re-interpretation and Re-building.

Heritage 2019, 2, 53 9 of 13

Re-inventing is the level dedicated to rare and unique sites. This type of UBH requires monitoring, preservation and control processes, which are the preliminary goals, followed by Re-use actions. When Re-inventing is applicable, emphasis is also placed on the communication of historical functions, even with the use of technological instruments to promote underground culture and allow virtual reconstructions of underground life. In this case, Re-use is virtual and happens to be a significant and qualifying element of the fruition of historical sites. Re-introduction is the level dedicated to very widespread and common historical artifacts, however is representative of local social and economic history. In these cases, restored spaces can re-host the same functions as in the past with respect to both the remains of UBH and the introduction of new standards such as those related to contemporary hygienic and security parameters. Re-interpretation of UBH is not a conservative approach with reference to the historical functions, however it refers to the location of new ones. In these cases, the sites are restored and new functions are located while the communication of historical uses is preserved, also with the support of contemporary design which often includes historical equipment being used in interior planning (Figure5b). Sometimes old pictures are utilized to recall the historical uses and occasionally the organization of the space includes new uses for dismissed facilities. This approach turns out to be a very stimulating challenge for and interior designers. The Re-building approach includes both the replication of the historical sites, in cases of extreme danger or vulnerability of the original ones, and the use of historical negative building methods—even implemented with the adoption of new technologies and materials—within environmentally friendly . The use of underground space to position the excavated buildings has become a popular worldwide trend in the last few decades as an effect of the stimulating input given in the 60s by Malcolm [22–24] who inverted guidelines of architecture for the very first time in contemporary ages. Because of his work, use and re-use of underground space was recognized as a solution to create sustainable urban development and has often been elevated in many countries to a strategic level for long-term economic and social development. The relationship between UBH and contemporary planning was a critical point for a long time. When Wout Broere [25], a worldwide famous specializing in underground infrastructure planning, consolidated the idea of the preservation of existing buildings and cultural heritage as a fundamental variable in underground planning, the general attitude changed; the coexistence between urban development and underground cultural heritage valorization became a priority. The application of the RE-USE classification in the Italian and Chinese underground settlements reveals substantial differences in the approach to their management, sustainable conservation, regeneration and touristic use. This is due to two main factors. First, while the villages of Southern Italy have been completely evacuated as an effect of the Italian Laws n.619 in 1952 and n.126 in 1967, yaodong are characterized by the coexistence of villages with continuity in use, voluntary abandonment and non-systemic evacuation. In Southern Italy, the demographic rise and the socio-economic decline of the underground built area turned those dwellings into the socio-environmental disaster described by Carlo Levi in his work “Cristo si è fermato a Eboli” (Christ stopped in Eboli) in 1946. Following the success of this book and much political pressure, Matera became the tangible, at times instrumental, symbol of the gap between Northern and Southern Italy, between urban and rural sanitary lifestyles. The fact that the city center was excavated in the underground was in itself considered to be incompatible with politically acceptable residential facilities in such a way that ground level was assumed to be the demarcation line between a politically correct approach to urban development and an unacceptable one. The Risanamento dei Sassi in Matera was part of a complex national program which included rational organization of rural areas, rural village sanitation and public investments in Southern Italy in order to support its economy and population [26]. The comparison of the two systems showed that unlike Matera, three different scenarios are possible in China: (a) currently populated villages, some of which present facilities and materials that have altered the natural equilibrium of excavated settlements; (b) abandoned and degraded yaodong and Heritage 2019, 2, 53 10 of 13 settlements already destroyed and (c) settlements destined to destruction to pave the way for the construction of new above ground urban settlements. The second factor is linked to the different levels of social acceptance of underground style living in Italy and China and to the involvement of local populations in the valorization of underground settlements as cultural sites. As an effect of the abovementioned factors, in Italy, underground sites are still perceived as poor locations, an expression of social exclusion, hygienic poverty and cultural degradation. An aboveground lifestyle in itself is considered a social upgrade, even when it requires the no-filter application of unsustainable architecture. Concrete buildings with glass and full sunlight exposure are considered the socially acceptable living standard and this perception stimulated the edification of skyscrapers in the suburbs, both in the public and in the private building sectors. Underground villages are only considered as an economic resource in the cultural and tourist sector. This attitude was also strongly influenced by the Sassi of Matera [27] which entered the UNESCO list in 1993 and are considered to be one of the most significant open air museums in the world. The recognition of Matera as the European Capital of Culture 2019 is the most evident example of how the focus of protection and enhancement measures is placed solely on the cultural value of negative building experience rather than on the potential within sustainable urban development. If one considers that even the house restored by Pietro Laureano for civil use in 2001 [28] and regarded as a symbol of contemporary sustainable use of underground Sassi as a living space was later re-transformed in a Boutique Hotel [29], the problem of gentrification seems to be a one-way process (Figure5b). In China, on the contrary, the widespread social housing policy and the architectural project competition has motivated academic research towards the evaluation of climatic conditions of underground settlements. Consequently, new-built yaodong villages are now believed to be at the forefront of sustainable rural development [30,31] and local communities actively participate in such projects. By way of contrast, there is little or no consideration of future touristic and cultural development of yaodong [32]. As a result, in 2014, the prestigious “Best Design Trophy”, held in Taiwan, was awarded to a project that conveys the tradition of Chinese negative building, viewed as the optimal solution to the very rapid and unsustainable urbanization of recent years, while in Southern Italy, almost all actions have the purpose of enhancing cultural heritage and thus, are directed towards the valorization of UBH as new facilities for tourists and local communities. The “Golden Pin Design Award” in Cina was the result of a long debate on sustainable construction that identified the historical typology of as the most suitable solution to the management of climate conflicts in the desert and depopulated regions of the Loess Plateau. For at least 20 years, the Chinese academic world has had a keen interest in underground settlements, mainly with the aim of recovering its value in terms of environmental sustainability, trying to replicate them in a contemporary way, as demonstrated by the new generation of yaodong. However, the role that these rural villages had in the past is still underestimated: these rural settlements are unique cultural landscapes full of character at the core of local identity and are examples of living heritage [33,34]. That said, very recently, focus has shifted, even in China, to enhancement as a “cultural asset” and the conservation and the valorization of yaodong villages. The Chinese approach to this specific element of UBH reflects the general approach resulting from the analysis of cultural heritage policies in China in the period of 11th and 12th Five Year Plans [35]. Indeed, among the actions planned and currently being implemented, the valorization of yaodong villages is mainly seen as a function of the economic development of the sites and aims at supporting and increasing the “lost” national cultural identity. As an effect of this approach, the application of cultural policies to yaodong villages, rather than being addressed towards the conservation of the correspondent UBH, focuses on their reconstruction (after the carried out during the Mao period) as instruments for the promotion of Chinese cultural identity. In this context, the envisioned future for underground Heritage 2019, 2, 53 11 of 13 settlements of the Loess Plateau can be a strategic backdrop to reinforce the resilience to loss of the cultural identity of the Chinese population which is not connected to troglodyte life, and is only marginally concerned for the intrinsic values of the corresponding elements of UBH. Notwithstanding the above considerations, this approach seems nonsensical considering that underground locations are typical of Chinese cultural heritage and not Italian heritage. This difference is strikingly apparent when considering the relative weight of troglodyte character present in the sites listed by UNESCO: 54 sites in Italy (of which 49 are cultural and five are natural) and 52 in China (of which 36 are cultural, 13 are natural and four are mixed). In fact, while in Italy, only nine sites (seven of which are cultural) include the word “rock” in the description, only one (natural) site description includes the word “underground” and four sites’ (three cultural) descriptions refer to “cave”, in China, this aspect is so remarkable that the word “rock” is present in the description of 18 sites (seven cultural, four mixed), “underground” in three (two cultural, one mixed) and “cave” in 13 (eight cultural, one mixed, five natural) descriptions. The comparative approach shows substantial differences in the processes adopted for the valorization of two very similar systems, stimulating new research questions. With such a remarkably rich cultural heritage and strong cultural identity, why is China lagging behind in terms of the valorization process? And why is sustainable underground not commonly practiced in Southern Italy, which is characterized by a wide range of temperatures, even though sustainability measures are in place for alternative energy sources, as the intensive installing of wind turbines in the area clearly demonstrates? Is it possible to define mutual adjustments as a result of the present paper?

4. From the Theoretical Approach to the Future for UBH On the basis of the considerations and results outlined above, our multidisciplinary research group carried out comparative research between Italy and China, dealing with the evaluation of best practices in the field of conservation, sustainable use and enhancement of underground rural villages with the aim of answering the above questions while stimulating mutual exchanges between China and Italy. On one hand, the research focuses on the historical, typological and functional aspects of underground spaces within the urban context. On the other, it evaluates the social structure and the hierarchies in the local rural community, with the aim of raising awareness of the common cultural heritage and evoking participation in strategic planning for sustainable growth, based on the enhancement of cultural assets. In China, this approach may be suitable for some rural villages of the Loess Plateau in Shanxy Region where historical UBH has not been compromised. The comparison between the two systems also suggests that the valorization of Italian underground settlements can be improved with the re-introduction of functional “living space” in dismissed settlements and with the adoption of underground lifestyle concepts reaching a new frontier for sustainable house design. Perhaps not in Matera, where the current situation appears to be solidified, however in the abandoned villages of Gravina, Ginosa, Laterza, Palagiano, etc., actions in this direction to support local urban and social development could be successfully planned and implemented.

5. Discussion and Conclusions Cave settlements in Italy and China—born as a common answer to the same natural and social obstacles, albeit in different historical, cultural and political contexts—are now at the core of projects that reflect substantial differences in approaches to cultural heritage and sustainable urban and rural development. While in Italy, all actions are geared towards Re-inventing, Re-introduction and Re-interpretation, in China, those three approaches are very weak and Re-building of new excavated settlements seems to be the nouvelle frontier of urban sustainability in karst habitat. This still depends Heritage 2019, 2, 53 12 of 13 on the underestimation of the role that these rural villages have had in the past and could have in the present as unique cultural landscapes that are full of character at the core of local identity and are exemplary of living heritage. The analysis of the case studies and the local cultural policies in place in both countries shows that a mutual exchange between the eastern and the western approach would be useful. The western approach, mainly aimed at protecting the historical heritage during the process of fruition, valorization and reinterpretation, could benefit from the introduction of the historical living function and experimenting the new troglodyte trends in abandoned villages and selected areas. The eastern approach, on the other hand, could benefit from focusing much more on the valorization of UBH as a significant element of material and immaterial local cultural heritage.

Author Contributions: Conceptualization, L.G., R.V., L.L., F.F.; methodology, R.V., F.F.; formal analysis, L.G.; writing—original draft preparation, L.G., R.V., L.L., F.F.; writing—review and editing, L.G., R.V., L.L., F.F.; project administration, L.G., R.V., L.L.; funding acquisition, L.G., R.V., L.L. Funding: This research was funded by the National Research Council of Italy (CNR) within the context of a Bilateral Agreement of Scientific and Technological Cooperation between the National Research Council of Italy (CNR) and the Chinese Academy of Cultural Heritage (CACH). Acknowledgments: We thank Patricia Sclafani (CNR-ISSM) for her assistance with the language editing that greatly improved the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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