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A multidisciplinary approach for the study and the virtual reconstruction of the ancient polychromy of Roman sarcophagi

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In this paper, we report a multidisciplinary approach for the analytic study and the reconstruction of the ancient colour used for Roman sarcophagi. For this purpose, we adopted the three-dimensional (3D) digital technology and found it to be a
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   JournalofCulturalHeritage16(2015)307–314  Available   online   at ScienceDirect  www.sciencedirect.com Original   article A   multidisciplinary   approach   forthe   study   and   the   virtualreconstruction   of    the   ancient   polychromy   of    Roman   sarcophagi Eliana   Siotto a , ∗ ,Matteo   Dellepiane a ,Marco   Callieri a ,Roberto   Scopigno a ,Corrado   Gratziu b ,Alessandra   Moscato b ,LuciaBurgio c ,Stefano   Legnaioli d ,GiuliaLorenzetti d ,Vincenzo   Palleschi d a VisualComputingLab,   ISTI-CNR,ViaG.Moruzzi1,   56124Pisa,Italy b DepartmentofEarthScience,Universityof    Pisa,ViaS.Maria18,56100Pisa,Italy c ConservationDepartment,VictoriaandAlbertMuseum,SouthKensingtonSW7     2RL,   London,UnitedKingdom d ICCOM-CNR,ViaG.   Moruzzi,1,56124Pisa,Italy a   r   ti   c   le   inf   o  Articlehistory: Received9August2013Accepted26May   2014Availableonline19    July2014 Keywords: Polychromy3DmodelMeshLabVirtualcolourReconstructionRomansarcophagiRamanspectroscopyOpticalpetrographicmicroscopy a   bst   rac   t In   thispaper,   we   reportamultidisciplinaryapproachfor   the   analytic   studyandthe   reconstruction   of    theancient   colourusedforRomansarcophagi.   Forthis   purpose,we   adopted   the   three-dimensional   (3D)   digi-tal   technology   and   found   itto   be   avaluabletoolfor   the   identification,documentation   andreconstructionof    the   ancient   colour.   This   technology   provedto   be   anexcellent   link   betweenarchaeologicalknowledgeandscientific   analyses.   Therefore,3Ddigitaltechnologies   would   effectively   facilitate   theexchangeof informationandcollaborationbetweenexpertsinvariousdisciplines.   Thisisextremely   importantinordertoobtain   demonstrable   resultsinanewareaof    study,   such   as   polychrome   Roman   sarcophagi   (andthe   ancientpolychromyandgilding   onthe   marble).   In   this   study,   the   digital3D   modelof    Ulpia   Domn-ina’s   sarcophagus   (NationalRoman   MuseuminRome,inv.   no.125891)   has   been   used   toidentify   boththepigmentsand   the   techniques   of    application   used,and   toexplore   the   potentialof    emerging   technologiesin   the   reconstruction   andvisualization   of    the   ancient   colour.©2014   Elsevier   MassonSAS.This   is   anopen   accessarticle   under   the   CC   BY-NC-SAlicense(http://creativecommons.org/licenses/by-nc-sa/3.0/). 1.Researchaims Theaimofthisworkistoshowtheresultsofa   practicalapplicationof    3Ddigitaltechnologyasa   linkingbridgebetweenarchaeologicalandscientificdata,in   ordertoprovideabetterknowledgeof    theoriginalpolychromyviatheproductionof    vir-tualreconstructions.Thisuseof    computer-basedtechnologieswithconsolidatedscientificanalysescouldsucceedin   obtaininga   com-monworkplatformfora   betterknowledgeof    ancientcolour(andgilding),andthedisseminationoftheresults. ∗ Correspondingauthor. E-mailaddresses: eliana.siotto@isti.cnr.it(E.Siotto),matteo.dellepiane@isti.cnr.it(M.   Dellepiane),marco.callieri@isti.cnr.it(M.   Callieri),roberto.scopigno@isti.cnr.it(R.   Scopigno),c.gratziu@tiscali.it(C.Gratziu),a.moscato@teletu.it(A.   Moscato),l.burgio@vam.ac.uk(L.Burgio),s.legnaioli@pi.iccom.cnr.it(S.Legnaioli),giulia.lorenzetti@pi.iccom.cnr.it(G.Lorenzetti),vincenzo.palleschi@cnr.it(V.Palleschi). 2.Introduction Romansarcophagihavebeensystematicallystudiedfromatypological,stylisticandiconographicpointof    viewsincethelatenineteenthcentury.Thishasgivenriseto   a   great Corpus andanextensivescientificproduction[1].   Conversely,thepapersrelatedtothepolychromyfieldarelessthantenin   a   century.Therefore,nowadaysthereis   onlylittleknowledgeaboutpigments,dyes,andbindersusedbyRomanartists,andaboutthepictorialstyleandthetechniquesusedto   applyboth   colourandgilding[2].   Forthisreason,theconsiderationsbyPietrogrande[3],   Gütschow[4]and Reuterswärd[5]f romthefirsthalfofthelastcenturyareof    fun-damentalimportance,althoughtheyareunsupportedbyscientificanalysis.However,in   thepastfewyears,therehasbeenarenewedinterestinancientpolychromythathasgeneratedseveralresearchprojectsandalsotwo   analyticalpublicationsonRomansarcophagi[6,7].Thisnewinterestinvolvedtheuseofcomputer-basedtechnolo-giesinthestudyof    theancientpolychromyandgilding.However,thepolychromereconstructionis   notaconsolidatedsubjectof research,sincea   lotof    workhastobedonestilltoimproveour http://dx.doi.org/10.1016/j.culher.2014.05.0111296-2074/©2014ElsevierMassonSAS.Thisis   anopenaccessarticleundertheCC   BY-NC-SAlicense(http://creativecommons.org/licenses/by-nc-sa/3.0/).  308 E.Siottoetal./JournalofCulturalHeritage16(2015)307–314 Fig.1. Sarcophagusof    UlpiaDomninaandsomepolychromedetails,NationalRomanMuseuminRome(inv.no.125891). knowledgeofthemethodsandtechniquesof    colourapplicationonGreekandRomanartwork[2,8].   Somepolychromereconstructionsareoftenbasedonthecommonlyknowncoloursfortheantiq-uity(i.e.redandyellowochre,leadwhite,cinnabar,carbonblack,Egyptianblue,etc.)withartisticinterpretations.Thetechnicalfocusof    thisworkisto   showthepotentialofdigitaltechnologiescoupledwithhigh-quality3Dmodelsintheexperi-mentationofseveralpaintingtechniquesandreproductionof    thealternativereconstructionhypothesesbasedonscientificanalysisdata.Ourreconstructionusesalltheinformationcollectedbyvisualobservationofthesarcophagusanditsvirtualphotorealistic3Dmodel,thehistoricalandstylisticknowledgederivedfromtheseobservationandtheresultsof    scientificanalyses.Itis   thankstomultidisciplinaryresearchthatitwaspossibletoproposeanaccu-ratevirtualreconstructionfromananalyticalaswellasa   historicalpointofview.Alongtestingcampaignwasalsoundertakentodefinethemorepropertoolsandpipelineforaneffectiveselectionandapplicationofmaterialstothedigital3Dmodelandtheimplementationof morerealisticrenderings[9].Asacasestudy,thesarcophagusdedicatedtoUlpiaDomnina(NationalRomanMuseuminRome,inv.no.125891)waschosenhere,mainlybecauseof    thefairstateof    preservationof    itssrcinalpolychromy(Fig.1)that   couldhavepotentiallyshedlightonthecolouringtechniquesused.ThissarcophagusismadeofProcon-nesianmarble[10]andwasfoundin1953duringtheconstruction ofabuildinginRome[11,12].   Froma   typologicalpointofview,itispartoftheserialproductionof    sarcophagiwithVictoriesholdingashieldandfuneraryCupids[12,13]. 3.Methodology  Alasertriangulationscanner[14]wasusedtoacquiregeo-metricdataandto   createa3Ddigitalmodeloftheartefact(seeSubsection3.1),whereasthecoloursoftheremainingfragmentsoftheoriginalpolychromiesweremeasuredbycalculatingtheRGB(Red–Green–Blue)encodingoftheselectedpaintedtraces(seeSubsection3.2andFig.2).In   additionto   historicalandiconographicresearches,anda   carefulobservationwithbinocularandportablestereoscopes,wealsoperformedmicro-andnon-destructiveinves-tigations,namelyOpticalPetrographicMicroscopyandRamanmicroscopy[15,16],inordertoacquireinformationonthepig- mentsandthetechniquesusedtoapplythemonthemarblesurface(seeSubsections3.3and3.4).Tothispurpose,fivesmallandrep- resentativesamples,namelysampleno.5,5A,6,   6Aand interno (Fig.3),   werechosenin   collaborationwiththeMuseum’srestorers,respectingthecriteriaof    minimuminvasiveness.  3.1.Acquiringthe3Dmodel Thedigitizationofthesarcophaguswasperformedfirstbypro-ducinga   geometrical3Dmodel(acquiredwitha   lasertriangulationscanner)andthenbyacquiringthecolourattribute[17]bymeans ofhigh-resolutionphotographs(usingaDSLRcamera).Thescan-ningsystemwaschosenin   laboratoryconsideringthematerialandreliefattributes;whilethe3D   scanningplanwasdecidedonsite,directlyevaluatingthespatialfeaturesofthesarcophagus[2].   Eachscanprovideddataona   maximumareaofnearly50 × 30cm;   there-fore,inorder   to   obtainthecompletereliefofthesarcophagus,itwasnecessarytoperformmultiplescansfromdifferentangles.A   totalof313scanswereacquired.Thescanningresolutionwaskeptatlessthanhalfa   millimetre,thusallowingthegenerationof    a   highly-detaileddigitalmodel.Aseriesof    photographswastakeninordertoenrich3Dmodelswithhigh-resolutioncolourinformation,inpar-allelwith3Dscanning[2].All   datawereacquiredin   approximatelyeighthoursbytwo   expertoperators.ThedatacollectedwereprocessedusingMeshLab,anopensourcemeshprocessingsystemdevelopedbyVisualComputingLab,ISTI-CNR [18,19].   Thestandard3Dscanningpipeline[14,17,20]wasfollowedtoproducea   final3Dmodelof19milliontriangles;otherlower-resolutionmesheswerecreatedfromthismodel,tosupportdifferentphasesof    thevirtualpolychromereconstructionprocess.In   additionto   thecharacterizationofthecurrentcolourtraces[9,14],   the3Dscanningcanprovidea   veryaccuratedigitalmodelof thesarcophagus.Theanalysisof    3Dgeometrydoesnotusuallybringadditionalhintsfortheanalysisof    theancientpolychrometraces.Nevertheless,we   shallseehowthe3Dmodelcanbeperfectlysuited  E.   Siottoetal./     Journalof    CulturalHeritage16(2015)307–314 309 Fig.2. Selectedtracesof    colourhavebeen   convertedinRGBcoordinates(a)   byusing   calibratedhigh-resolutionimagesthat   areacquiredbyDSLRcamerawithaMiniMacbethColorchecker TM insertedintheimagesarea,intheproximityof    thesampledsurface;b:thecreationof    acorrespondingcolourpaletteinMeshLab. Fig.3.   Micro-samplingpointsof    colourstracesoverUlpiaDomnina’ssarcophagus(inv.no.125891). tosupportthecolourinvestigationin   allphasesof    theknowledgeprocess(seeSubsection4.3).  3.2.DetectingRGBcoordinatesofcolourtraces Aimingatcreatingdigitalreconstructionsof    thehypotheticalpolychromedecoration,we   neededthereferencevaluesfortheoriginalcolourscorrespondingtothepolychrometracesthathavebeendetectedonthesarcophagussurface.Thus,high-resolutionandcalibratedimagesoftherepresentativeareaswereacquiredon   UlpiaDomnina’ssarcophagusbyusinga   DSLRcameraandacolourcalibrationtable(aMiniMacbethColorchecker TM ).Fromtheimages,colour-correctedusingtheknowncoloursonthechart,theRGBcoordinatesoftheselectedcolouredtraceswerederived(Fig.2)   [21,22].  3.3.Opticalpetrographicmicroscopy Opticalmicroscopyallowsa   preliminaryanalysisof    thesamples,whichwereinvestigatedwithanOlympusSZX10stereomicro-scopeinterfacedto   a   PCusingdigitalcameraOlympusColourViewI.Thepolishedsections,preparedwithepoxyresin,werethenobservedusinganOLYMPUSBX51microscopewithvisibleandultravioletlightandinterfacedtoaPCusinganOlympusDP71digitalcamera.Forallthefivesamples(Fig.3),wewantedtoinvestigateand verifythetypeof    pigmentandtheapplicationtechniqueadopted.Therefore,weperformedtheirpetrographicanalysisasthinsec-tionswithapolarizingmicroscope.Inparticular,thethinsectionswereobservedusinga   LeitzOrtoplan-polmicroscopeequippedwithaPloemOpakfiltercubes(ACBP340-380nm).  3.4.Ramanspectroscopy Subsequently,allthesampleswerealsoexaminedbyRamanspectroscopyusinga   RenishawRamanInviainstrumentandanXploRAHoribaJobin-Yvonmicroscope.Thefirstdevicewasequippedwithan1800grooves/mmdiffractiongrating,a   CCDdetectoranda   50 ×   magnifyinglens.Thelasersources,a   HeNelaser(    =   633nm)   andanNd:YAGlaser(  =   532nm),wereselectedaccordingtothekindof    sampleanalysed.Thesecondinstru-mentwas   equippedwithtwo   diodelasers(638nmand532nm,respectively)andanOlympusmicroscopewitha   10 ×   anda   50 × objective.  310 E.Siottoetal./JournalofCulturalHeritage16(2015)307–314 Fig.4. Images(aandb)   presentthinsectionsof    sampleno.5parallelnicolsat100  and50  lengthscale;thestratigraphyshowsthattheredochreis   appliedoveragroundlayer   ofgypsum,red  pozzolana andlime.Image(c)presentsa   thinsectionofsampleno.6parallelnicolsat50    lengthscale;thestratigraphyshowsthatthered   ochre(1)is   appliedoveralayerof    mixedEgyptianblueandboneblackpigments(2).Image(d)showsthattheEgyptianblue(1)andboneblack(2)granulesare   visiblebetterinthesectionof    sampleno.6Acrossednicolsat50  lengthscale.Finally,theimages(eandf)   presentthinsectionsofsamplenamed interno (e)   paralleland(f)crossednicolsat300    lengthscalethatshowthepresenceof    mortarcomposedof    gypsum(1),red  pozzolana (2)andlime(3).  3.5.Archaeologicalandhistoricalresearches Atthesametime,we   performedarchivalandliteraturesearchestosolvedoubtsabouttheauthenticityof    somepictorialele-mentsbyevaluatingthedescriptionof    thesarcophagusinthehistoricalarchiveandin   thecatalogueof    theNationalRomanMuseum[12]. 4.Resultsanddiscussion Theresultsofthescientificanalyseshavebeenindeedvalu-ableindeterminingthepigmentsandstratificationof    somecolours;however,theywerenotenoughto   understandthatthissarcophagusis   characterizedbytwodifferenttechnicalapplica-tionsof    colourwithouta   visualobservationsandarchaeologicalknowledge.Alldata(scientificanalyses,archaeologicalinforma-tionandvisualobservationsupportedbyopticalandcomputergraphic3D   tools)suggestedthatthesarcophaguswas   colouredintwodifferentmomentsof    itshistory.Thefirstschemewas   appliedintheLateAntonineperiod(160–180AD)whenthesarcophaguswasmade.Acloserlookof    somedetailsofthecornucopiasandwreathofoak   revealedthat   thesarcophaguswas   paintedwhenitwasnot   completelyfinished(Fig.1)[2,20].   Thesecondaryschememostlikelycamefroma   repaintinginthemiddle3rdcentury,whenthesrcinalinscriptionwas   re-carvedanddedicatedtoUlpiaDomnina,loweringthesrcinallevelof    thereliefbackground[2,20].  E.   Siottoetal./     Journalof    CulturalHeritage16(2015)307–314 311 Fig.   5. Ramanspectroscopyof    sample5:(a)Goethite,(b)   Haematiteand(c)Quartz. Thissarcophagusisthereforeveryimportantbecauseitis   a   firstanalyticallydocumentedcasestudyof    reuseandrepaintduringtheantiquity[2].Untiltoday,thestudiesofRomansarcophagi neverconsideredthepossibilityoftheancientre-colouringpres-ence[3–5]. 4.1.Scientificanalysesandvisualobservationresults The   visualobservationof    sample5showsa   redcolourappliedoveralayerofwhitecolourwithredandblackinclusions;sample5Ashowsawhitelayerwithredandblackinclusions.Thepetro-graphicanalysisofthethinsectionsof    samplesno.5and5Ashowsthatredochrewasappliedovera   finegroundlayerof    gypsum,red  pozzolana (aRomanporousvarietyofvolcanictuff)andlime(Fig.4aandb).Samplesno.6and6Aarebluein   colourandtheanalysisoftheirthinsectionshowsthatthebluecolourisEgyptianbluemixedwithboneblack,applieddirectlyontoa   smoothmarblesurface(Fig.4c).Tracesofironoxidesintheupperpartof    thethinsectionsampleno.6suggestthatredochrewasappliedoverEgyp-tianblue(Fig.4d).Thelastsample(named interno )   is   a   fragmentremovedfromtheinternalsurfaceof    thesarcophagus.Thethinsec-tionshowsthattheinteriorof    thesarcophaguswascoveredwithacoarsemortarof    gypsum,red  pozzolana andlime(Fig.4eandf). Subsequently,thefivesampleswereinvestigatedbyRamanspectroscopy.Thespectrawereinterpretedusingreferencespectratakenfromtheliterature[23,24].Generally,thespectrahavecon- firmedthepresenceof    someofthepigmentsalreadyobservedinthethinsections.Fig.   5showsspectraof    sampleno.5,   inwhichthetypicalpeaksof    goethite,haematiteandquartzareclearlyidentified;theRamanspectrumofthesampleno.6showsthetyp-icalpeaksofEgyptianblue,thusconfirmingthefindingsof    theopticalanalysis(Fig.6a).Concerningthe interno sample,Ramanspectroscopyhasrevealedthepresenceof    gypsumandhaematite(Fig.6b).Thismay   suggestthepossibilitythat   theinteriorofthesarcophaguscouldhavebeencolouredusingredochre.Anyway,  pozzolana doescontainironoxides,sothisattributioncouldbequestionable.Therefore,atthemomentofresearch,weare   notcer-tainthattheinsideof    thesarcophaguswaspaintedredbut   itwascertainlyplasteredwitha   coarsemortarofgypsum,red  pozzolana andlime.However,thisdiscoveryisa   veryimportantachievement,sinceitrepresentsa   noveltyinthestudiesof    marblesarcophagi[2]. 4.2.Archaeological,historicalandiconographicresults Thearchivaldatafounddidnotprovidetheamountof    informa-tionwehopedfor,   butwasstilluseful.Thetextof    thecataloguegaveadetailediconographicdescription,theyearandplaceofdiscov-eryandtheindicationtoreferencepublications.SomeinformationreportedbyFellettiMaj   atthetimeof    itsdiscovery(1953)isveryimportanttounderstandandsolvesomedoubtsraisedduringthecurrentinvestigation.Inparticular,shewrote:“Thesarcophaguswasnotfound insitu ,butwithinasectionof    amodernwallthatwasdemolishedinthatcontext.Itappearedasa   rectangularblockcoveredin   limethatfortunatelywas   soonrecognizedasasarcoph-agusandtransportedtotheNationalRomanMuseum.Hereitwascleanedanditappearedasa   boxwithouta   lid,withmarblereliefsandexceptionallywell-preservedcolour”[11].   Thisreportinformsusthat   thesarcophaguswassubjectedtooneormoremodernrestorations.Thisstrengthenstheresultsof    ourvisualobservationsandallowsustoconfirmtheattributionof    theblacklines,identifiedonthereliefandmadeofasubstanceseepingthroughthemarble(Fig.   7),tomodernrestorations,a   characteristicthatwas   alreadyidentifiedbyReuterswärdin1960[5].Thisalsosolvestheproblem oftracesofmortarpresentabovethelayerof    redcolour,asseeninthelettersof    theinscriptionorin   thefeathersofwings(Fig.7)[2]. Thestudyof    thepolychromywas   alsobasedontheiconographiccomparisonwithsimilarsubjects,suchasotherRomansarcophagithatstillpreservegoodtracesof    colour,orpaintingsandmosaics(seeSubsection4.3).Theseassociationshaveprovidedusefulsup- portforthedetectionof    coloursthatarenolongervisibletothe Fig.6. (a)   RamanspectrumofEgyptianbluein   sampleno.6;and(b)spectrumof    HaematiteandGypsum(peak1007cm − 1 )   insamplenamed interno .
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