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Corrosion investigation of fire-gilded bronze involving
high surface resolution spectroscopic imaging
Giulia Masi, Cristina Chiavari, José Avila, Jérôme Esvan, Simona Raffo,
Maria Chiara Bignozzi, Maria C. Asensio, Luc Robbiola, Clara Martini
To cite this version:
Giulia Masi, Cristina Chiavari, José Avila, Jérôme Esvan, Simona Raffo, et al.. Corrosion investigation
of fire-gilded bronze involving high surface resolution spectroscopic imaging. Applied Surface Science,
Elsevier, 2016, 366, pp.317-327. �10.1016/j.apsusc.2016.01.101�. �hal-01494641�
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Eprints ID : 16707
To link to this article : DOI:10.1016/j.apsusc.2016.01.101
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http://dx.doi.org/10.1016/j.apsusc.2016.01.101
To cite this version : Masi, Giulia and Chiavari, Cristina and Avila,
José and Esvan, Jérôme and Raffo, Simona and Bignozzi, Maria
Chiara and Asensio, Maria C. and Robbiola, Luc and Martini, Clara
Corrosion investigation of fire-gilded bronze involving high surface
resolution spectroscopic imaging. (2016) Applied Surface Science,
vol. 366. pp. 317-327. ISSN 0169-4332
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Corrosion
investigation
of
fire-gilded
bronze
involving
high
surface
resolution
spectroscopic
imaging
G.
Masi
a,∗,
C.
Chiavari
a,b,
J.
Avila
c,
J.
Esvan
d,
S.
Raffo
e,
M.C.
Bignozzi
a,
M.C.
Asensio
c,
L.
Robbiola
f,
C.
Martini
gaDipartimentodiIngegneriaCivile,Chimica,AmbientaleedeiMateriali,UniversitàdiBologna,viaTerracini28,40131Bologna,Italy
bC.I.R.I.(CentroInterdipartimentaleRicercaIndustriale)MeccanicaAvanzataeMateriali,UniversitàdiBologna,Bologna,viaTerracini28,40131Bologna,
Italy
cSynchrotronSOLEIL,L’OrmedesMerisiers,91190Saint-Aubin,France
dCentreInteruniversitairedeRechercheetd’IngénieriedesMatériaux,UniversitédeToulouse,4alléeEmileMonso,31030Toulouse,France eDipartimentodiChimicaIndustriale“TosoMontanari”,UniversitàdiBologna,vialeRisorgimento4,40136Bologna,Italy
fTRACESLab(CNRSUMR5608),UniversitéToulouseJean-Jaurès,5,alléesAntonio-Machado,31058Toulouse,France gDipartimentodiIngegneriaIndustriale,UniversitàdiBologna,vialeRisorgimento4,40136Bologna,Italy
Keywords: Firegildedbronze Corrosion Chemicalimaging Synchrotronradiation Photoemission Artworks XPS Decuprification
a
b
s
t
r
a
c
t
Gildedbronzesareoftenaffectedbyseverecorrosion,duetodefectsintheAulayerandAu/Cualloy galvaniccoupling,stimulatedbylargecathodicareaofthegildedlayer.Galvaniccorrosion,triggeredby gildingdefects,leadstoproductsgrowthattheAu/bronzeinterface,inducingblisteringorbreak-upof theAulayer.Inthiscontext,fire-gildedbronzereplicaspreparedbyancientmethods(useofspreadable Au–Hgpaste)wasspecificallycharacterisedbycompilingcomplementaryspectroscopicandimaging informationbefore/afteracceleratedageingwithsyntheticrain.Fire-gildedbronzesampleswere chem-icallyimagedincross-sectionatnano-metricscale(<200nm)usinghighenergyandlateralresolution synchrotronradiationphotoemission(HR-SRPES)ofcorelevelsandvalencebandafterconventional characterisationofthesamplesbyGlowDischargeopticalEmissionSpectroscopy(GD-OES)and conven-tionalX-rayphotoelectronspectroscopy(XPS).WehavefoundanetsurfaceenrichmentinZnandSn afterfire-gildingandpresenceofmetallicHg,PbandCuwithintheAulayer.Moreover,thecomposition distributionoftheelementstogetherwiththeiroxidationhasbeendetermined.Itwasalsorevealedthat metallicphasesincludingHgandPbremaininthegildingaftercorrosion.Moreover,selective dissolu-tionofZnandCuoccursinthecraterduetogalvaniccoupling,whichlocallyinducesrelativeSnspecies enrichment(decuprification).Thefeasibilityadvantagesanddisadvantagesofchemicalimagingusing HR-SRPEStostudyartworkshavebeeninvestigatedonrepresentativereplicas.
1. Introduction
1.1. Systemunderinvestigation
Inoutdoorconditions,gildedbronzes(widelyusedin histori-calmonumentsandinarchitecturalelements)areoftenaffected byseverecorrosionofthebronzesubstrate,duetothepresence ofporesanddefectsinthegildinglayer.Inparticular, degrada-tion of gildedbronzeis induced by galvanic coupling between
∗ Correspondingauthor.Tel.:+390512090361;fax:+390512090322. E-mailaddresses:giulia.masi5@unibo.it(G.Masi),cristina.chiavari@unibo.it (C.Chiavari),jose.avila@synchrotron-soleil.fr(J.Avila),jerome.esvan@ensiacet.fr (J.Esvan),simona.raffo2@unibo.it(S.Raffo),maria.bignozzi@unibo.it
(M.C.Bignozzi),maria-carmen.asensio@synchrotron-soleil.fr(M.C.Asensio), robbiola@univ-tlse2.fr(L.Robbiola),carla.martini@unibo.it(C.Martini).
bronzeandgold.Defectssuchasthicknessvariationandporesin thegildinglayeraremostlyduetothegildingprocedure.Themain gildingmethodsusedinantiquitywerebased(i)onthe applica-tionofgoldfoilorleaf,(ii)onselectiveleachingofelementsnot asnobleasgold(theso-called“depletiongilding”method)or(iii) ontheuseofspreadablegold-mercuryamalgampaste,followed bymercuryevaporation,asdescribedinseveralpapers[1–3].In thepresentpaper,theinvestigationisfocusedonamalgam-gilded (or“fire-gilded”)bronzeforwhich,aftermercuryevaporation,the mattegold-richlayermustbe“burnished”(i.e.plasticallydeformed and compacted)by a smooth and hard tool (e.g. agate stone), soastoobtaina shinysurface [4].Thistechnologicalsequence producesagildinglayerwithpeculiarfeatures.Themostrelevant characteristicisthepresenceofresidualmercury(usuallyHg con-centration ranges fromabout 7 to10wt.% in the gildinglayer, although valuesup to17wt.%were alsoreported[4]), but also
Cu–Auinterdiffusionduetoheatingformercuryevaporation[1]
andthedetrimentalinfluenceofalloyingelementssuchasPbon thegildingprocess[4]shouldbementioned.
Inapreviouspaper[5],wecharacterisedquiteextensively a fire-gildedquinarybronze(Cu–Sn–Zn–Pb–Sb)intheas-gilded con-dition,obtainedaccordingtoancientrecipes.Specifically,Focused Ion Beam (FIB) milling followed by cross-section observations, both by field emission SEM and by ion channelling contrast withGa+,clearly showed themorphology ofthe Au-richlayer, partlycompactedbyburnishingbutstillretainingsub-micrometric porosity. It wasalso possible to reveal annealing twins in the recrystallisedmicrostructureoftheAu-richlayer,aswellasthe strain-hardened(henceveryreactive)layerinthebronze imme-diatelybeneath thegilding.Thislayerconsistedof smallgrains elongatedin the direction of material flow due to mechanical polishingofthebronzebeforegilding.However,itwasnot pos-sibletomakedetailedinvestigations,mainlyduetothelimitsof theconventional X-rayspectrometry technique usingMg or Al anodes.
Asregardsthecorrosion of fire-gildedbronzes, theParadise Door(i.e.theEasterndooroftheBaptisteryofFlorence,created byLorenzoGhiberti, 1425–1452)is a famousexample [6].This masterpiecewasexposedtooutdooratmosphereinFlorence,Italy, sinceitsproductionuntiltheArnofloodin1966.Asformany oth-ersgildedmonuments,suchasthehorsesofSaintMark’sbasilica inVenice[7],thegrowthofcorrosionproductsatthegold/bronze interface(withtheformationofcharacteristiccorrosioncraters) ledtoblisteringorbreak-upandlossofthegoldlayer[6,8].The corrosionproductsdetectedbeneaththegoldlayerinthecaseof Ghiberti’sParadiseDoormainlyconsistedofcopperoxide,basic sulphatesandcoppercarbonates[9],withlesseramountsof chlo-rides and nitrates [10]. Furthermore,due tothe action of acid rains,coppersaltsalsodissolvedanddiffusedabovethefire-gilded surface, where brochantite, antlerite, paratacamite and copper hydroxylnitrate[11]wereidentifiedbyX-RayDiffractionandIR spectroscopy.Thepresenceofsolublenitrate-basedcompoundsis oftenattributedtointeractionwiththepollutedurbanatmosphere, buttheywerealsohypothesisedtoderivefromresiduesofbronze picklingreagentsusedin theancient pastbygoldsmithsbefore applicationofthegold-mercurypaste,forimprovingtheadhesion ofthegildinglayeronbronze[11].
Thepresenceofseveralcorrosionproducts, including hygro-scopiccompoundsatthegold/bronzeinterface,makesfire-gilded bronzesquiteunstableandcomplexsystems[9,12,13],requiring specificrestorationandconservationprocedures[8].Inparticular, cleaningproceduresbasedbothonchemicalandonlaser meth-odswereapplied[11,14,15].Aftercleaning,conservationactions mustbeimplementedongildedbronzeartworkstoensuretheir long-termcorrosionprotection.Bothpreventivetechniques,such asmicroclimatecontrol[9,16]anduseofcoatingsandinhibitors
[12,17,18],wereinvestigated.Inordertohelpmicroclimatecontrol, specificsensorsbasedonthemeasurementofgalvaniccurrentsin gold/bronzecouplesprovedtobeusefulinfindingthemost suit-ableclimaticparameterswhichensurenegligiblecorrosionrates
[16].
Evenifthemain phenomenologicalaspectsof gildedbronze corrosionareknown,animprovedcharacterisationofthis multi-layersystemandtheinvolvedphenomenaof decayisrequired, in order to pursue a deeper understanding of the corrosion mechanism,whichisalwaysabasicrequirementfordeveloping effectiveconservationstrategies. Therefore,inthepresentwork thesamefire-gildedbronzeasinthepreviouspaper[5],was pre-paredaccordingtoancient methods,soastomimicthetypical microstructureandmorphologyofamalgam-gildedbronzes. Age-ingin conditionsclosely simulatingoutdoorexposurein runoff conditions(droppingtests[19])wasappliedandthesefire-gilded
bronzesampleswerepreliminarycharacterisedbymicroscopyand micro-spectroscopytechniques(FIB-FEG/SEM,EDS,micro-Raman)
[5].
1.2. GeneralXPSbackground
X-rayphotoelectronspectroscopy(XPS)hasbeenappliedsince morethanthirty yearstoancientbronzes andpatinas,inorder toanalysethechemicalstateofelementsontheoutermost sur-facesand (by Ar+ sputtering)to explore depthprofile[20–22]. However,itsuseinculturalheritagematerialssciencehasbeen ini-tiallyrestricted,duetoexperimentalconstraintslinkedtosample size,analysedarea(typicallylargerthan300mindiameter)but alsotochargeeffectslinkedtoweaklyconductingsurfaces[21,22]. ThistechniquewasoftencoupledtoscanningAugermicroscopy (Augerelectronspectroscopy—AES)asanassociatemethod high-lightingthedifferencesinthemicrochemicalcompositionthanks toasmallerspatialresolutioncomparedtoXPStechniques[20–24]. Sincethelastfifteenyears,theimprovementindetection sen-sibility of XPS analyser and to the needs to accurately relate chemical binding toelemental or structural information ledto moreinvestigationsonsyntheticmaterialstobecarriedout.These investigationsweremainlyfocusedonchemicaltransformationof polishedorartificiallycorrodedCu–Sn(Zn,Pb)bronzesindifferent conditions—asforexample,inatmosphereunderSO2 orH2Sgas pollution[25–28],inNaClaqueoussolution[29,30],insoil[31,32], fromparticularchemicalsolutions[33,34],butalsorelatedtothe identificationofspecificcopperororganicproducts[24,35,36]or ontheefficiency of conservationprotective treatments [37,38]. On ancient artefacts, XPS investigationwas oftenassociated to otheranalyticaltechniques(EDS–SEM,XRD,PIXE,SIMS, LA-MS)
[31,39–44].XPSanalysisusuallywereperformedonsurface frag-ments,sometimescoupledtodepthprofilemeasurementfromAr+ sputtering[21,22,39,40]orlaserablation[44].Itshouldbenoticed thatseveralstudiesinvestigatedtheeffectofX-raybeamor(Ar+) ionsputteringonthesurfaceevolution[21,24,26,28,39,41].They revealedthatleadcarbonatespeciesaswellascoppersulphateor copperoxidecanbereduced,whichcancausetheinterpretationof XPSandAEStobeproblematicforcopperspecies.XPSwasalso sel-domconductedonthickcorrosionlayersfromcross-sections[31]. Asregardsmercury-amalgamplatingonancientbronzes,onlyvery fewstudiesapplyingXPShavebeenperformed.In [45],surface analysisofseveralfragmentsofgildedcopperbasealloysclearly demonstratedtheimportancetocombinesurfaceanalytical tech-niqueshelpingtorefinedataonboththemanufacturingprocess andthedegradationstate.
Recently, a multi-analyticalapproachincluding conventional XPSandsynchrotronradiation—(SR)basedtechniqueswasapplied totwoleadedbronzes,revealingalltheinterestforgetting map-pingofthesurfacechemistryofthebronzeatasub-micrometric spatialresolution[41,42].Thisglobalapproachhasbeenrecently extended tosynchrotron X-rayphotoelectronspectroscopy and X-rayabsorptionspectroscopyonmercurygildedsilverplates, dif-ferentlycolouredaccordingtodifferentappliedtreatmentsbased onmediaevalwritings[46].Thisstudypointedouttheinterestto useSRasahighlypowerfulX-raysourceforperformingXPSsurface measurements.
1.3. Aimofthework
Inthepresentpaper,themainaimistoevaluatetheapplication ofhighlyenergyandlateralresolutionsynchrotronradiation pho-toemission(HR-SRPES)forobtainingspatiallyresolvedinformation oncorrodedbronzesamplesatasub-micrometriclevel(<500nm). Inparticular,specialattentionwaspaidtothemappingof chemi-calspeciesandtheirstateofoxidation.Tothisaim,stateoftheart
chemicalmappingusingHR-SRPEShasbeentestedhereforthefirst timeoncross-sectionsoftypicalcorrosioncraters,attheinterface betweenthegoldlayerandthebronzesubstrate.
Here,alsobasedonamulti-analyticalapproachinvolvingthe useofotheranalyticalmethods(EDS-SEM,GD-OESand conven-tional XPS), from surface as wellas cross-section analysis,the objectiveswere:(i)toclarifythedistributionofelementsinthe as-gildedbronze,inordertounderstandtheroleofbronze allo-yingelements(Cu,Sn,Zn,Pb)aswellastheroleofresidualHgin thestructureofthegildinglayerand(ii)toinvestigatethe interac-tionsbetweenelementsinthealloy/gildinglayersystemandthe environmentalelements,mainlyO,andalsoSandClelements.
2. Materialsandmethods
2.1. Preparationoffire-gildedbronzesamples
Thecompositionofthebronze,castinstonemouldsandthen fire-gilded,issimilartothoseofRenaissanceartefacts:Sn:3.3,Zn: 1.5,Pb:1.0,Sb:0.2wt.%,Cutobalance(Sn:1.81,Zn:1.49,Pb:0.31, Sb:0.11at.%).Thealloyexhibitsadendriticmicrostructureofcored ␣-Cusolidsolution,typicalofas-castbronze(withSn-andSb-local enrichmentintheinterdendriticspaces),alsoincludingglobules ofPbasdescribedin[47].Residualporosityandshrinkagecavities wereuniformlydistributedinthealloy.
Fire-gildingwascarriedoutaftermechanicalpolishing(SiC180 grit)of thecastbronzesurface followed bynitricacidpickling (10wt.%ofHNO3indeionisedwater)andrinsinginwater.The pol-ishedandpickledsurfacewasthencoveredwithAu–Hg(1:8wt.%) amalgampaste(accordingtotherecipebyBenvenutoCelliniin histreatisesongoldsmithingandsculpture(1568)[48])andthen heatedbyanopenflame,soastovaporiseHg.Subsequently,the gildedsurfacewasburnishedbyanagatetool(usingoliveoilas alubricant),in ordertodensifytheporouslayerandproducea smoothandshinysurfacewithagoldenappearance.Noother gold-colouringprocedure,asthosedescribedin[46],wasappliedonthe as-gildedsurface.Thethicknessofthemercurygildinglayerwas estimatedbySEMobservationofcrosssections.
2.2. Acceleratedageinginrunoffconditions(droppingtests) Fire-gildedbronzespecimens(2.5×5.0cm2)wereexposedto syntheticrain (pH=4.3)by droppingtests witha total Timeof Wetness(TOW)of30days.Thedroppingtestssimulate unshel-teredexposuretorainwater(runoffconditions).Inthistesttherain solutionwasperiodicallydroppedonto 45◦ inclinedspecimens, withalternatedcyclesof2-daysdropping/1-daydryingand3-days dropping/1-daydrying,selectedonthebasisofenvironmentaldata fromthemonitoringstationsinBologna,Italy[19].
Asreportedin[5],theartificiallyaged samplesproducedby droppingwithTOWof30dayscanbeusedforadeeper inves-tigationof corrosion process. This methodproduces crater-like corrosionfeaturesattheinterfacebetweenthegildinglayerand bronze,comparabletohistoricalreferenceartefacts.
2.3. Characterisationoffire-gildedsamples
The free surface of fire-gilded bronzewas observed by 3D-Digitalmicroscopy.Onthefreesurfaceofgildedbronze,alsoGlow Discharge-OpticalEmissionSpectroscopy(GD-OES)concentration profilesweremeasured,aftergentlerinsingwithethanol,soasto removesurfacecontamination.GD-OESanalysiswascarriedout withaGrimm-styleglowdischargelampinDCmode.The inter-naldiameterofthetubularanode(hencetheanalysedareaineach measurement)was2.5mmandthein-depthspatialresolution dur-ingdepthconcentrationprofiles(perpendiculartothesurface)is
about50nm.TheGrimm-typeatomisation/excitationsourcewas evacuatedbyarotarypumptoafinalpressureof0.05hPa.After evacuation,flowingargonasaworkinggas(99.995%purity)was introducedtoaconstantpressureof3hPa.Constantdischarge con-ditions(voltage 900V andcurrent 9mA) wereusedinorder to maintainaconstantsputteringrate.Thecraterdepthswere mea-suredbystylusprofilometryaftertheGD-OESmeasurements,in ordertocheckthesputteringrates.
Conventionalmetallographictechniques(i.e.coldmountingin conductiveresin)wereused,forthepreparationofcrosssections. The gildinglayerandthecorrosion features atthegold/bronze interface were characterised by a coupled system SEM/EDS-RamanSCA(ScanningElectronMicroscopywithEnergyDispersive Spectroscopy and Ramanmicroprobes, Structuraland Chemical Analyser,=514.5nm,power:5mW).
Thecharacterisationoftheinterphasesinthecross-sectionswas performedbyHR-SRPESwithanexcitationenergyof850eV (there-foreitwasnotpossible,byHR-SRPES,toexplorebindingenergy contributionssuchasthosefromCu2p3/2andZn2p3/2atabout 932–934and1022eV,respectively).HR-SRPESexperimentswere conductedintheANTARESbeamlineattheSOLEILsynchrotron.
ConsideringathicknessoftheAulayerlowerthan10m, cross-sectionofthesampleswereanalysedtakingadvantageofANTARES beamresolution.Imagingofpolishedcross-sectionswascarried outwiththehighresolutionR4000Scientahemisphericalanalyser andasetofFresnelZonePlates(FZP),abletofocalisethebeamspot uptoafewtenthsofnanometersinspatialresolution.Themain differenceoftheANTARESmicroscope fromotherconventional ARPESinstrumentsisthatthespecimenscanbemountedona high-precisionplatethatensurestheirnanoscalepositioninginthex,y andzdirections[49,50].Thepassenergywasfixedat100eVwitha stepat0.05eVforcorelevels.Investigatedcorelevelswerelimited toafewlocalisedspots,duetotheavailablesynchrotronbeamtime, whichwasmostlyusedfortime-consuminghigh-resolution photo-electronspectroscopy(HR-SRPES)maps.Asurfacecleaningusing HeionbombardmentwasappliedpriorXPSmeasurements.
Complementaryconventional XPSanalyseswerealsocarried out on the top of the fire-gilded surfaces. The photoelectron emissionspectrawererecordedusingamonochromatisedAlK␣ (h=1486.6eV)sourceonaThermoScientificK-Alphasystem.The X-rayspotsizewasabout400m.Thepassenergywasfixedat 30eVwithastepof0.1eVforcorelevels.Ionicsputteringofthe surfaceswasmadebyAr+ionbeamacceleratedunder2keVlow current.XPSspectrawererecordedindirectN(Ec).
ForbothXPSsystems(HR-SRPESandThermoScientificK-Alpha XPS), thein-depthresolution isof a fewnm.The spectrometer energycalibrationwasmadeusingtheAu4f7/2(BE=84.0±0.1eV) photoelectron line and the C 1s value of adventitious carbon (BE=284.8±0.2eV)whennoAuwaspresent.Thedifferentcore levelsinvestigatedwereC1s,O1s,S2p,Cl2p,CuandZn:3p(2p onlyforK-AlphaXPSsystem),Sn3d,Sb3d,Au4fandPb4f.The backgroundsignalwasremovedusingShirleymethod.The photo-electronpeakswereanalysedbyGaussian/Lorentzian(G/L=30%) peakfitting.Theatomicconcentrationsweredeterminedwithan accuracyof10%fromphotoelectronpeakarea,usingtheatomic sensitivity factorsreported byScofield, takinginto account the transmissionfunction oftheanalyser accordingtotheenergies applied(850eVforHR-SRPESand1486.6eVforK-AlphaXPS).
3. Results
3.1. As-gildedbronze
Thetopographicimageoftheas-gildedsurface(Fig.1a)shows residualgrooves(fromabout5to10mdeep)duetomechanical
Fig.1.Fire-gildedbronze:3-Ddigitalmicroscopytopographicimageofthefreesurface,withindicationofaporousarea,correspondingtogroovesinthebronzesubstrate, wheregoldwasnotcompactedbyburnishing(a).SEM/EDScross-sectionimages(backscatteredelectrons,BSE))intheregionscorrespondingtosmooth(bandc)andporous areas(d).Thefire-gildedlayeristhewhitelayerinthecentralareaofeachBSEimage.
polishingbeforegilding.Thispeculiarfeatureleadstotwotypical surfacemorphologies:smoothandflatareas(wheretheburnishing proceduresuccessfullycompactedthegildedlayerafterHg evapo-ration)andporousareas(duetoinsufficientburnishing).Infact,in theporousareas,somegroovesweretoodeeptoallowfull densifi-cationofthegildedlayer.Cross-sectionimagesinFig.1b–dshows thatthethicknessofthegildedlayerisremarkablyhigher(around 10mthick)inporousareasbycomparisontothesmoothones (about2m).
Itmustbetakenintoaccountthatthegoldlayerthicknessmay beoverestimatedduetosmearingofsoftgoldonhardbronze dur-ingmetallographicpolishing[1]),whereas,forthesamereason, thegoldporositymaybeunderestimated.Asdetailedin[5],FIB millingandFEG-SEMobservationsalloweddetailedmorphological characterisationintermsofcompactnessandporosity,confirming thatsmoothareas(i.e.areaswheretheburnishingprocedurewas fullyeffective)displaylowerthicknessandhigherdensityby com-parisontoporousareasonthegrooves.
The typical elemental distribution of the fire-gilded bronze beforecorrosionisshowninFig.2.Mercury(Hg)wasdetectedin thegildedlayer,butalsoalloyingelementsfromthebronze sub-strate,suchasSnandZn.ThepresenceofNinthegildedlayeris moreprobablyduetothepreviouslydescribednitricacidpickling procedure.
However,itisdifficulttodetermineaccuratelytheelemental distributionofAu,Hg,Pb,SnandSbinthegildedlayerby SEM-EDS,duetothelimitedspatialresolutionoftheX-raysourcebut alsototheoverlappingoftheprincipalemissionpeaks(Mpeaksof Au,HgandPbandLpeaksofSnandSb).Therefore,amoredetailed characterisationwascarriedoutbyGD-OESconcentrationprofiles aswellasbyconventionalXPSandbyHR-SRPESimaging.
TheGD-OESconcentrationprofiles aregiveninFig.3.Inthe outermostpartof thegilding(<0.25m),an enrichmentof Hg wasrevealed:Hgconcentrationincreaseswithdecreasingdistance fromthesurface,mostlikelyduetodiffusionofHgtowardsthe outersurfaceduringthefireheatingprocedure.ThissuperficialHg
enrichmentisaccompaniedbyamarkedenrichmentinSnandZn bycomparisontoCuandPbalsopresentinthegildedlayer(Fig.3, inlay).ThisphenomenonvanishedafterthemaximumofAu con-centration(thickness:0.4–0.5m)anditwasmostlikelylinkedto theeffectofthemercuryevaporationduringtheheatingstep.The gradualvariationofthealloyingelements,coupledwiththegradual decreaseofAuacrosstheinterfacebetweenthegildinglayerand thebronzesubstrate,issurelyalsorelatedtothelargesize(2.5mm) oftheanalysedsurfacearea,averagingthevariablethicknessofthe gilding.
ConsideringCuandAuGD-OESprofilesacrossthegildinglayer, showingagradualincreaseofCuconcentrationstartingfromthe surface,coupledwiththegradualdecreaseofAu,Cu–Au interdif-fusioncanbehypothesised,becausetheseelementsareknownto intermixreadilywhensolid-state diffusiontakesplace[51].On goingfromtheoutermostsurfacetowardsthebulk,theGD-OES profilesshowthepresenceofCualsointhemostexternalregions ofthegilding(farfromtheinterface,henceinregionswherethe variablethicknessofthegildingdidnotaffecttheGD-OESdata),so confirmingthehypothesisofinterdiffusion.Theabilityofthe heat-ingstep(carriedoutheresoastoremoveHgfromthefire-gilded layer)toinduceinterdiffusionof CuandAu,wasalsoobserved byotherauthorsinthegeneralcaseofgildedartefactsinvolving heating[1,4,6,52–54].
TheresultsofXPSmeasurementsperformedontheas-gilded bronzesurface,beforeandafterAr+etching,aregiveninTable1, whichhighlightsahighcontentofenvironmental-relatedelements includingC,N,O,SandSi,whichdecreasedinintensityafterAr+ etching.C1sislinkedtocarbonylpollutantsalsoincludingC N species.Itisinterestingtonoticetherelativelyhighproportionof nitrogen(2.4at.%),whichcannotbeascribedtonitratebutcouldbe relatedtoCandH(BEcorelevelat∼399.5eV[55]).Alsoregarding sulphur,thepeakofS2pisintwostates,correspondingtosulphate andsulphide.Nochloridesweredetected.
Onthegildedsurface,ahighamountofHgwasalsorevealed, decreasingafter theion bombardment (Table 1), in agreement
Fig.2. Fire-gildedbronze:SEM-BSEimage(topleftcorner)andEDSX-Raymapsshowingelementaldistributionincrosssection.Possibleresidualoverlappinginterference forEDSX-rayMpeaksofAu,HgandPb,aswellasforLpeaksofSnandSb.
Fig.3.Fire-gildedbronze:GD-OESconcentrationprofilesofAu,Hgandbronzealloyingelements(Cu,Sn,Zn,Pb)measuredfromthefreesurface.Theinlayshowsadetailof theconcentrationvariationfortheouterzoneofthegilding(<1.5m).
withGD-OES data (Fig. 3).Alsotheenrichment in Znby com-parisonto theCu initialcontent of the bronzewasconfirmed, (Zn/Cu atomic ratio increases from 0.015 (alloy) to 0.17 (top surface)).
XPScorelevelmeasurementsshowedthattheseelementsare inthemetallic(Au,Hg)orintheoxidisedstate(Sn,Zn)orunder bothstates(Cu,Pb).ThemetallicCu(0)componentinthe outer-mostlayerofthegildedsurfaceisconfirmedfromthecalculation
Table1
XPSatomicquantificationofas-gildedbronze(monochromatisedAlK␣(hv=1486.6eV)).Analysisofthetopsurfaceofthegilding,beforeandafterAr+ionetching.Each
valuewasnormalisedtoAuatomiccontent(Au=1).
Au4f7/2 Hg4f7/2 C1s N1s O1s S2p Cu2p3/2 Sn3d5/2 Zn2p3/2 Pb4f7/2
Metal Metal N C Sulfide/sulfate Cu◦ Cu(I)/Cu(II)oxide/Cu(II)hydroxide Sn(IV) Oxide Metal/oxide
Beforeetching PeakBE(eV) 84.0 100.0 284.6 399.7 531.9 162.4/169.0 932.5*/933.5/934.9 486.7 1022.1 136.9/138.6 at.% 4.9 0.5 74.0 2.4 15.4 0.7/0.4 0.5/0.3/0.1 <0.2 0.3 0.1/0.2 Afteretching(10sAr±) PeakBE(eV) 84.0 100.0 284.7 399.2 532.0 162.9/168.9 932.5*/933.4/934.9 486.8 1022.3 136.8/138.7 at.% 12.7 0.8 69.2 2.9 9.6 1.1/0.3 1.1/1.1/0.2 <0.2 0.4 0.1/0.3
Normalised(/Auat.%)
Beforeetching 1.00 0.10 15.10 0.49 3.14 0.14/0.08 0.1/0.06/0.02 <0.04 0.06 0.02/0.04 Afteretching 1.00 0.06 5.45 0.22 0.75 0.09/0.02 0.09/0.09/0.01 <0.01 0.03 0.01/0.02
* KE(CuLMM)=918.1eV˛=1850.6eV.
oftheAugerparameter(˛,Table1)andissignificantlyhigh.ForPb, oxidisedstateismainlyfoundbutaminorcomponentismetallic evenbeforeionetching,clearlyindicatingthepresenceofametallic phase,probablyalloyedwithHg,withinthegoldlayer.ForSn,the signalintensitywasatthelimitofdetection(around0.2at.%after ionetching),thereforetheexactoxidationstateandthepossible relativeenrichmentofSninthesurfacebycomparisontoCucould notbedetermined.
Theseresultspointoutthatthefire-gildingprocessinducedthe Hgenrichmentintheouterlayerofthegilding,butalsofavoured thediffusionofSnandZnbycomparisontoCu(inlay,Fig.3).The XPSdataalsohighlightthepresenceofmetallicphasesinvolvingPb butalsoCu.ThemetallicCu(0)componentonthetopgildedsurface remainsthemainoxidationstate,asconfirmedbytheevaluationof itsAugerparameter(˛—Table1):˛=1850.6eV(bothbeforeand afteretching),ingoodagreementwithCu(0)state(1851±0.5eV)
[56].
Thesealloyingelements(Sn,Zn),presentintheoutermostpart ofthegildinglayer,arepotentiallyandpreferentiallyreactiveto preferentialcorrosion[19].ThepresenceofmetallicSnandZnin theouterlayerofthegildingmayberelatedtostrongerinteractions oftheseelementswithbothAuandHg.Inparticular,bothSnandZn formmercuryamalgamsandintermetalliccompoundswithAuand Hg[57].Specifically,Au–Sndiffusionandcompoundsformationis well-documentedinthefieldofelectronicdevices[51].ForPb,only verylowamountsofmetallicPb(0.07at.%)weredetectedbyXPS afteretchingonthetopsurface(Table1).Pbwasdetectedonlyin verylowconcentrationbyGD-OESintheouterpartofthegilding layer(Fig.3,inlay),notshowingthesamesuperficialenrichment trendasSnandZn,notwithstandingtheabilityofthiselementto formHgamalgamandAu-basedintermetallics[57].Thelow con-centrationofPbintheouterpartofthegildingcouldbeduetothe lowamountandtotheinhomogeneousdistributionofPbinthe bronzealloy.
3.2. Artificiallycorrodedgildedbronze
Afterartificialageing,XPSmeasurementsonthetopofthegilded surface,beforeandafterAr+ionbombardment(Table2),revealeda highcontentoflightelementsbycomparisonwiththeuncorroded surface.ThemaindifferenceisrelatedtoanincreaseofC,OandS elements,duetocorrosioninducedbytheartificialrainsolution. Forcarbonandoxygen,amarkedincreasecorrespondingtoC O bindingisfound,whichcanbemainlyascribedtocarbonatespecies (Cat∼289.3eVandOat∼532eV)[36].Afteracceleratedageing,it wasalsofoundthatHgremainsinthemetallicstateinthetopgilded layer.
As regards the alloying elements of the bronze, Cu and Pb arealwaysfoundwithinthesurface.ForCu,themetallic compo-nentremainspresentaccordingtotheAugerparameterestimation
(Table2,˛=1850.6eVafteretching[56]).ForPb,several contrib-utionsarepresentandcanberelatedtooxidisedspecies(hydroxi-, sulphates and probably oxide) but also to metallic Pb, which remainsonthegildinglayerinaverylowamount(0.08at.%after etching).ForZnandSn,alwaysbycomparisontotheuncorroded sample,adecreaseoftheiramountswasobserved,asrevealedby theirnormalisedatomicvalues(/Au)ofTable1comparedtoTable2. Snisoxidised,butthesignalwasbarelydetectableandnoprecise informationcouldbegiven.However,theseresultsshowedthat acceleratedageinginducedadecreaseofZncontentonthetop surface,e.g.theoxidisedZnconcentrationbeforeageingwas0.03 (/Au),(Table1afteretching)anddecreased downto0.01(/Au),
Table2).NoremarkablevariationwasobservedforHg.Similarly, alsoCuwasnotsignificantlyaffectedafterageing:thenormalised Cuatomicratio(/Au)was0.19fortheas-gildedsample(Table1) and0.22whencorroded(Table2).Therefore,bothHgandCu, prob-ablyinsolidsolutionwithAu,remainedinthetoplayerandwere notcompletelydissolvedafterartificialageing.
Thecross-sectionsoftheartificiallycorrodedfire-gildedbronze revealed large craters under the fire-gilded layer as usually observedforthistypeofmaterials[9,10].Elementaldistributionis illustratedbyEDSX-raymapsinFig.4.Concerningthefire-gilded layer,thecontributionsofAu,HgandPbwerenoteasytoseparate onthebasisofEDSdata.Withinthecrater,corrosionwasmarked byadepletionofCuandZn,withconsequentrelativeenrichment ofSnandincorporationofoxygenspecies(Fig.4).
HR-SRPESinvestigationswereperformedonarepresentative corrosioncraterinthecorrodedgildedbronzesample.
Inparticular,tworepresentativepointswereidentified:pointA inthegildinglayerandpointBinthecorrosioncrater(seeFig.5for thelocalisationofpointsintheHR-SRPESmaps).
TheHR-SRPESanalysisofthegildedlayerconfirmedthe pres-enceofPbandCu,butalsoSnandZn(nearthedetectionlimit) withinthegoldlayer.Fig.6showscorelevelspectraforHginthe gildedlayeraftercorrosion,indicatingthatHgisstillpresentalso inthemetallicstate(BEHg4f7/2:99.8eV).Thesameinterpretation ofcorelevelwasappliedtoPb(BEPb4f7/2:136.5eV),notshownin
Fig.6.
Asregardsenvironmentalelementsinsidethegildedlayer,the overviewXPSspectradidnotshowthepresenceofSandCl;only CandOweredetected.ThepresenceofCandOinthegildedlayer couldberelatedtothecorrosionprocessbutalsotomountingresin contamination.
Asregards alloying elementsin the gildinglayer,HR-SRPES showedthepresenceofmetallicphaseswhichcanbestillpresent aftercorrosion,asalreadyobservedalsobyconventionalXPS sur-faceanalysisonlyonthetopsurface(Table2).Infact,intheupper partofthegildinglayer(atafewhundredsofnmfromthesurface, pointAinFig.5),anAu/Hgatomicratioofabout15wasdetermined byatomicquantificationfromcorelevelspectra(Table3),ingood
Table2
XPSatomicquantification(monochromatisedAlK␣(hv=1486.6eV)oncorrodedfire-gildedbronze(artificialcorrosion:TOW=30d).Analysisofthetopsurfaceofthegilding, beforeandafterAr+ionetching.EachvaluehasbeennormalisedtoAuatomiccontent(Au=1).
Au4f7/2 Hg4f7/2 C1s N1s O1s S2p Cu2p3/2 Sn3d5/2 Zn2p3/2 Pb4f7/2
Metal Metal Sulfide/sulfate Cu◦ Cu(I)/Cu(II)oxide/Cu(II)hydroxide Sn(IV) Oxide Metal/oxide
withoutetching PeakBE(eV) 83.8 99.9 285.1 399.6 532.5 161.8/168.9 932.3*/933.6/935.1 486.2 1023.1 136.8/138.0 at.% 4.7 0.2 66.1 2.3 22.7 1.3/1.0 0.5/0.5/0.3 <0.2(LD) 0.1 0/0.1 Afteretching(10sAr±) PeakBE(eV) 83.8 99.8 284.9 399.5 532.5 162.0/168.8 932.3**/933.5/935.1 486.7 1022.3 136.8/138.4 at.% 29.8 1.0 42.8 3.9 13.4 1.9/0.1 4.5/1.7/0.2 <0.2(LD) 0.2 0.1/0.2
Normalised(/Auat.%)
Beforeetching 1.00 0.04 14.06 0.49 4.83 0.28/0.21 0.1/0.1/0.06 <0.04 0.02 0.00/0.02 Afteretching 1.00 0.03 1.43 0.13 0.45 0.06/0.03 0.15/0.06/0.01 <0.01 0.01 0.00/0.01 LD:limitofdetection.
*KE(CuLMM)=918.1eV˛=1850.4eV−Cu◦/Cu+component. **KE(CuLMM)=918.3eV˛=1850.6eV−Cu◦.
Fig.4.Corrodedgildedbronze(runoffconditions,TOW=30days):SEM-BSEimage(topleftcorner)andEDSX-raymapsshowingelementaldistributioninthecrosssection ofacorrosioncrateratthegold/bronzeinterface.
agreementwiththeGD-OESconcentrationprofilesat300–500nm fromthetopsurface(Fig.3).
Insidethecorrosioncrater,theelementsdetectedapplying HR-SRPESwereC,O,Cu,Zn,Sn,SbandPb,whileSandClwereonly foundattracelevelsandwillnotbeconsideredinthefollowing discussion.
HR-SRPESmapsinFig.5showtheAu,Cu,SnandPb elemen-taldistribution ata submicrometric scale.It is clearly revealed thattheformationofthecorrosioncraterisrelatedtoa discon-tinuitywithinthegoldlayer,asshownbytheAumap.Another
importantpointis thatSnandCuHR-SRPESmapsclearly high-lightaSnrelativeenrichmentinducedbydecuprification(internal oxidationofthebronzeandCuselectivedissolution[58]). Decupri-ficationisdemonstratedalsobymeasurementsofCureleaseinthe ageingsolutions,reportedin[5,59].
Moreover,thePbmapinFig.5confirmsthatPbisstillpresent intheAu-richlayeraftercorrosion.
Moredetailed chemicalinformation ontheelementsspecies involvedinthecorrosionlayersinsidethecraterwasobtainedfrom corelevelanalysis.Inparticular,quantificationfromcorelevelswas
Fig.5. Cross-sectionofcorrodedgildedbronze(samecraterasinFig.4);SEM-BSEimage(topright)andHR-SRPESmapswiththedifferentkineticenergyranges:Cu 764–785eV;Au754–764eV;Pb696–715eVSn345–366eV;combinedimageCu,AuandSn.TheHR-SRPESmapsweremeasuredintheareaoutlinedinredintheSEMimage. Thedottedlinemarksthemountingresin/gildinglayerinterface.
Table3
XPSatomicquantificationoncorrodedfire-gildedbronze(artificialcorrosionTOW=30d),dataobtainedbyHR-SRPESat850eV.Cross-sectionanalysisofatypicalcorrosion crater,afterAr+ionbombardment.PointAcorrespondstothegildinglayer(at300–500nmfromthetopsurface)andpointBcorrespondstothecorrosioncrater(seeFig.5
forthelocalisationofpointsinthemaps).EachvaluehasbeennormalisedtoAuatomiccontent(Au=1).
PointA(gildinglayer)
Au4f7/2metal Hg4f7/2metal C1s O1s Pb4f7/2metal Sb3d3/2metal
PeakBE(eV) 84.0 99.8 284.5 530.5 136.5 527.3
at.% 12.2 0.8 75.0 11.8 0.2 0.1
Normalised(/Auat.%) 1.0 0.06 6.1 0.96 0.01 0.0 PointB(corrosioncrater)
Au4f7/2metal C1s O1s Cu3p
metal Sn3d5/2oxide Sn3d5/2metal Pb4f7/2 metal Pb4f7/2oxide Sb3d3/2metal PeakBE(eV) 84.0 284.8 530.8 75.3 486.5 484.8 136.9 138.7 527.8 at.% 0.3 37.4 14.1 46.5 0.9 0.5 0.2 0.1 0.2 Normalised(/Auat.%) 1.0 149.4 56.4 185.9 3.7 1.9 0.6 0.5 0.6
performedonpointBlocatedinsidethecrater(Fig.5)andatomic quantificationresultsare reportedin Table3.Aprecise attribu-tionanddeconvolutionofthecurveswascomplex,notonlydue tothepresenceofnumerousspecies,butalsobecauseprolonged Ar+cleaning,carriedoutinordertoreducecarboncontamination fromthemountingresin,inducedanimportanteffectofreduction, accordingtotheatomicbalanceobtainedfromcorelevelsanalysis forpointB(Table3).ItwasalsorevealedthatAuwasdetectedin averylowamount,probablylinkedtothemetallographical prepa-ration.
ArepresentativeexampleoftheinfluenceofAr+cleaningcanbe observedinHR-SRPESmapsforSnspeciesinthecorrosioncrater, reportedinFig.7(a–c).Analysisofcorelevelspectrashowedthat themaincontributionwasduetooxidisedSnspecies(Sn(ox);BEat 486.5eV),likelytobeascribedtothehighestoxidationstate(+4).It alsorevealedthepresenceofSninitsmetallicstate(BE484.8eV). AsalsometallicSn(0)wasidentifiedinsidethecorrosioncrater,this
chemicalstateshouldbeconsideredasanartefactduetoapartial reductionofoxidisedSn,linkedtotheAr+cleaningstep.Thiskind ofartefactwasalreadyobservedbyotherauthors:thereduction phenomenonwasalsoobservedforPb[21,40,42,60],butpossibly alsoforCu(3p)[22,26].
Inthesamearea,ObondedwithCwasdetectedwithinthe cor-rosioncrater,asshowninFig.8.Also,aremarkableeffectofcarbon pollutionwithahighintensitycorrespondingtoseveralC Cand C O(284.8eVand286.2eV)compoundswasobserved,likelydue tomountingresincontamination.Thus,unfortunately,inthiscase, apreciseattributionofpeaksfromspecificbindingenergieswas notachievable.ForpointB(Fig.5,Table3),basedonthecorelevel spectrum(Fig.8d),theO1sregionwasfittedwiththreepeaksat aboutBE=529.8eV(O2−),BE=531.7–531.8eV(OH−,O C O)and BE=533.1eV(C OandH2O)[61].Thespatialdistributionofeach oxygenspecieswasreportedinHR-SRPESmapsofFig.8a–c.In thiscase,themapsshowedthatonincreasingdepth,thelow-BE
Fig.6.Hg4f7/2curve-fittedregionswithinthegoldlayer(ataround300–500nm
fromthetopsurface(pointAinFig.5)–corrodedgildedbronze–cross-section).
componentgainsintensityattheexpenseofhigh-BEcomponents. Inparticular,H2O(533.1eV)ismainlylocatedintheupperlayersof thecrater(nearertothemetal/environmentinterface),whilstOH− (531.7eV)andO2−(529.8eV)concentrateinthemostinternal lay-ers.ThepresenceofawidespreaddistributionofOH−canbealso relatedtothecontaminationofthemountingresin(becauseofthe overlappingofsignalsfromO C ObondsintheresinwithOH−
[61]).Bycomparisontotheexpectedmetallicoxidisedspecies,the amountofoxygenisveryhigh,probablyduetotheabove men-tionedcontaminationeffects(differentO ClinkedtoO Hbonds weredetected).
However,evenifspecialcarehastobetakenwhenpreparing sampleinordertoavoidsuchcontaminationeffects,thecombined imageinFig.8eallowstohighlightthespatialdistributionof differ-entoxygenspeciesthroughoutthecorrosioncrater.Interestingly, itshowsthatthemainspecieswithinthecrater,involvedinthe corrosionprocess,couldberelatedtotheformationhydroxi-oxide compoundsratherthantopureoxide.Forbronze(Cu–Snalloys), thishastoberelatedtotheoxidationofthealloy,linkedwiththe selectivedissolutionofthemainalloyingelements.Aspointedout incaseofatmosphericcorrosion[62],butalsoincaseofanodic corrosion[30]whichistypicallythecaseofgalvaniccouplingwith gold,Snspeciesarenotdissolvedintheenvironmentbutremain intheinnerlayerasaninsolubleproductwhichissuspectedtobe (hydrated)hydroxi-oxides[62].Thesespeciesaresupposedtoform anetworkasforSnoxidexerogel,throughwhichcopperionsand anionsmigratetowardsthesurface[30].
Fig.8HR-SRPESmapsofacorrosioncraterofafire-gildedbronze (sameasforFig.4).Oxygenspatialdistribution(O1s)measuredat differentkineticenergy ranges:(d)corelevelofOxygen 1s cor-respondingtoa typicalpointin thecratercorrosion(pointBin
Fig.5),revealing differentbindingenergiesin relationwith dif-ferentchemical statesand(a)O2−313.8–315.0eV;(b)OH− and O C O313.0–313.8eV;(c)C OandH2O310.8–312eV,soasto
Fig.7.HR-SRPESmaps(measuredinthesamecorrosioncraterofFig.4)withdifferentkineticenergyranges:(a)Sn0359–360eV;(b)Sn4+356.8–358eV,(c)combinedimage
ofthetwodifferenttinspeciesshowninaandb,soastohighlightthedistributionofdifferenttin-basedspecies;corelevelofSn3dcorrespondingtoatypicalpoint(point BinFig.5)inthecorrosioncrater,revealingdifferentbindingenergiesinrelationwithdifferentchemicalstates.
Fig.8.HR-SRPESmapsofacorrosioncraterofafire-gildedbronze(sameasforFig.4).Oxygenspatialdistribution(O1s)measuredatdifferentkineticenergyranges:(d)core levelofOxygen1scorrespondingtoatypicalpointinthecratercorrosion(pointBinFig.5),revealingdifferentbindingenergiesinrelationwithdifferentchemicalstates and(a)O2−313.8–315.0eV;(b)OH−andO C O313.0–313.8eV;(c)C OandH2O310.8–312eV,soastohighlightthedistributionofdifferentoxygen-basedspecies;(e)
combinedimageofthethreedifferentoxygenspeciesshownin(a–b–c).
highlightthedistributionofdifferentoxygen-basedspecies;(e) combinedimageofthethreedifferentoxygenspeciesshownin a–b–c.
4. Conclusions
Thespatiallyresolvedapplicationofsynchrotronradiation high-resolutionphotoelectronspectroscopy(HR-SRPES)wasfoundto beverypromisingforunderstandingcomplexstructureasthose foundonculturalheritagemetallicmaterials.Contaminationdue tomountingresinandconsequentelementreductionduetosputter cleaningherewereidentifiedasamaincriticalissues.
From this investigation of artificially corroded fire-gilded bronze,severalmainconslusionscanbedrawn:
• Heating during the fire-gilding procedure hasbeen found to inducearemarkableenrichmentinSnandZninthetoppartofthe gildedlayer.Thisnewresulthastobecarefullytakenintoaccount forconservationpurposesofthesebronzes,asthesechemical ele-mentsusuallyexhibitdifferentcorrosionbehaviourinoutdoor conditions.
• HR-SRPES,appliedforthefirsttimeoncorrodedgildedbronze, alloweddetailedmappingofthespatialdistributionand oxida-tionstateofelementssuchasSnandO.
• MetallicphasesincludingHgandPbstillremaininthegilding aftercorrosion.
• SelectivedissolutionofCu andalsoZnoccurredin the corro-sioncratersduetogalvaniccoupling,whichinducesrelativeSn speciesenrichment(decuprification).
• Thedistributionmappingofthechemicalstatesrevealedthe for-mationofhydroxi-oxidecompoundsofSnandO.
Acknowledgements
This research has been financially supported by PRIN 2009 MIUR Funds (Italy), grant number is 2009L55TFF003. The
financial support of Emilia-Romagna Region, Italy (POR-FESR funds)aswellasoftheFP7-CALIPSO(CoordinatedAccessto Light-sourcestoPromoteStandardsandOptimisation),grantnumberis 20140530TransnationalAccessProgrammeisgratefully acknowl-edged.Theauthorswish tothank:Mr.AlessandroPacini (Mon-tepulciano,Italy) forproducing fire-gildedbronzesaccordingto ancientrecipes(https://sites.google.com/site/archeometallurgia/). Dr. Iuri Boromei, at Dept. Industrial Engineering,University of Bologna, for his valuable help in sample preparation and for SEM/EDSinvestigations.AllthepersonnelfromtheSOLEIL syn-chrotron facility and the IPANEMA laboratory (Gif Sur Yvette, France)isgratefullyacknowledgedfortheirveryeffectivetechnical support.
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