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Original article

Evaluation of vibrational spectroscopic techniques for consolidants’

penetration depth determination

Polonca Ropret

^a,b,∗

, Lea Legan

^a

, Klara Retko

^a

, Tanja ˇSpec

^a

, Andreja Pondelak

^c,d

, Luka ˇSkrlep

^c

, Andrijana Sever ˇSkapin

^c

aConservationCentre,InstitutefortheProtectionoftheCulturalHeritageofSlovenia,Poljanska40,1000Ljubljana,Slovenia

bMuseumConservationInstitute,SmithsonianInstitution,4210SilverHillRd.,Suitland,MD20746,UnitedStates

cSlovenianNationalBuildingandCivilEngineeringInstitute,Dimiˇceva12,1000Ljubljana,Slovenia

dUniversityofLjubljana,FacultyofChemistryandChemicalTechnology,Veˇcnapot113,Ljubljana,Slovenia

a r t i c l e i n f o

Articlehistory:

Received23November2015 Accepted14July2016 Availableonline12August2016

Keywords:

Penetrationdepth Consolidants Carbonate Silicate

RamanandFTIRspectroscopy

a b s t r a c t

Thepenetrationdepthofconsolidantsappliedtoculturalheritageobjectsplaysacrucialroleinasuc- cessfulconservationandprotectionofthem.IntheframeofHEROMATFP7projectnewconsolidants forcarbonateandsilicatebasedmaterialsweredeveloped.Amongmanyotherinvestigatedproperties, thepenetrationdepthwasdefinedbyRamanandFTIRspectroscopies,forwhichtheirabilitywasalso evaluated.Duetotheformationofcalciumcarbonateintheconsolidationprocessofcarbonateforming consolidants,theadditionofsodiumnitroprussideindicatorsupportedRamandifferentiationoftreated andnon-treatedareasinthecalciumcarbonatebasedsubstrate.Furthermore,thecombinationofthe indicatorreactionandRamanresultsgavemuchmoreprecisepenetrationdepthestimationthanthe visualassessmentalone.ForfollowingthepenetrationdepthofmodifiedTEOSbasedconsolidantsfor silicatebasedsubstrates,FTIRspectroscopyturnedouttobeverysuccessfulwithoutanyindicatorappli- cationorpre-treatmentofsamples.Furthermore,thepenetrationdepthrelatedtodifferentapplication methods,suchasbrush,cellulosepulp,airlesssprayandroller,wasalsostudied.Thedeepestpenetration wasachievedby8hofapplicationofconsolidantsincellulosepulp,whileincomparingoneapplication byroller,airlesssprayandbrushwithinthesamesubstrate,thedeepestpenetrationcanbeachievedby brush.

©2016ElsevierMassonSAS.Allrightsreserved.

1. Introduction

Sincethemiddleofthelastcenturydifferentsyntheticpoly- mershavebeenusedforconsolidationoflimestonesurfaces.Due totheiraestheticalalteration,changeinthepolymerssolubilityand degradation,whichcouldalterthephysico-chemicalproperties, suchascreaking,flakingandglossyeffect,theirpotentialapplica- tionishindered.Inthisrespect,theuseof“compatible”materials ispreferablesincethis waydurabilityand long-termstability is granted[1,2].Inthelastdecademuchoftheresearchhasbeenmade inthefieldofsynthesisofmacro-andnano-particlesofCa(OH)₂in differentalcohols[1].DispersionsofCa(OH)₂ nanoparticleshave beenproventobeeffectiveforconsolidanttreatmentduetotheir smalldimensionsandeaseofthepenetrationaswellasincreased

∗Correspondingauthorat:ConservationCentre,InstitutefortheProtectionofthe CulturalHeritageofSlovenia,Poljanska40,1000Ljubljana,Slovenia.

E-mailaddresses:polona.ropret@zvkds.si(P.Ropret),lea.legan@zvkds.si (L.Legan),klara.retko@zvkds.si(K.Retko),tanja.spec@zvkds.si(T. ˇSpec), andreja.pondelak@zag.si(A.Pondelak),luka.skrlep@zag.si(L. ˇSkrlep), andrijana.skapin@zag.si(A.Sever ˇSkapin).

reactivitytowardsCO2[2,3].However,therestillexistimportant obstaclesfortheirusesuchasthevariableparticlesize,incomplete carbonization(duetotheneedofatmosphericCO₂)andthelow concentrations(5–25g/L)[1,3].Infact,aconsequenceofthelow concentrationandvolatilityofsolventsusedinconsolidantsisthat multipleapplicationsareneeded[1,2].

Foreffectiveconsolidationof deeperlayersofdegradedsub- strates,useofsolublestartingmaterialsisdesirable.However,since thesolubilityofcalciumcarbonateislow,thepreparationofsucha materialisverydifficult[4].InthescopeofHEROMATFP7project anewsolutionofcalciumacetoacetateforconsolidatingcarbon- atebasedsubstrateswasdeveloped[5].Sincetheconsolidantis a solution,it canpenetrate deeperintothematerial,wherere- cohesionbetweenparticlescanbeestablished.Additionally,dueto higherconcentrations(from5–100g/L)thenumberofconsolidant applicationscanbestronglyreduced.

Ontheotherhand,forconsolidationofsilicatesubstratescon- solidantsbasedonalkoxylanesor tetraethoxysilane(TEOS)are used[6,7].Theycanhavea pooraffinitytomaterialswithlarge pores,forexamplesuchasfoundinsomesandstones.Namely,dur- ingthehardeningphaseorundermechanicalstressthickandbrittle http://dx.doi.org/10.1016/j.culher.2016.07.004

1296-2074/©2016ElsevierMassonSAS.Allrightsreserved.

silica-gelstructuresof consolidantsfilltheporeswithsilica-gel resultinginamaterialthatislesspermeabletothewatervapour [8,9].For thesilicate basedsubstratesmodified formulationsof commercialproductbasedonsilicateesterwasdevelopedinthe scopeofHEROMATFP7project.Duetoitsbalancedcombinationof polysilicate,dioxalane,mixtureofC11-C13alkanes(liquidparaffin) anddiethylethanolamine,theconsolidanthasalowdrymassand thereforedoesnotreducetheporosityofconsolidatedmaterial.

Furthermore,ithasagoodretainedwatervaporpermeability,effi- cientanduniformconsolidationthroughtheprofileofthesubstrate withoutskinforming.

Chemicaland/or physical response of the substrate(i.e. cal- careous and silicate materials) varies strongly due to their heterogeneousproperties(e.g.differentporosity, distributionof components).Therefore,tovalidatetheconservationtreatmentof thematerialscomposingculturalheritageobjects,acarefulselec- tionoftheapplicationmethodoftheconsolidantandtheanalytical techniquefor monitoringand assessingtheperformance ofthe consolidationshouldbemade.Thenon-destructivemethods,in which a sample is left intact,such as Environmental Scanning ElectronMicroscopy(ESEM-EDS),spectrophotometry,ultrasound velocity,NuclearMagneticResonance(imagingandrelaxometry) andOpticalSurfaceRoughnessanalysiscouldofferaplatformto determinenotonlythechemicalcharacteristics,butalsothemor- phological, physical and the hydricproperties of thematerials beforeandafterconsolidation[10].Oneoftheparametersbesides thecompatibility,durability,effectonappearance[11]etc.,which shouldbetakenunderconsideration,whenevaluatingnewcon- solidantformulationsisalsothedepthofconsolidantpenetration.

Regardingcompatibilityoftheconsolidantwiththematerialbeing consolidated,theinsitucarbonationusingthesolutionsofcalcium hydroxideisgainingmuchattention,ashasbeenpreviouslymen- tioned.Despitethefactthattheformedcarbonatecouldexistin differentgrainsizeanddifferentcrystal-aggregatetexture[11],itis difficulttodifferentiatethemfromtheoriginaloneconstitutingthe culturalheritageobject.Withtheassumptionthatthemagnesium hydroxideparticleshavethesamecapabilityasthecalciumhydrox- idefor thepenetration, Dei andSalvadori [12] showedthat by usingthemagnesiumhydroxideasthemarker,SEM-EDXanalysis andsuperficialareaanalysis(BET)couldbeusedforfollowingthe depthofpenetrationoftheconsolidantinadditiontowaterabsorp- tioncapillaritymeasurementsforevaluationofwater-interaction properties.PintoandRodriguez[13]quantifiedtheconsolidation effectandtheroleoftreatmentprocedures(i.e.bycapillaryabsorp- tion,byimmersion andbybrushing)foron-sitebuildingstones (monuments)incomplementationtolaboratorystudiesofdifferent carbonatesampleswithdifferentintrinsicpropertiesbyweighing, microdrillingresistance(DRMS),ultrasonicvelocityandflexural strength.Inaccordance,alsothepenetrationdepthcouldbecon- sequentiallycorrelated withthose results. Moreover,Pinto and Rodriguez[14]showed,thatDRMSmethodologyisoneofthemost promisingfortheassessmentoftheconsolidationrateinthesoft stones,whilethedepthofthestrengtheningactioncouldbefurther successfullydeterminedbyothertechniquessuchasbythecollec- tionoflongitudinalultrasoundvelocityprofiles.Forevaluationof penetrationdepthofthepolymerconsolidantwithinporousstone substrates,severaldirectanalyticalmethodssuchasstainingtest usingforexampleiodinevapor[15]orfluorescentdyesforvisu- alizingthelocationoftheconsolidant,Fouriertransforminfrared spectroscopy(FTIR),SEM-EDXand XPS,andalsoindirect,which includesmeasurementsofcontactangle,bendingstrength,mod- ulusofelasticityetc.couldresultingainingvaluableinformation oftheinteractionbetweentheconsolidantandthesubstrate[16].

␮Raman mapping proved to be an effective tool for assessing thepenetrationdepthofconsolidantproductsbasedonoxalate formation.Consolidationwithammoniumoxalate(oxammite)in

differentplastersmadeoflimeandcarbonateaggregatewasstud- iedbyConti etal.[17].Itwasshown thatduringconsolidation process,oxammiteinducesthetransformationofcalciumcarbon- atetocalciumoxalate(whewellite),whichgivesabetterresistance todecayphenomena.Strongandcharacteristicbandsat1462cm⁻¹ (whewellite)andat1085cm⁻¹(calcite)havebeenconsideredfor following thepenetration depthby ␮Ramanmapping. Further- more,neutronradiographyandtomographyallowthevisualization ofpolymerizedaswellasnon-polymerizedconservationmaterials insidetheporousmaterials.WiththesemethodsCnuddeetal.[18], Diericketal.[19]andMasschaeleetal.[20]successfullydefinedthe impregnationdepthofsilicate-basedmaterialsinsidethenatural buildingstonesandresultingeffectsontheuptakewater.Alter- nativetechniquefortheinvestigationofdepthprofile,whichalso allowsthevisualizationofthepresenceanddistributionofdiffer- enthydrophobicproductinstonematerial,ismagneticresonance imaging (MRI) [10,21].The structure of naturalbuilding stones aswellas thevisualizationof consolidantsand/orwater repel- lentswithinthestonescouldbeinvestigatedusingnon-destructive X-ray radiography computed with micro-tomography [22]. As theresolution ofmicro-tomography dependsontheamountof treatmentmaterialsinsidetheinvestigatedsample,theaddition of contrast agent (e.g. 3-bromopropyltrimethoxysilane) is often needed[22].Anothernon-destructivemethodforobservationof penetrationdepthisthetabletophigh-resolutionX-rayradiogra- phy,whichbecameusefulaftertheintegrationofpixeldetectorsof Medipixtype.Thistechniquehasprovedtobeanoptimaltoolfor observationofpenetrationdepthoforganosiliconconsolidantsin theOpukastone[23].

WhenusingRamanandFTIRspectroscopy,whichareusually available inconservation laboratories, thedifficultiesmayarise duetotheoverlappingofmodesofconsolidantsandthematrix.

For that reason our study was focused on exploring theabili- tiesofvibrationalspectroscopictechniquesforpenetrationdepth evaluation,wherethesamecompositionisformedintheconsoli- dationprocessasitispresentinthematrixofthematerialthatis consolidated.

2. Researchaims

Inthiswork,intheframeworkofHEROMATFP7project,spectro- scopictechniques,RamanandFTIR,wereevaluatedforfollowing thepenetrationdepthofdevelopedcarbonateandsilicateform- ingconsolidantsforcarbonateandsilicatesubstrates,respectively.

This,however,wasspecificallychallengingsincethesamecompo- sitionisformedintheconsolidationprocessasitispresentinthe matrixofthematerialthatisconsolidatedthatcanresultinover- lappingofspecificvibrationalmodes.Toovercomethedifficulties, inthecaseofcarbonateconsolidantsanindicatorsodiumnitro- prussideisproposed,whileinthecaseofthesilicateconsolidant, thenewformulationitself,consistingalsoparaffin,offersthesolu- tion.Furthermore,differentapplicationmethods,usuallyapplied inrestorationorinbuildingsrefurbishmentweretested.Thework wasdoneonpreparedmodelsubstratesthatmimicthecompo- sitionofoutsidematerialsatDornavaManor,oneoftheselected testsitesoftheHEROMATproject,inordertoprovideresultsrele- vantforlateron-siteapplicationof,withintheproject,developed consolidants.

3. Experimental

3.1. Consolidants

Forcarbonatesubstratesnewconsolidantformulations,thor- oughlydescribedinthepatentPCT/SI2014/000028[5]andbasedon solublecalciumcompoundcalciumacetoacetateCa(OAcAc)2were

developedinthreedifferentsolvents:methanol(CF1),mixtureof ethanolandmethanol(CF2)andwater(CFW).Additionallycatalyst (0.05wt.%ethylenediamine,99+%,AcrosChemicals)wasaddedinto consolidantbeforetheapplication.

For silicate substrates new consolidant formulation HERO- MAT CF4 was prepared by mixing ethyl polysilicate (WACKER TES40WN,WackerSilicones),solvent(1,3-dioxolan–DOX,99%, SigmaAldrich),catalyst(diethylethanolamine–DEEA,99.5%,Sigma Aldrich)and the mixtureof C11-C13 alkanes(paraffin, Samson Kamnik, CAS: 64742-48-9). Catalysts were always mixed into ethanol and added last to the rest of the mixture. Mixture of C11–C13alkanes(liquidparaffine)wereusedtoreducethesolubil- ityofcatalystandvarioushydrolysedspeciesofethylpolysilicate aftertheevaporationof1,3-dioxolane.Thiswouldpreventforma- tionofthegelinthewholevolumeofthepore.MixtureofC11–C13 alkanes(liquidparaffine)haslowvolatility,butwillevaporatefrom thesubstrate.

3.2. Samples

3.2.1. Carbonatesubstratesamples

Onthebasisofpetrographicanalysisofasampletakenfromthe outsidecarbonatesurfaceofthegardenfenceatDornavaManor themostappropriatequarryforaggregateswasselected,andchar- acteristicsoflimeandcementwere defined.Thesilicate gravel fraction0/4mmfromHoˇcequarry wasacquiredandsieved out togetsimilargradingcurveastheaggregatefromtheanalyzed carbonatesample.Forbindercleanpucolancementwasused.In ordertoweakenmechanicalproperties,apartofthecementwas replacedwithaspecial pucolanadditive.Alsotraditionallypre- paredslakedlimefromlocalproducerswasused.Thesubstrates werepreparedcombiningcoarseandfinecarbonatetoachievethe appropriatethickness.Onapproximately3.5cmthickcoarselayer 3–4mmthicklayerof finecarbonatewasapplied.Steelmoulds of16cm×4cm×4cmand15cm×15cm×4cmdimensionswere usedfor modelspreparation. The modelswerethen cut outto therequireddimensionsfortesting.Wettingandcuringwasdone foraminimumof14daysbeforethecuttingwasperformed.The porosityofthecarbonatemodelsubstratewas39.34%,measured byHg-porosimetry.

3.2.2. Silicatesubstratesamples

AbulksampleofsilicateknownasPtujskaGorasandstoneor VunduˇseksandstonewastakenfromtheJelovicequarry,whichis locatednearthetownofMajˇsperk,intheeasternpartofSlovenia.

ThissilicatebelongstotheMiddleMiocenegeologicalperiod.It isacoarsegrained,moderatelywellsortedsandstone,comprising quartz,dolomite,feldspar,muscoviteandunusualmineralssuch aszirconand apatite.It alsocontainsafinegrained porefilling matrix/cement of a high Fe content, of uncertain mineralogy.

Secondaryinterstitialcalcitecementisalsopresent.Theporosity ofthesilicatewas14.20%,measuredbyHg-porosimetry.

3.2.3. Preparationofsamplesforpenetrationdepthinvestigation For investigationof the depthof consolidation in carbonate substrates three different consolidants (CF1, CF2, CFW), devel- opedwithinHEROMATproject,wereapplieddirectlyontheupper surfaceofthecarbonatemodelsubstrates.Forthesilicatemodel substrateasilicateformingHEROMATCF4consolidantwasused.

Themodelsubstrateswerethencut acrossthedepthprofilein ordertoprovideasuitablesurfaceforthepenetrationinvestigation.

Table1gathersthedescriptionofinvestigatedsamplesrelatedto differentdevelopedconsolidants.Thepenetrationdepthsofconsol- idantsweretestedonthecarbonateandsilicatesubstrates.Inthe caseofcarbonateformingconsolidantsindicatorsodiumnitroprus- side,Na2[Fe(CN)5(NO)]·2H2O(5%watersolution;SigmaAldrich)

Table1

Investigatedsamplesrelatedtodifferentdevelopedconsolidants.

Sample Consolidant Consolidantcomposition

Carbonate CF1 9.6wt.%Ca-acetoacetateinmethanol Carbonate CF2 8.4wt.%Ca-acetoacetatein

ethanol/methanol(9:1) Carbonate CFW 9.6wt.%Ca-acetoacetateinwater Silicate CF4 Diethylethanolamine(DEEA)5wt.%,

1,3-dioxolan(DOX)20wt.%,alkanes C11–C13(P)25wt.%,ethylpolysilicate 50wt.%

wasappliedoverselectedareaofthepenetrationdepthprofileof eachsubstrateaftercuringperiod(threedaysatRT).Intheperiod offewminutestheredcolourappeared.AllRamanmeasurements ofpenetrationdepthwereperformedwithin1h(theredcolour wasvisiblemorethan1h).Thechemicalidentificationofthemate- rialcomponentswasperformedonthepenetrationdepthprofileof thecarbonatesubstratesusingmicro-Ramanspectroscopy.Inthe caseofsilicateformingconsolidantFTIRmicroscopywasusedon thepenetrationdepthprofileofthesilicatesubstrateswithoutany indicatorapplication.

3.3. Applicationmethods

Oneofthedevelopedcarbonateformingconsolidants(CF1)was applieddirectlyontheuppersurfaceofthecarbonatemodelsub- strate,and silicateformingconsolidantCF4 wasappliedonthe silicatemodelsubstrate,usingdifferenttechniques:roller,airless spray,brushandcellulosepulp.

Table2gathersthedescriptionofinvestigatedsamplesrelated todifferentapplicationmethodused.Theapplicationofthecon- solidantinthecellulosepulpwasdonebythefollowingprocedure:

8.6gofcellulosepulpwasmixedwith43gofconsolidant(previ- ouslymixedwithcatalyst)andlaidoverthesubstratewhichwas coveredwithJapanesepaper.Thicknessofthepulpcontainingthe consolidantwasapproximately1cm.Toinvestigatethetimeofthe applicationinfluenceonthedepthofpenetration,thepulphadbeen appliedoverthesubstratefor1and8h,andthenremoved.Sam- pleswerethenlefttodryforthreedays.Inthecaseofthecarbonate modelsubstratealsodifferentnumbersofapplicationsbythesame methodweretested.

3.4. Instrumentation 3.4.1. Ramanspectroscopy

Raman analysis of consolidated carbonate model substrates was performed using 633 and 785nm laser excitation lines witha Horiba Jobin YvonLabRAM HR800Raman spectrometer

Table2

Descriptionofinvestigatedsamplesrelatedtodifferentapplicationmethodused.

Sample description

Application technique

Numberof applications/time ofapplication

Consolidant

CarbonateR1 Roller 1 CF1

CarbonateR2 Roller 2 CF1

CarbonateR3 Roller 3 CF1

CarbonateA1 Airlessspray 1 CF1

CarbonateA2 Airlessspray 2 CF1

SilicateA3 Airlessspray 1 CF4

CarbonateB1 Brush 1 CF1

SilicateB2 Brush 1 CF4

CarbonateP1 Cellulosepulp 1h CF1

CarbonateP2 Cellulosepulp 8h CF1

SilicateP3 Cellulosepulp 1h CF4

SilicateP4 Cellulosepulp 8h CF4

coupledtoanOlympusBXFMopticalmicroscope.Thespectrawere recordedusing×50lwd(longworkdistance) objectivelensand 600grooves/mm grating,which gave thespectral resolution of 1.3cm⁻¹/pixeland0.83cm⁻¹/pixel,respectively.Thepoweratthe sampleswassetbetween0.1and35.8mWusingneutraldensity filters.Amulti-channel, aircooledCCDdetectorwasused,with integrationtimesbetween5and20s,andthespectralrangewasset between50and4000cm⁻¹.Thewavenumbercalibrationwasper- formedusingasiliconwafer.Spectraobtainedonsamplestreated withCF1andCF2aretranslateduponverticalaxisforeaseofcom- parison,andthespectraobtainedonsubstratestreatedwithCFW arebaselinecorrected(LabSpec,polynom8degrees)andtranslated uponverticalaxisforeaseofcomparison.

3.4.2. FTIRspectroscopy

FTIRanalysisofconsolidatedsilicatemodelsubstrateswascar- riedoutwithaPerkinElmerSpectrum100FTIRspectrophotometer coupledtoaSpotlightFTIRmicroscopeequipped withnitrogen cooledMCTdetector.Spectrawererecordedin reflectionmode overa4100–500cm⁻¹wavenumberrangeat4cm⁻¹spectralreso- lutionover64scans.Thespatialresolutionwasabout50␮m².After applyingtheCF4consolidantFTIRanalyseswereperformedonthe penetrationdepthprofilesofsilicatesubstrates.

4. Resultsanddiscussion

4.1. EvaluationoftheusefulnessofRamanspectroscopyin carbonatesubstrates

Theprincipleofconsolidationwithnewconsolidantisthespon- taneouslydecarboxylationofcalciumacetoacetateinthepresence of water what leadsto calcium carbonate, carbon dioxide and acetone [5]. Decarboxylation of calcium acetoacetate is greatly influencedbythetreatmentconditions,suchastemperatureand

relativehumidity.Ontheotherhandbyvaryingtreatmentcondi- tions,arangeofdifferentcrystallinepolymorphscanbeobtained.

Xuetal.[24]reportedthatamorphousCaCO₃hasbeenstablemore thanthreemonthsunderroomtemperatureanddryconditions, whileSpanosetal.[25]mentionedalongtime(severalmonths) solid-state vaterite transformationinto furtherpolymorphs. For thisreason,weassumedthatinpresentedexperiments,allthree developedconsolidantvariations,designatedasCF1,CF2andCFW, formunstableamorphous CaCO₃,which transformintovaterite form.

Anotherpolymorphofcalciumcarbonate,calcite,isthemost commonformpresentinculturalheritageobjects,andisacon- stituentofthepreparedmodelsubstrates.VateriteexhibitsRaman bandsat305,671,1078,1089cm⁻¹ [26],whilecalcitebandscan befoundat156,283,713,1087cm⁻¹[27].

The strongest bands assigned to non-degenerate symmetric stretchingvibrationsofvateritecarbonateion(␯1)whichappear at1078andat1089cm⁻¹couldpotentiallybeselectedtofollow thepenetrationdepthofconsolidants.Theotherbandstypicalfor vateritehaveweakintensityusingthechosenexcitationlineandin additioncouldbehinderedbythefluorescenceemissionand/orby theinterferingsignalbelongingtoothercomponentsofthehet- erogeneoussubstrate.Moreover,thenon-degeneratesymmetric stretchofcalcitecarbonateion(␯1)at1086cm⁻¹ causesdifficul- tiesintheinterpretationduetothepossibleoverlappingwiththe signalofvaterite.

It is worth toemphasize that transformation of amorphous CaCO3 tovateritephase isslowandtransformation maynot be uniformthroughthedepthof thesubstrate,thereforefollowing thevibrationsofvateritecarbonateiontodeterminepenetration depthisquestionable.Theevaluation ofpenetrationdepthonly throughthevateriteRamananalysissuffersofalimitation,notonly forthevaterite/calcitebandsoverlapping,butalsobecausewecan- notexcludeacalcitecrystallizationinducedbythereaction,which

Fig.1. Penetrationdepthofcarbonateconsolidantappliedincellulosepulpfor1and8h:(a)Ramanspectraafterconsolidantandindicatorapplicationrelatedtodifferent penetrationdepth;(b)crosssectionofthetreatedcarbonatemodelsubstrate(8hofconsolidantincellulosepulp)aftertheindicatorapplication(0=633nm).Obtained bluecolourisaresultofreactionbetweenindicator,calciumacetoacetateandcalciumhydroxideduetonotfullycarbonizedsubstrate.

cannotbedistinguishedfromthecalciteofthesubstrate.Forthis reason,followingthepenetrationdepthwithtracingcalciumace- toacetateseemsabettersolution.Itturnsoutthatsufficientamount ofcalciumacetoacetateisunreactedunderusedconditions.Forfast andsimpleidentificationofpenetrationdepthofconsolidantsCF1, CF2andCFWweintroducethecombinationofindicatormethod andRamanmappingofacetoacetate.BeforeRamananalysisindi- catorsodiumnitroprussidewasappliedonconsolidatedsample.In theconsolidationprocess,calciumacetoacetatedecomposesinto calciumcarbonate,carbondioxideandacetone.Itisknownfrom theliterature,thatthesodiumnitroprussidecouldbeusedasthe analyticalreagentforthedetectionofmethylketonesanditcan alsodetectacetoacetate[28].Actuallysodiumnitroprussidemainly reactswithacetoacetateionandispracticallynotreactivetoace- tone,iflatteritisinlowconcentrations[29].Itiscommunicatedby Swinehart[30]thatduringthereactionofacetoacetatewithnitro- prussideredcolourationtakesplace.Consequently,asthesolution oftheindicatorgivesdifferentcolourreactionsonlywithground carbonatethanwithgroundcarbonatemixedwiththeconsolidant, itcanbeappliedtodetectthecalciumacetoacetateinthesubstrate.

Thereforeitcanbeusedforthedeterminationofthepenetration depthoftheconsolidantwithinthecarbonatesubstrate.Theresults indicatethattheconsolidatedcarbonatesubstratesimpregnated withsodiumnitroprussidemostlygiveredcolourorsometimes bluecolour(asitisvisibleinFig.1b)inthecaseofcalciumhydroxide presenceinthesubstrate.Ontheotherhand,theimpregnationof not-consolidatedcarbonatesubstrateswithsodiumnitroprusside hasusuallynoeffectoncolouroronlyinthecaseofthepresenceof calciumhydroxideslightlyyellowhuecanbeobserved.Thecom- binationtechniqueofvisualdeterminationtogetherwithRaman spectroscopywasestablishedasthemethodwhichcanofferamore preciseevaluationofthepenetration.Fig.2showsRamanspectra ofgroundcarbonatemixedwithindicator,withandwithoutcon- solidant,andFig.3showsthechangeoftheindicatorcolourfrom yellowtored-violetinthepresenceofconsolidant.

Intheabsenceofconsolidant,thestrongestbands,typicalof theindicatorsodiumnitroprussideappearat2096and2122cm⁻¹ (Fig.2b).Whenaddingtheconsolidant,theCNstretch[31]ofthe indicatorshiftsto2101and2178cm⁻¹,andbandsat563,647,901, 996,1465and1569cm⁻¹appear(Fig.2a).ThecharacteristicRaman modesat2178,2101,1569,1465and996cm⁻¹ canthereforebe usedforfollowingthepenetrationdepthofcarbonateformingcon- solidants.

Fig.4showsthecomparisonofthedevelopedconsolidantsand theirpenetrationdepthinthesamesubstrate.AsevidentinFig.4,

Fig.2. Ramanspectraof(a)groundcarbonatesubstratewithconsolidantandindi- catorand(b)groundcarbonatesubstrateandindicator.0=633nm.

CF2showedthehighestpenetrationof2.8mmintothecarbon- atemodelsubstrate,whilethelowestpenetrationof0.8mmwas identifiedfortheCF1consolidant.

4.2. EvaluationoftheusefulnessofFTIRinsilicatesubstrates Several FTIR techniques (e.g. ATR, diamond cell, etc.) were tested for monitoringthepenetration depthof silicateforming consolidantCF4.Thebestresultswereachievedwiththemicro- reflectionFTIR spectroscopy. Indeed, thespectraobtained with thistechnique,intheparticularspectralarea(3000–2800cm⁻¹) mostclearlyrevealed characteristicCHstretching bandsofsili- cateformingconsolidantCF4amongallotherFTIRtechniques.The penetrationdepthforHEROMATpromisingimprovedTEOS-based formulation,CF4,maybethereforeevaluatedonCHgroups’vibra- tionbandsfromboth,TEOSandparaffin.

Fig.5showsFTIRspectraofsilicateconsolidantmixedtosilicate substrate(Fig.5a),pureconsolidant(Fig.5b),andsilicatesubstrate (Fig.5c).Immediately,aftermixingtheconsolidanttothesilicate substrateonlythebandsoftheTEOSarevisibleintheCHstretch- ingregionat2978,2930,2894cm⁻¹(Fig.5aandb).However,the hydrolysisreactionis veryfastafterapplicationandthesignals fromparaffinat2959,2925and2855cm⁻¹becomemorevisible andmaybecomemoreintensethatthoseofTEOSaftertheveryfirst days.Paraffinhaslowvolatilityandthereforegraduallyevaporates

Fig.3. Left:groundcarbonatesubstratemixedwithconsolidantCF1andindicatorgivingred–violetcolour.Right:groundcarbonatesubstratemixedwithindicatorgiving yellowcolour.

Fig.4. RamanspectraofCF1(0=633nm),CF2(0=633nm)andCFW(0=785nm)consolidantsafterindicatorapplication,appliedtocarbonatemodelsubstraterelated todifferentpenetrationdepth.

fromthesurface.Thestrongestsignalat2925cm⁻¹fromparaffin overlapsthediagnosticTEOS-assignedbandat2930cm⁻¹.Never- theless,thisdoesnotleadtoproblemssincebothsignalsarerelated toconsolidantitselfandmaybeusedasdiagnosticbandsfordeter- miningthepenetrationdepth.Thefingerprintregioninthespectra inFig.5revealsalsoothercharacteristicbandsofsilicateforming consolidant,CF4,aswellasbandsbelongingtosilicatesubstrate.

Spectrum ofpureconsolidant(Fig.5b)shows strongandsharp signalsintheregionbetween1500–1300cm⁻¹[32].Thesecharac- teristicbandsareassignedtoTEOSbendingvibrationofCHgroup andarepresent alsointhespectrumofmixtureofsilicatesub- strateandconsolidant(Fig.5a).AdditionalbroadenedTEOSsignals intheregionbelow1200cm⁻¹correspondtodifferentcombina- tionbandsofstretchingvibrationsofSi–O–Si,Si–OH,Si–O⁻aswell

Fig.5.FTIRspectraof(a)silicateconsolidantmixedtosilicatesubstrate,(b)pureconsolidant,and(c)silicatesubstrate.

asbendingvibrationofCHgroup(Fig.5b)[32].Mentionedstrips areinlesserextentvisiblealsointhespectrumofmixtureofsilicate consolidantandsilicatesubstrate(Fig.5a).

Silicatesubstrateismostlycomprisedofclayminerals.Accord- ingly,astrongandbroadenbandintheregionof1150–900cm⁻¹ correspondsto Si–O stretchingvibration [33]. Anotherbroaden bandofasymmetricstretchingvibrationofcarbonateion(␯3)inthe rangebetween1500and1400cm⁻¹canbeascribedtocalciumcar- bonate.Spectralfeaturesintheregionfrom2600to1700cm⁻¹are correlatedtoovertoneandcombinationbandsofcarbonateandsil- icatebasedcompounds[34,35].Anothermineralogicalcompound foundinthesilicatesubstrateiscalciumhydroxide(Ca(OH)2).Its characteristicsharpsignalintheOHstretchingregionissitedat 3645cm⁻¹(Fig.5aandc)[36].

4.3. Penetrationdepthofconsolidantsappliedbydifferent methods

Differentmethodsofapplicationofconsolidantsareusedinthe conservationpractice,dependingonthenatureofobjectstreated, andthescopeoftreatment.Thereforeseveraltechniques,indiffer- entcombinationsofapplication“runs”weretestedinthiswork,to ascertainthepenetrationdepthofthesaidcombinations.Results onpenetrationdepthofCF1andCF4consolidants,relatedtodiffer- entapplicationmethodaregatheredinTable3.InthecaseofCF1 penetrationintothecarbonatemodelsubstrate0,0.4and0.5mm depthwasachievedfor1,2and3applicationsbyroller,respec- tively.Oneortwoapplicationsbyairlesssprayshowedthesame penetrationof 0.1mm, while oneapplication by brushshowed 0.2mm penetration. In the case of CF1 consolidant application incellulose pulp for1 and 8h0.3mmand 4.4mm penetration depthwasachieved,respectively. Fig.1a shows Ramanspectra ofCF1consolidantappliedbycellulosepulptechniqueatdiffer- entpenetration depths into the carbonatesubstrate. Following

Table3

PenetrationdepthofCF1andCF4consolidantsappliedbydifferentmethodsonthe surfaceofcarbonateandsilicatemodelsubstrates,respectively.

Sample description

Application technique

Numberof applications/time ofapplication

Depthof penetration/

mm

CarbonateR1 Roller 1 0

CarbonateR2 Roller 2 0.40

CarbonateR3 Roller 3 0.50

CarbonateA1 Airlessspray 1 0.10

CarbonateA2 Airlessspray 2 0.10

SilicateA3 Airlessspray 1 3.65

CarbonateB1 Brush 1 0.20

SilicateB2 Brush 1 4.35

CarbonateP1 Cellulosepulp 1h 0.30

CarbonateP2 Cellulosepulp 8h 4.40

SilicateP3 Cellulosepulp 1h 19.10

SilicateP4 Cellulosepulp 8h Bottom-through

theabove-mentionedRamanmodes at 2178,2101,1569,1465 and996cm⁻¹afterthereactionofconsolidantwithsodiumnitro- prusside(Fig.1a)thepenetrationdepthwasdefined.Interestingly, whencomparingtheredcolouroftheindicatorspreadoverthe depthprofileofthetreatedcarbonatemodel substrate(Fig.1b) withtheRamanresultsitwasevidentthatthepositiveidentifica- tionoftheconsolidantwaspossibleevenonthebluishareasbelow.

Therefore,thevisualpenetrationdepthassessmentonthebasisof theredcolourwassupportedbyRamanresults,whichgaveeven moreprecisepenetrationestimation.

Fig.6showsFTIRspectrainCHstretchingregion(between3250 and2750cm⁻¹)wherebandsofboth,paraffinandTEOSarevisible, forCF4appliedbydifferentapplicationmethods,relatedtodiffer- entpenetrationdepth. AsevidentinFig.6,CF4wasdetectedat 3.65and4.35mmdepthwhenappliedinoneapplicationbyspray andbrush,respectively.Theapplicationincellulosepulpfor1h

Fig.6. FTIRspectrainthespectralrangebetween3250and2750cm⁻¹ofCF4consolidantappliedtosilicatemodelsubstratebyspray,brushandcellulosepulp(1and8h) techniques,relatedtodifferentpenetrationdepth.

gave19.1mmpenetration,whilein8htheconsolidantpenetrated throughthewholesilicatemodelsubstrate.

5. Conclusions

Vibrationalspectroscopictechniquesturnedouttobeuseful inthedeterminationofpenetrationdepthofconsolidants,which compositionissimilartothesubstratematrix.Ramanmicroscopy incombinationwithindicatorapplication,suchassodiumnitro- prusside,canbeusedtofollowthepenetrationdepthofcalcium carbonate formingconsolidant within calciumcarbonate based substrates.The three, slightly different, developed consolidants alsoshowedsome differencesrelated to thepenetration depth withinthesamesubstrate.

Furthermore, different application methods were tested for oneoftheselectedconsolidant.Itwasshownthatincreasingthe numberofconsolidantapplications,adeeperpenetrationcanbe achievedbyrollertechnique.Inthecaseofapplicationbyairless spraythenumberofapplicationsdoesnotseemtoinfluencethe depthofpenetration.

Thedepthofpenetrationalsogreatlydependsontheselected methodofapplication.Comparingoneapplicationbyroller,air- lesssprayandbrushtechniques,thedeepestpenetrationcanbe achievedbybrush(ofabout200␮m).Furthermore,itwasshown thatby8hofapplicationofcarbonateconsolidantincellulosepulp thedepthof4.4mmcanbeachieved,thehighestdetermineddepth ofcarbonateformingconsolidanttotheselectedcarbonatesub- strateinthisinvestigation.However,thesetrialsarenotenough tomakethefinaldecisionofwhichapplicationisthebestwithout amoredetailedstudy,whichwouldincludeahighernumberof differentsubstrates,differentconsolidantstobetestedandstatis- ticallyhighenoughnumberofapplicationsbydifferentmethods.

Inthecase ofsilicate formingconsolidantFTIRspectroscopy turnedout tobe very useful. Again,a deeperpenetration was achievedby brush technique when comparingto airlessspray, while 1h of consolidant application in cellulose pulp already showed19.1mmofpenetration,and8hwerealreadyenoughfor consolidanttopenetratethroughthewholeselectedsilicatesub- strate.

Thispreliminarystudyshowedthatthepenetrationdepthusing cellulosepulpcanbecontrolledbythetimeoftheapplication,while forotherapplicationtechniquesthedepthofpenetrationcanbe controlledbythenumberofapplications.Controllingthedepthof penetrationbythetimeorthenumberofapplicationsgivesthe opportunitytoadjusttheapplicationmethodforawiderangeof substratesofversatilecompositionand/orphysicalproperties,and canbeadjustedaccordingtothedamageoftheobject.Successive applicationsarenotalwaysfavourable.Inthecaseofsurfacedam- ageonly,acarefulapplicationof consolidantbybrushcouldbe sufficient,whileinthecaseadeeperpenetrationisneeded,several successiveapplicationsbyrollercanbeusedorlongertimeusing thecellulosepulp.Therefore,accordingtothedamageofanobject, acarefuldecisionofconservatorsneedstobemade.

Acknowledgements

Theresearchleadingtotheseresultshasreceivedfundingfrom theEuropeanUnionSeventhFrameworkProgramme(FP7/2007- 2013) under Grant Agreement N^◦ 282992; project acronym:

HEROMAT; projecttitle: Protection of culturalheritage objects withmultifunctionaladvancedmaterials;coordinator:Facultyof Technology, University of Novi Sad, Novi Sad, Serbia; research area:ENV-NMP.2011.3.2.1-1Developmentofadvancedcompatible materials and techniques and their application for the protec- tion, conservation and restoration of cultural heritage assets;

type of funding scheme: SME targeted Collaborative Project;

http://www.heromat.com/. The presented research was also in partsupportedbytheSlovenianResearchAgency(GrantL1-5453 andProgrammeP2-0273).A.PondelakisgratefultotheSlovenian ResearchAgencyforherPh.D.grant.

TheauthorsareespeciallygratefultoDr.CostanzaMilianifrom IstitutoCNR di ScienzeeTecnologie Molecolari(CNR-ISTM)for thefruitfuldiscussiononthedevelopmentofpenetrationdepth methodologyusingRamanandFTIRspectroscopies.

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