PotentialApplicationsofSupercriticalCarbonDioxideExtraction
andImpregnationfortheStabilizationandConservation
ofIndustrialHeritageArtifacts
FinalProjectReport
January15th,2016
Grant#P11AP60840
GrantContractNumberMT-2210-12-NC-05
NovelTechniquesforStabilizationandConservationof
FerrousMetalsinIndustrialHeritage
MichiganTechnologicalUniversityProposal#1110065
TimothyJamesScarlett,PrincipalInvestigator
ProjectTeam:
GerardCaneba,Co-PI
AlexanderAtkinson
EricPomber
StephanieTankersly
ShubhamBarole
KatherineTrotter
AlexanderWhydell
StevenMoray
BrendanPelto
AlejandraAlverez
MarkDice
Withcontributionsbyorsupportfrom:
GeraldAnzalone
OwenMills
EdwardLaitila
TableofContents
ListofFiguresandTables
1. ExecutiveSummary
2. Introduction
2.1 NationalNeed
2.2 IronandFerrousMetalsCorrosion
2.3 TraditionalConservationPractices
2.4 ThechallengeofSalts
2.5 SupercriticalandSubcriticalFluidTechniques
3 Methodsand/ormaterials
3.1 SC-CO2Extraction
3.1.1 Assessment:ExtractionwithSEM
3.2 PolymerImpregnation
3.2.1 Assessment:Scanningvs.SEMEBSImaging
3.3 Assessment:Weathering
3.4 ComparativeTraditionalConservationPractices
4 ResultsandDiscussion
4.1 AssessmentofLongTermEffectiveness
4.2 StructuralStabilityandComparativeStudies
4.3 ChangestoMineralogicalStructures
4.4 TheProblemofExtractingChlorides
4.5 ExtractionofHazardousCompounds
4.6 CorrosionAssessment
4.7 ScalingUptheProcess/UseintheField
5 Conclusions
6 Acknowledgments
7 References
3
4
5
5
7
10
11
12
16
22
23
25
25
27
28
31
32
34
36
37
40
42
42
43
44
45
2
TablesandFigures
Figure1:Awroughtironmachinebolt.
8
Figure2:Pressure-TemperaturePhaseDiagramforCO2.
14
Figure3:Ageneralizedpressure-temperaturephasediagram.
15
Figure4:DiagramoftheSCFextractionsystemequipmentatMTU.
17
Figure5:ShubhamBarolemonitorsthebombpressureinMTU’sCEBFM. 18
Figure6:SamplesESS003andESS004,afterchillinginliquidnitrogen. 22
Figure7:SixbackscatterSEMimages,cross-sectionofcorrodediron.
24
Figure8:TwoSEMbackscatterImagesofsampleESS002C.
26
Figure9:SteelwasherscoatedwithParaloidTMB72andPolyurethane.
28
Figure10:ExemplaryFerrousMetalObjectssuitableforSC-CO2.
30
Table1:ListofSamplesfromformaltrials.
21
Table2:Traditionalconservationplansexecutedduringtheproject.
28
3
1.ExecutiveSummary(400words)
AninterdisciplinaryteamofstudentsfromMichiganTechnological
UniversitycompletedaseriesoftestsusingSupercriticalCarbonDioxide(SC-C02)to
extractwaterandvolatilecompoundsfromsamplesofcorrodedarchaeologicaliron
artifacts.TestsampleswerethencrackedandexaminedusingbackscatterSEM.
Qualitativevisualinspectionshowedthatporesandmicrofissureswereclearand
openedafterSC-C02extraction.AnotherSC-CO2treatmentthenimpregnatedthe
testobjectswithanenvironmentally-benignpolymer(Acryloid/ParaloidTMB-72)to
consolidatefragilestructuresandsealobjectsagainstfuturewaterabsorption.
Followingtreatment,thesesampleswerealsocrackedandexaminedwithSEM.
ChemicaltracesofthepolymerdemonstratedthecompletediffusionoftheB72into
theporesandmicrofissuresofthesample.Thesetestswereparalleledbythedesign
andexecutionoftraditionalconservationplansfortwentyferrousmetalartifacts.
Theprojectteamdevelopedessentialcomparativeperspectivesonexisting
techniques,whilebeingengagedinconsiderationsofprofessionalethics,
practicality,andeconomicvalue.
ThisstudyshowedthattheemergingapplicationofSupercriticalFluid(SCF)
Extractioncanbeusedtorapidlystabilizebatchesofcorrodedferrousmetal
artifacts,includingcastandwroughtironandsteel,aswellascompositeartifacts.
Thiswillallowlabstoavoidorsafelydelaytraditionalelectrochemicaltechniques
thatcanrequiremonthsofexperttreatment.Thistechniqueworksasabatch
operation,allowinggroupsofsmallartifactstobequicklystabilizedand
consolidated,potentiallyeveninfieldsettings.Theprocessmayalsobetailoredto
treatobjectsfromsiteswithsoilcontaminationormaterialsfromheritage
collections,wherepotentiallyhazardouschemicalsarealsoextractedduring
dewatering.Whilecriticallyimportantquestionsremaintobeaddressedbeforethe
techniquecanbemadesystematicallyoperational,particularlythosesurrounding
chloridesalts,thetechniquehastremendouspotentialtoimprovebestpracticesin
metalsconservation.Ourproof-of-conceptexperimentsallowedustoimproveour
proceduresforthenextphaseofdevelopment.
Studentsandfacultydisseminatedtheirfindingsatfourseparate
archaeologicalandconservationconferenceswhereresultscouldbedisseminated
todifferentprofessionalcommunities.Teammembersalsopreparedblogposts
abouttheirexperimentstargetedtopublicaudiences.Theprojectestablisheda
sustainedcollaborativerelationshipbetweentheDepartmentsofSocialSciences,
MaterialsScienceandEngineering,andChemicalEngineeringatMichigan
TechnologicalUniversity.Projectscientistsalsodevelopedprofessionalconnections
withpracticingconservatorsforfuturecollaborations.
4
2.Introduction
Between2013and2015,MichiganTechnologicalUniversityassembledasmall
interdisciplinaryteamofundergraduateandgraduatestudentstoundertakea
proof-of-conceptstudyinnovelconservationtechniqueswhilebuilding
collaborativecapacityamongunits.WorkingonresearchsupportedbytheNational
ParkService’sNationalCenterforPreservationTechnologyandTraining,the
studentsundertookexperimentsapplyingSupercriticalCarbonDioxide(SC-CO2)
treatmenttonovelapplicationsofdewatering,stabilizing/consolidating,andsealing
corrodedferrousmetalartifactsfromarchaeologicalsites.SupervisedbyTimothy
JamesScarlettandGerardCaneba,thestudentteamundertookexperimentswith
sacrificialironartifactswhilealsolearningtraditionalconservationtechniques.
TheirworkrevealedthatSC-CO2treatmentsholdexcellentpromisefornew
treatmentpracticesthatareethical,highlyfunctional,andnotablycosteffective.
Historicpreservationprofessionalsneedimprovedtechniquesformanaging
ironandsteelartifactsfromarchaeologicalsites,museums,buildings,monuments
andotherheritageresources.Theseexperimentsrepresentedanewcollaboration
fortheresearchersatMichiganTech,buttheUniversityisacompellingplacetohelp
meetthisnationalneed.Whiletherearenoconservationprofessionalsonthe
faculty,theDepartmentofSocialSciencesfocusonIndustrialHeritageand
IndustrialArchaeology(IAorIH&A)providedauniquestartingpointforresearch
andtraining.Theuniversity’sconsiderableexpertiseinchemicalengineering,
includingtheresourcesoftheCenterforEnvironmentally-BenignFunctional
Materials(CEBFM),providedguidanceonpolymerizationandsupercritical
chemistry.Thiscollaborationestablishedaworkingrelationshipbetweenthese
faculty,createdopportunitiesforarchaeology,history,materialsscience,and
chemicalengineeringstudentstoearntechnicaltraininginconservation,exposed
studentsinengineeringtoappliedproblemsofheritagepreservation,and
connectedtheUniversity’sfacultywithprofessionalconservatorsworkingwith
industrialheritage.Thiscollaborativeworkisleadingtonewproposalsseeking
fundstosupportapost-docpositioninconservationscienceatMichigan
TechnologicalUniversityandestablishedtheuniversityasaqualifiedconservation
laboratoryinaregionoftheUnitedStateswherethenearestsimilarfacilityis600to
1,000milesaway.
2.1NationalNeed
IndustrialHeritageincludes“traditionally-scaled”archaeologicalsitesandartifacts
concernedwiththerecentpast,butalsoincludesengineeringlandmarks,historic
andarchitecturalmonuments,andcollectionsofmaterialoftennicknamed“Big
Stuff.”Therearegrowingnumbersofindustrialheritagemuseumsandsitesinthe
UnitedStates,manyoperatedbylocalhistoricalsocieties,butincreasinglyalso
operatedbystateandfederalagencies,includingmorethanadozenNational
HistoricalParksandNationalHeritageAreas.Thisisagrowinginternational
movementwithrootsinIndustrialArchaeology.Attheinternationallevel,UNESCO
andICOMOScollaboratewithTheInternationalCommitteeforthePreservationof
5
theIndustrialHeritage(TICCIH)toincludesitesintheWorldHeritageList.Ofthe
702CulturalWorldHeritagesites,morethan5%areexemplarsofindustrial
heritagelikecoalandironmines;steelandtextilemills;transportationnetworks;
andthecommunitiesthatsupportedthem.
Theseheritagesitespresentcomplexstabilizationandpreservation
problems,particularlythoseassociatedwithiron,steel,andferrousmetalmaterials.
Theobjectsrangefromhandtoolsandmachineparts,thetraditionalscalefor
museumconservationprofessionals,topurpose-builtstructureslikeblastfurnaces,
architecture,andinfrastructure(i.e.monuments).Unlikethearchaeologyof
households,excavationsatworkplacesintheindustrialeraoftenproduceverylarge
assemblagesofcorrodedferrousmetalartifactsthatoverwhelmrepositories.
Traditionalconservationstrategiesforferrousmetals(desalinization,electrolytic
reduction,chemicalorthermaldewatering,andsealing)areoftenunsatisfactory,
overlyintrusive,slowandexpensive,orimpractical(Rodgers2004;Newman2002;
Watson,Fell,andJones2008).Newmaterialsandprocesses,particularlysiliconebasedpolymersforobjects(Smith2003)andanti-corrosivepaintsandtransparent
coatingsfortechnicalmonumentsandarchitecturehavebeenofferedaspotential
improvements(Shashoua,TaubeandHolst2009;Conrads2008;Mottneretal.
2008;andolderworkbySeipelt,Pilz;andKiesenberg1998;Brüggerhoffetal.2008;
Kiesenberg1997;SeipeltandBrüggerhoff1997).
Professionalinterestsinindustrialheritagehaveexpandedwiththebasic
economicdriversofthistypeofheritage:urbanrevitalization,post-industrial
adaptivereuseofstructures,brownfieldsremediationandsuperfundprograms,etc.
Bigbooststo“Technicalobjects”conservationand“Industrialmonument
conservation,”spreadthrougheffortsliketheEU’screationoftheEuropeanRoute
ofIndustrialHeritage,whichconnectedmuseumsandheritagesitesthroughthe
network.ThishasspreadtoNorthAmericaastheinternationalinteresthas
coalescedaroundthe“BigStuff”conference.Thisoccurredconcurrentwiththerise
ofthe“curationcrisis”inSHPOapprovedcollectionsmanagementfacilitiesaround
theUnitedStates.TheUnitedStatesfederalgovernmentalonecuratesmorethan60
millionarchaeologicalartifacts,figuresthatdonotincludeantiquesorobjectsheld
bystate,local,andnot-for-profitinstitutionsinthecountry.1Anytreatmentthatcan
savetimeandmoneybringingcollectionsintostatutorycompliancewillprovide
tremendousfinancialbenefittothesystem.
Museumsandcollectionsfacilitiesclearlyneedconservatorstrainedtowork
withindustrialheritageandtheyneedmoreoptionsforconservationtreatments.As
indicatedabove,manyindustrialheritagemuseumslackextensiveprofessionalstaff
orfacilities,sostaffrequirelow-costconservationtreatmentsappropriatefortheir
geographicandeconomicsituations.Inaddition,theusewear,gouges,grease,and
corrosiononartifactsareoftenimportanttotellingthestoryofminework,for
example,whichleadsprofessionalstodebatetraditionalcleaningandpaintingof
metalartifactsandsurfaces.Giventheseissues,supercriticalandsubcriticalfluid
1Noreliableestimatesexistaboutthissituation.Someinformationcanbefound
here:http://www.nps.gov/archeology/collections/repos_pr.htm.
6
treatmentswillundoubtedlyplayanimportantroleinfuturepreservationofthe
industrialheritage.
2.2.IronandFerrousMetalsCorrosion
“RustNeverSleeps”isasloganoftheRust-OleumCorporationandbecameauseful
catch-phraseforaParksCanadaPublication(Ankersmitetal.2008)intendedto
helpnon-professionalandvolunteerstaffatmuseumsandheritagesitesunderstand
thecorrosionofironandferrousmetalobjects.Thisreportisnottheappropriate
placeforafulldiscussionoftheoriginsofirontechnologyandthecharacterof
ferrouscompounds,buttheformsofironandsteelthathumansfindusefulinour
livesarefundamentallyunstable.Inambientenvironmentalconditions,ironand
steelwillbegintodecaythroughelectrochemicalreactionwithwater,oxygen,and
salts.Ferrousmetalartifactsrecoveredfromarchaeologicalsites,asaconsequence,
arealwaysdecayed.Slowingorhaltingthatdecayisathornyproblemfor
conservationprofessionals.DonnyL.Hamilton(1997)provideddetaileddiscussions
ofmetalscorrosion,whichisbrieflysummarizedbelow(seealsoWatkinsonand
Lewis2005;Navrotsky,Mazeina,andMajzlan2008).Varioustraditional
conservationtreatmentscanalsobefoundinNorth(1987),Rodgers(1992,2004),
Cronyn(1990).
Oxidationisthemostcommonformofironcorrosion,anelectrochemical
processduringwhichironatomslooseelectronsandbecomepositiveions.Electron
flowisessentialforoxidation,asironironswillthenbondwithoxygen,hydroxyls,
orothermoleculestoenteramorestableambientstate.Ironishighlyreactive
becauseofitselectromotiveforce(EMF)anditdecaysintoalargenumberof
corrosionproductsdependinguponitspost-manufacturestructureandthe
environmentalandcontextualconditions.Anythingthatfacilitatesthemovementof
electronsintheenvironment,suchassaltsinsolution,willfacilitatedecay.Decay
canalsobebioelectrochemical,suchaswhenmicroorganismsinanaerobicoranoxic
settingsrichinsulfates(likeseawater)reactwithirontoformIronSulfide.Iron
oxidedecayproductsincludea“bewilderingarray”ofpolymorphs,includingatleast
20majortypesdiscussedintheliterature(CookandPeterson2005;Gonzálezetal
2004;Majzlan2008,1635;Matthiesen,Hilbert,andGregory2003;Navrotsky,
Mazeina,andNeff2012;RefaitandGlenin1997;Régueretal2015):
AnhydrousFerricOxides:
Common:
Alpha-phasehematite(α-Fe203)“BrownRust”moststableatambient
conditions
Gamma-phasemaghemite(γ-Fe203)
Iron(II,III)Oxidea.k.a.magnetite“BlackRust”(Fe3O4)
HydratedIronOxidea.k.a.“RedRust”(Fe203•2H20)
7
Figure1.AwroughtironmachineboltfromtheWestPointFoundrysitecuratedat
MichiganTechnologicalUniversity.Thecorrosionvisiblehereistypicalofdecayof
ferrousmetalsfromthatsite.Activedecayattheinterfacebetweenthecorrosion
patinalayerandtheremainingironcoredestabilizestheadhesionofthecorrosion
producttotheironcore.DelaminationsuchasthisresultswhenRHisaboveabout
20%.Photo:TimothyScarlett
8
Lesscommon:
Epsilon-phaseironoxide(ε-Fe203)
Beta-phasemagnetite(β-Fe203•Fe304)
Iron(II)Oxide(FeO),Wüstite(Fe1-xO)
Siderite(FeCO3)
Chukanovite(Fe2(OH)2CO3)
IronOxyhydroxides,HydratedIron(III)Oxide(FeOOH)
Goethite(α-FeOOH)moststableatambientconditions
Lepidocrocite(γ-FeOOH)
Akaganéite(β-FeOOH)
Hydratedforms,“YellowRust”(FeOOH•H20)
IronHydroxides
FerrousHydroxide(Fe(OH)2)
Ferrihydrite(Fe(OH)3
Otherpolymorphs
Cl-andS-products:
Ferrouschloride(FeCl2)
Ferrouschloridedihydrate(FeCl2•2H20)
Ferrouschloridetetrahydrate(FeCl2•4H20)
FerrousHydroxychlorideβ-Fe2(OH)3ClsometimescalledFerrous
Hydroxysalt.
Ferricchloride(FeCl3)
Rozenite(Fe(S04)•4H20)
Greigite(Fe3S4)
Ammoniojarosite(NH4Fe3(S04)2(OH)6)andotherjarositecompounds.
“GreenRust”amixed-valenceferrous(Fe+2)andferric(Fe+3)hydroxide
andoxyhydroxideofvariableCl-1content.(North(1982:81)
providesanexamplethatrangesfrom“4Fe(OH)•FeOClto2.17
Fe(OH)2•1.83FeO(OH)•FeOClwithachloridecontentof7.5%
to8%byweight.”)
Conservatorsareconcernedbythesedifferentoxidesofironbecausetheyallhave
differentstructuresandvariousreactivityandstability.HematiteandGeothiteare
mostlystableatambientconditions,wherethecorrosionlayerformsaprotective
patinaonanobject.AkaganéiteandFerricChloridewillcontinuetoreactwiththe
remainingironcoreattheinterfacewiththecorrosionlayer,convertingitintonew
oxideorhydroxidecompounds,unlessinterruptedthroughaconservation
intervention(Selwyn,Sirois,andArgyropoulos1999;CookandPeterson2005).
Fromtheperspectiveofboththearchaeologist’sandconservator’s
professionalethics,decayshouldideallybepreservedaspartoftheartifact.Decayis
a“natural”partofthelifecycleofferrousmetalartifacts.Assuch,thecorrosion
productcanbepartoftheuse-lifeofanartifactormayfossilizesurfaceorstructural
details.Ongoingandactivecorrosionbecomesaproblembecausethenewelectron
bondsleadtotheformationofdifferentmolecularstructures,whichcause
consequentchangesinshapesofmoleculesandcrystalforms.Thesechanges
deformobjectsbycausingswelling,delamination,blistering,flaking,andother
9
changesinshape/volume;changesincompressiveandtensilestrengthofthe
material;shiftsincolor,textureandappearance;andsoon.
Theethicalpracticeofbothconservatorsandarchaeologistscallfora
minimalinterventionstrategy(Hamilton1999,6-7).Preferredmethodsinvolve
stabilizing,consolidating,andpreservingartifacts,andthereforeminimalcleaningis
preferredandirreversiblechemical,material,orstructuralalterationstoobjects
shouldbeavoided.This“donoharm”perspectivemustbecompromisedforother
needs,includingdestructiveinvestigativeanalyses,preparationandrestorationfor
exhibition,financialpracticalities,conflictingsocialvalues,orlegalmandates.
2.3.TraditionalConservationPractices
Bestpracticesfortheconservationofferrousmetalobjectshavebeenchanging,as
conservatorsshiftawayfrommoreintrusivepractices.Becauseirondecaysso
quicklyandtheeffectsofthatcorrosioncanbesodamaging,pastpracticeof
conservatorshasbeentointervene.Inabasicsense,thesepracticesevolvedbefore
professionalshadaclearunderstandingofthedifferentkindsofcorrosionproducts
andthedifferentprocessesthatgoverntheirformation(Rodgers200471-104).
Asmentionedabove,thekeytomostcorrosionisthepresenceofliquidor
vaporwater,whichfacilitatesthemovementofelectrons,providesoxygenand
hydroxylatoms,andeasilydissolvessaltsintoelectrolyticsolutions.Whencorroded
ironseemsdrytothetouch,liquidwateroftenremainssealedunderneaththe
corrosionlayers,trappedinporesandfissures,weaklybondedtomolecules,and
evenadheredasathinfilmtheobject’ssurfaces.Conservatorshavethereforefelt
theneedtoremovethecorrosionlayerfromtheobject,becausedoingsoallows
themtoextractthewatertrappedattheinterface,extractsaltsorotherionsfrom
thatsurface,andcreateandstabilizeanewpatinasurface.Historically,conservators
haveaccomplishedthesetaskswithvariousmethods:
Removalofcorrosionviamechanicalmechanism(abrasion,microbead
abrasion,laserablation,hydrogenplasmaoxidation,etc.)orchemical
mechanism(electrolysis,galvanic,etc.)
Removalofsaltsbysoakingindistilledwaterorchemicalbath.
Creationofpatinausingtannicacidorrelatedproducts.
Sealingobjectstoprotectnewpatinausingwax,polyurethane,orother
products.
Whileconservatorshavegenerallyfollowedtheprojectionsof“donoharm”
andminimalistintervention(Rodgers2004,12-16),theirneedtobethoroughand
costeffectivewhenremovingthecorrosionlayerpushedprofessionalstouse
electrolyticcleaningdespiteitsirreversibility.Sincemostsmallinstitutionssonot
haveaccesstolaser,plasma,ormicrobeadabrasioncleaningsystems,electrolysisis
commonasitrequiresonlysimpleequipmentandbasictraining.Theresultsofthis
interventionhavebecomelesspopularincurrentprofessionalpractice,however,
becausethetreatmentdestroysthesurfacecharacterofmaterialbyalteringits
colorandtexture,platinganartifactwithnewmetalwhileexacerbatingpittingon
castironanddelaminatingwroughtiron.Tannicorotheracidtreatmentsalso
dramaticallydarkenobjectsandobscuresurfacedetails,whichisfurther
exacerbatedbycoatsofpolyurethane,wax,orothersealants.Conservatorsarenow
10
movingawayfromthesetreatments(Wang2007).Finally,doingelectrolytic
cleaningwellrequiresagreatdealoftimeandexpertise,particularlyforlarge
objectscommonlyheldbyindustrialheritagemuseumsandfacilities.Professional
conservatorshaveshiftedtoalightertouchstressingmechanicalcleaningover
chemicalorelectrochemical(Newman2002,44-46),oftenfocusingonhow
corrosioncanbecontrolledbycarefulmanagementoftheconditionsinwhich
objectsarestored,particularlyhumidityandtemperature.Thoseworkingwith
objectsfrommaritimesiteshowever,suchasshipwrecksandinundatedlandscapes,
generallystilluseelectrolyticprocessingbecauseofproblemsspecifictosaltwater
contamination(Watkinson2010,3320-3321;Rodgers2004,71-104).
2.4.TheChallengeofSalts
Thisstudywasnotdesignedtoexaminetheproblemofchlorideandsaltsinartifact
conservation.Inrecentyears,theterrestrialindustrialsitesstudiedbyMichigan
TechnologicalUniversity’sarchaeologistshavebeenpartoftemperatefreshwater
environmentswithlittlecontamination.Whileironartifactsareconservedinour
lab,teammembersuseChlorideTestStripsforrepeatedtestingofthedeionized
waterandelectrolyticbathsolutionsinwhichartifactssoak.Artifactsfromsiteslike
theCliffMinehavenotyetexhibiteddetectiblelevelsofchloridecontamination.
Thatsaid,theproblemofsaltsmustbereviewedherebecausechloride
contaminationanditsroleincorrosioniscriticaltotheconservationprocesses
discussedinthisreport.Supercriticalandsubcriticalfluidtreatmentsmaynow
finallyhelpresolvechallengesofchloride“washing”inarapidbatchprocesses(see
Section4.cbelow).Conservatorshavelongrecognizedtheexpenseandtime
demandsofextractingchlorides,forexample,experimentingwithbatch-styleheat
treatments(NorthandPearson1977).Butwhilebatchtreatmentslikeheattreating
canbeshowntoeliminatechloridecontamination,theyalsoalteredthe
metallurgicalstructureofartifactsandthuswereconsideredunacceptablesolutions
totheproblem.
Chloridesaredestructivetoironandtheirremovalwhenpresentmustbea
partofanyconservationplan.Theseionsfacilitateongoingdecaywhentheyare
trappedattheinterfacebetweencorrosionnodulesandtheremainingironbody
andtheycanalsobecomeincorporatedintomineralizations(Wang2007).Once
mineralizedintoakaganéite,forexample,thechloridesbecomeinsolubleinwater
andarenearlyimpossibletoremovewithouthighlycausticchemicaltreatments
(CookandPeterson2005;deViviésetal2007).Akaganéite(β-FeO(OH))isa
“chloride-hostingironoxyhydroxide”,andthechloridesheldbyitareamongthe
mostdifficulttoremovewithoutradicalintervention(Näsänen,González-Pereyra,
andCretté2011).
Theremovalofchloridesiscurrentlyaslowprocesswhereartifactsare
soakedinabathofdeionizedwater.Duringthisbath,whichmaylastweeks,
months,orevenyearsforlargeobjects,thechloridescontinuallydiffuseintothe
waterthroughosmosis.Whilethissoakingbathcouldbeabatchprocessforgroups
ofartifacts,suchtreatmentpreventsdirectattributionofchloridelevelsinsolution
toindividualobjects.Thediffusionischaoticbuthasbeenmodeled(Selwyn,
McKinnon,andArgyropoulos2001)andcanbegenerallycharacterizedasatwo
11
stageprocess,relativelyrapidatfirstandthenmuchmoreslowlyinthesecond
stageprocess(WeizhenandChunchun2005),unlessthebathisrefreshedregularly.
Thisprocesscanbeacceleratedbytheuseoflithiumhydroxide,sodiumsulphite,
sodiumsydroxide,sodiumcarbonate,oraqueous1,2-diaminoethane
(ethylenediamine,EN)solutionsinsteadofpuredeionizedwater,butalltheseare
causticsolutions.Someofthesechemicalspresenthealthandsafetyrisksforlab
staffandareburdensomeforwastedisposal(Gonzálezetal2004,Näsänen,
González-Pereyra,andCretté2011,SelwynandArgyropoulos2005).Higherratesof
desalinationcanalsobeachievedthroughtheapplicationofpulsatingcurrentinan
electrolytebath(Dalard,Gourbeyre,andDegrigny2002;Liu,Li,andWu2008).
Intryingtomitigateforchloridesandothercorrosionprocesses,
conservationscientistshavetriedothermethodstoslowreactions.Theyhavemade
greatprogressidentifyingdifferentcorrosionproductsandmodelingtheirevolution
betweenvariousphasesandintoalternatecompounds,overtimeandundervarious
conditions(Neff2012;Neffetal2005;Navrotsky,Mazeina,andMajzlan2008).With
thesenewmodels,whichoftendifferentiatehowmaterialschemiabsorbor
physisorbatmosphericmoisturetocreatenewelectrolyte,conservatorsknowwhy
differentcorrosionproductsarestableorbecomeactiveincertainconditions.No
matterwhattreatmentsareusedtostabilizeironartifacts,forexample,theyshould
laterbestoredatorbelow12%relativehumidity(RH).At15%RH,akaganéite
beginscorrodingiron,withincreasingRHproducingafastercorrosionrate.Above
20%RH,ferrouschloridetetrahydrate(FeCl2•4H20)startstoformandaddtothe
overallcorrosionrate.At25%andaboveRH,therateofdecayincreases
dramaticallyforalltypesofreactions(WatkinsonandLewis2005).Manyindustrial
heritagefacilitiesthatholdartifactcollectionsdonothavecuratorialstoragethat
meetsprofessionalstandardsandcannotaffordtobuildfacilitieswithoperational
HVACthatcanholdRHcontinuallyatsuchlowlevels.Inmanycases,decaywill
resumeafterconservationandsealingandironmustbemonitoredcloselyfor
ongoingdecay(c.f.Pingitore2015).
2.5.SupercriticalandSubcriticalFluidTechniques
Professionalsarepushingtodevelopnewtechniquesofconservationforferrous
metalsthatcanextractwater;removechlorides,salts,andotherharmfulions;
developthinfilmcoatingstosealsurfacesagainstreabsorbtionwithoutdetracting
fromsurfacedetail;arereversible;andwhicharefast,longlasting,andtherefore
costeffective.SupercriticalandSubcriticalFluidtechniquesareperhapsthemost
promisingofallthoseindevelopment.Whilethetechniqueenteredprofessional
practicemorethantwentyyearsago(KayeandCole-Hamilton1994),high-pressure
treatmentshaveattractedmoreinterestinthepastdecade.
Supercriticalandsubcriticalfluidextractions(SFE)arewidelyusedin
industrialprocesses,includingdecaffeinatingcoffeebeans.J.Memet(2008)
explainedthatconservatorsfirstbecameawareofsupercriticalandsubcriticalfluid
treatmentsfromFrenchindustrialapplicationsforcleaningsheetmetalsand
manufacturingpharmaceuticals.WhiletheindustriesstartedapplyingSC-CO2
extractioninthe1970stodecaffinatecoffeebeans,thetechniqueisnowverywidely
usedtoextract,separate,andpurifynaturalandsyntheticchemicals;cleanclothing
12
andprecisionmachinery;synthesizeandprocesspolymers;andapplyawide
varietyofcoatings(Crettéetal2012,3).Mostrecently,Roweetal.(2013)applied
SC-CO2extractionasanon-destructivemethodforremovingcontaminatingorganic
residuesfrommummifiedhumanremainspreparedforAMSdating,replacingharsh
acidtreatments.Thesetechniquesinvolveusingafluidasasolvent,butby
controllingtemperatureandpressuresothatthefluidentersasub-orsupercritical
phaseand,inthelatter,thefluidisnolongeraliquidoragas(Figure2).Inthis
phase,fluidsexhibitpropertiesofhighdiffusivity,lowdensity,lowviscosity,and
lowsurfacetension.Matterinasupercriticalphase2caneffuseintoanothersolidas
agaswhileatthesametimeactingasaliquidsolvent.Thelackofsurfacetension
meansnobehaviortointerrupteffusingmovement.Usingasupercriticalfluidlike
carbondioxideasasolventthenallowsthefluidtoeffusethroughasolidwhereit
candissolvevolatile(andsomenonvolatile)liquidsfromexteriorandinterior
surfacesofporousmaterials.Aspressureisreleased,theCO2entersintoagasphase
andtheextractedcompoundsareevacuatedfromthepressurechamberalongwith
thegas.
Supercriticalfluidextractionisapowerfultoolinindustry,wheretechnicians
cangenerally“tune”theprocesstocontrolwhichelement(s)areextractedfroma
complexsample.Laboratorieswillcontroltemperature,pressure,flowrate,and
processingtimeinordertoadjusttheextractionprocess.CarbonDioxideisthemost
commonsupercriticalfluidusedinindustry,followedbywaterasanotherpopular
choice.Thesupercriticalprocessallowsindustrialapplicationstoreplacecaustic
solventsolutionswithmorebenignfluidsusedathighpressure.
SupercriticalFluidExtraction(SFE)hasbeenusedwithcarbondioxidefluid
(SC-CO2)bysmallgroupsinconservationscience.Weinitiallydesignedour
experimentsbaseduponthestudybyEricSchindelholz(2007),undertakenwhen
hewasattheNationalParkService’sHarpersFerryCenter.Thatstudywasbased
uponthepioneeringworkbyateamattheUniversityofSt.Andrews(Kayeand
Cole-Hamilton1994,1998;Kaye,Cole-HamiltonandMorphet,2000).Since
beginningourproof-of-conceptstudy,wealsofoundthepublicationsbythe
conservationgroupatClemsonUniversity’sWarrenLaschConservationCenterwho
haveappliedsubcriticalprocessesinworkonthesubmarineCSSH.L.Hunley(see
citedpublicationsbyNäsänen,González-Pereyra,Cretté,Mardikian,Drews,Bayle
andothers).TheyarenowcollaboratingwiththeFrenchconservationfirmA-Corros
(deViviés2007,Memet2008,andNeff2012).AteamworkingontheUSSMonitor
hasalsorunSC-CO2experiments(CookandPeterson2005).
2Allmattercanmovebetweenvariousphases,thefundamentalphasesaregas,
liquid,solid,andplasma.Amaterialmovesbetweendifferentphasesasconditions
oftemperatureandpressurechange.Sowhilewatershiftsfromaliquidtoasolidat
32°F/0°C,whenat1atmosphereofpressure,ifpressureweretoincrease,water
willremainintheliquidphaseatthattemperature.Carbondioxideisasolidat109.3°For-78.5°Cwhileat1atmosphere,aphasecommonlyknownasdryice
(Figures2and3).TheCO2willbegintosublimateintogasasitwarmsup,unlessthe
pressureisincreasedproportionallytoforceitintotheliquidphase(orevenback
intothesolidphase).
13
Figure2.Pressure-TemperaturePhaseDiagramshowingthepointatwhichcarbon
dioxidecansimultaneouslyexistasaliquid,gas,andsolid(the“TriplePoint”)and
theareawhereitenterssupercriticalphase,abovecriticaltemperatureof31°C
(≈88°F,≈300°K)andcriticalpressureof73atmospheres(≈74bars)."Carbon
dioxidepressure-temperaturephasediagram"byBenFinneyandMarkJacobs.
LicensedunderCC0viaCommons-
https://commons.wikimedia.org/wiki/File:Carbon_dioxide_pressuretemperature_phase_diagram.svg#/media/File:Carbon_dioxide_pressuretemperature_phase_diagram.svg
14
Figure3.Ageneralizedpressure-temperaturephasediagram,illustratingthe
relationshipbetweentripleandcriticalpoints,howthefreezingpointcanvarywith
pressure(representedbythesolidgreenline),howtheboilingpointcanvarywith
pressure(representedbytheblueline),andthesublimation/depositionboundary
(representedbytheredline).Some“non-compressible”liquidsexhibitoddfreezing
behavior,suchaswater,indicatedherebythedottedgreenline."Phase-diag2"by
Matthieumarechal.LicensedunderCCBY-SA3.0viaCommons-
https://commons.wikimedia.org/wiki/File:Phase-diag2.svg#/media/File:Phasediag2.svg
15
MostconservationexperimentsthusfarhavefocuseduponusingSC-CO2to
treatorganicmaterials,particularlywoodandcork.Morerecentlyteamshavetried
toapplysuper-andsubcriticaltreatmentstoiron,primarilywiththegoalof
chlorideextractionfrommaritimeremains(thesewillbediscussedfurtherin
Section4dbelow).ThisstudyexaminedthequestionofhowSC-CO2couldbeusedto
establishabatch-treatmentprocesstodewater,consolidate,andimpregnateand
coatcorrodedironwithpolymersealant.
3.Methodsand/ormaterials
Drs.ScarlettandCanebarecruitedateamofstudents,includingundergraduates
fromSocialSciencesandMaterialsScienceandEngineering,alongwithagraduate
studentfromChemicalEngineering.Theteamdesignedandranaseriesof
experimentstoassesstheappropriatenessandeffectivenessofSC-CO2conservation
treatmentsforironandmeasurethepotentiallifespan.SupervisedbyDrs.Scarlett
andCaneba,thestudentswerealsoassistedininstrumentworkbyGeraldAnzalone,
OwenMills,andEdwardLaitilaintheDepartmentofMaterialsScienceand
Engineering.
ShubhamBaroleassembledtheSCFextractionsystemequipment,illustratedin
Figure3.ThesyringepumppressurizedCO2intothec.350ccstainlesssteel“bomb.”
Thebombisaboutthesizeandshapeofa12ozsodacan(ontheleftsideofthe
illustration).Thebombwasequippedwithaninletvalve,apressuregage,anda
thermocoupletomeasureinternaltemperature.Thebombwasalsowrappedwith
electricheatingstripsandsatinachillerthatallowedfortemperatureregulation.
Thepumpcontrollerallowedforaprogrammedrateofpressurizationandwould
thenholdthemaximumpressureuntiltheextractioncyclewascomplete.
16
Figure4.DiagramoftheSCFextractionsystemequipmentassembledinMichigan
TechnologicalUniversity’sCenterforEnvironmentallyBenignFunctionalMaterials
lab.Credit:ShubhamBarole/MichiganTech.
17
Figure5.ShubhamBarolemonitorsthebombpressureintheCEBFM.Photo:Eric
Pomber.
18
Duringoperationofallthetrials,theCO2syringepumpfilledovernight,
buildingpressure.ThesyringetheninjectedthesupercriticalCO2fluidintothe
bombwithin1-5minutes,rapidlyraisingthepressureinthechamberto950-1000
Pound-ForceperSquareInch(about68ATMor69Bar).Thebombwasleftatpeak
pressurefor12-24hourstoallowforeffusionofCO2intosamplesanddiffusionof
waterintoCO2SCF.Thebombtemperaturestartedabout23°Candpeakedandheld
at60°Covernight.Researchersthenreleasedpressureinthebomb,startingslowly
andthenincreasingtherateaspressuredroppedinthebomb.Thepressure
droppedrelativelyrapidly,usuallyinabout1minute.Thisdecompressionisvery
rapidcomparedtothoseusedincomparativestudies(Teshirogietal.2002).The
rapidpaceencouragedthequickevacuationofCO2asitshiftedtogasphase,
openingporeandfissurestructures,andcarryingdissolvedmoistureandother
compoundsoutofthesampleandintothereleasedgas.Therationaleforthese
proceduresisdiscussedbelow.
Beforeinitiatingtheformalstudy,theteamrananumberoftrialswiththe
injectorsysteminordertofamiliarizethemselveswithitsoperationandassessthe
effectsoftheprocessoncorrodediron.Itquicklybecameclearthattheferrous
metalsamples,nomatterthestateoftheirdecay,weremuchmoreresistanttothe
shrinkingorwarpingeffectsobservedinwoodsubjectedtothesametreatments.In
allcases,therewerenonoticeablechangestothesamplesaftertheyhadbeenrun
throughSC-CO2extraction.Theteamthereforedecidedtodroptheirplansfor
digitalscansofthetestsamplesbecausethedeformationanalysiswouldnothave
beenuseful.Instead,theteamshiftedefforttoextendtheirexperimentsinto
polymerselectionandthelong-termstabilityofpolymersealants.
Theteamintendedthefirstsetofsamplestoassesstheabilityofthesystem
toextractwaterfromthesamplesandopentheporesandcracks.Afterthosetrials
werecompleteandSEMimaginghadbeendone,theteamranaseriesof
experimentstoseeifSC-CO2couldbeusedtoeffuseapolymerthroughoutthe
samples,asbothastructuralconsolidantandasealanttocreateamoisturebarrier
againstrehydrationandrenewedcorrosionprocesses.Duringthesetrials,theteam
experimentedwithdifferentpolymers,examiningtheeffectivenessandsurface
appearanceoftwomaintreatments,Acryloid/ParaloidTMB-72andPolyurethane.
Theteamselectedthesepolymersbecauseconservatorstrustthemforapplication
tometalsusingtraditionaltechniques,sousingthemwouldallowtheteamtofocus
ontheapplicationofSC-CO2.Followingthosesoakingtestsandtheassessmentof
theSC-CO2impregnations,however,theteamdesignedsimulationstogetanideaof
theexpectedlifespanofvariouspolymersealantsthatmightbeusedinsupercritical
conservation.
AcryloidTMB-72(alsocalledParaloidTM,particularlyinEurope)isethyl
methacrylate,adurableandnon-yellowingacrylicco-polymer,commonlyusedin
conservationsciences.Atvariousconcentrations,AcryloidTMisusedasaconsolidant
forpaintings(1%-5%),tostabilizewood(5%-20%),andasanadhesive(50%+)for
manymaterials.B-72issolubleinacetone,tolueneandisopranol.Many
conservationlaboratoriespreferthispolymerforawidearrayofapplications
becauseitisresistanttodiscoloration(evenathightemperatures),isverydurable,
isresistantto“water,alcohol,alkalis,acid,mineraloil,vegetableoils,andgrease”
19
(Hamilton2010,12).Itisalsoreversiblebecauseitremainssolubleoverlong
periods,probably100to200years,whilealsoretainingconsiderableflexibilityover
time(DavidsonandBrown2012,99-100).
Polyurethane(inthiscasebyMinwax)isathermoplasticpolymerdilutedin
mineralspiritssolvent(48%-50%wt.).Variouspolyurethanemixturesareusedin
ironconservationforsealingandprotectingcleanedmetals,generallywhenobjects
aretoolargetobeimmersedinandsealedbywaxblends.Polyurethaneisapplied
bybrushandriesquicklyintoaclear,tough,andflexiblecoating.Thedriedcoating
ishighlyresistantto“moisture,saltwater,acids,alkalis,abrasions,andweathering”
(Hamilton2010,5-6).Coatingscanberemovedwitharomaticandchlorinated
solvents,suchastolueneorethylenedichloride,althoughcompleteremovalcanbe
difficult.
AftersoakingtestsamplesinpolyurethaneandAcryloid/ParaloidTMB72,
formulatedatdifferentweights,theteamselectedAcryloid/ParaloidTMat10%
weightsolutioninacetonefortheirtests.Theteamthenchoseartifactsfortesting
thatwouldfitintothec.350ccbombchamber.Theyalsoselectedobjectsthatthey
couldthencrackandexamineincrosssection,settingonagroupofnailsofvaried
manufactureandasmallcastironbar.Thesampleswereruninbatchesduringeach
cycleoftheinjectorsystem,wheretheywereplacedtogetherintoaglassbeaker
andthensetwithinthebombchamber.Afteraparticulartreatment,researchers
fracturedthesamplesbyimmersingtheminliquidnitrogenandthencuttingthem
withahammerandchiseluponananvil.Aftercutting,oneportionofthesample
wasretainedforanalysisandtheother(larger)portionwasreturnedforthenext
stepintesting.
Eightsamples(Table1.a)wereinitiallyselected,includingfourcutnailsand
fourwroughtnailsfromthesamedepositionalcontext.Oneofeachnailtypewas
retainedasanuntreatedcontrolsample(sampleset“a”).Thensixremaining
sampleswererunthroughaSC-CO2extractioncycletodewatertheobjects,andone
ofeachnailtypewasretainedafterdryingwithnopolymertreatment(sampleset
“b”).OneofeachnailtypewasreturnedtothebombtobeimpregnatedwithB72in
theacetonesolutionduringSC-CO2treatment(sampleset“c”).Thefinaltwo
samplesweresoakedfor24hoursinthesamepolymersolutionimmediatelyafter
drying(sampleset“d”).Examplesfromsetscanddwerecracked,inthesame
mannerdescribedabove,inordertoexaminethecrosssection.
20
Table1:ListofSamples,includingonlythosefromformaltrials.
a.MainTestSampleList(CliffMine,KeweenawCounty,Michigan):
Sample# Material/Object
Treatment
Detail
ESS001a WroughtIronNail untreated
ESS001b WroughtIronNail SC-CO2dryonly.
ESS001c WroughtIronNail SC-CO2dry,SCimpreg. 10%wt.B72inAcetone
ESS001d WroughtIronNail SC-CO2dry,24hr.soak 10%wt.B72inAcetone
ESS002a CutIronNail
Untreated
ESS002b CutIronNail
SC-CO2dryonly
ESS002c CutIronNail
SC-CO2dry,SCimpreg. 10%wt.B72inAcetone
ESS002d CutIronNail
SC-CO2dry,24hr.soak 10%wt.B72inAcetone
ESS003
Wrt.IronSpike
SC-CO2dry,SCimpreg. 10%wt.B72inAcetone
ESS004
CastIronRod
SC-CO2dry,SCimpreg. 10%wt.B72inAcetone
AllofthemaintestsampleswerefromtheCliffMineArchaeologicalProject
collection(StampMillComplex,QuadA,Level2,Bag23/174).
b.ArchiveSamples(WestPointFoundry,ColdSpring,NewYork):
04-03-9B-9Box440 WroughtIronBolt,SC-CO2dry,24hr.soakin10%wt.
B72inAcetone.
04-04-04F50
WroughtIronBoltw/fusedWasher.SC-CO2dry,SC
Impreg.Using10%wt.B72inAcetone,withCutnail.
21
Figure6.SamplesESS003andESS004,afterchillinginliquidnitrogen,cracking
(A),andplacementinthebomb(B)priortoSCFtreatment.PhotosbyStephanie
Tankersley.
3.1SC-CO2Extraction
SampleswereselectedfromsurplusartifactsfromtheCliffMinesite(20KE53),
includingwroughtandcutnailsandcastironsamplesfromtheexcavationofthe
1850-1869StampMillComplex.Thesampleswerechosentoestablishacross
sectionofirontypesandproductionprocesses,includingnailsmadefromiron
rolledintoplatesandcutintoshapeandthosethathadbeenhandwrought,along
withotherobjectsthathadbeenmanufacturedbycasting.Allsampleshadbeen
catalogedfollowingfieldworkandculledfrompermanentcollection.Thelab
maintainsacollectionoftheseartifactsforuseasexpendableexperimentalsamples.
Theteamraninitialtrialswithvarioussmall,corrodedsamples,mostof
whichwereintendedtoassesstheeffectoftheSC-CO2treatmentonthecorroded
ironstructuresothattheycoulddesignexperimentsandassessmentsofthe
technique’sabilitytodewaterandthensealcorrodedartifacts.AllSC-CO2extraction
includedseveralsamplesinsidethebombprocessedinsmallbatches.Groupsof
sampleswereplacedintoasmallglassbeakerthatwasthensealedinsidethebomb
fortreatment.Thisallowedthebeakertocatchanylooseflakesornodules
corrosionandholdliquidsolutions.
22
Thesamplesshowednosignsofcracking,warping,delaminating,orany
otherchangesinshape,regardlessoftheextentofcorrosion,evenwithrapid
decompression.Thebombchambercontainednoloosegrainsorflakesof
delaminatedcorrosionmaterial.Asaconsequenceofthis,theteamdecidedthatthe
photogrammetricor3DlaserscanningdonebySchindelholz(2007)was
unnecessary.Theironsamples,evenwhencorrodedintoironoxides,wererobust
enoughthatwarpingduetopressurechangeswouldbenegligibleduringthisstudy.
3.1.1AssessmentofExtractionwithESM
Inordertoassessthemicroscopiceffectsofextraction,twosamples(oneSC-CO2
treatedandoneuntreated)werefrozenwithliquidnitrogen,thencrackedtoexpose
thecrosssectionforSEManalysis.MountedsampleswereexaminedwithanFEI
PhilipsXL40EnvironmentalScanningMicroscopeinaLowVacMode,equipped
withanEDAXenergydispersivex-rayanalyzersystem(EDS).Inthisfirststudy,the
examinationwasonlytomakeaqualitativeassessmentofanyeffectstheSC-CO2
extractionupontheironsamplesatincreasingdepthsbelowthesurface.
WhencomparingSEMviewsofanuntreatedsample(ESS-002A)witha
samplethatwassupercriticallydried(ESS-002C,beforeimpregnation),the
supercriticaltreatmentappearstohaveopenedmoreporesandexhibitedcleaner
cracksthantheuntreatedsample(Figure7).FromthisweexpectthatSC-CO2
extractedsampleswillhavemoresurfaceareaforthepolymertopenetrate,embed,
andadhere.
23
Figure7.SixbackscatterSEMimagesoftwocorrodedironnailsincrosssection,
showingfissures,pores,andcorrosionstructures.A-Carefromanuntreatedsample
(ESS-002A).D-FarefromasamplethatwasrunthroughanSC-CO2extraction(ESS002C).Thequalitativeanalysisofthesesamplesshowedthatthepost-extraction
sampleexhibited“cleaner”and“moreopen”surfacesthatindicategreaterpotential
fordiffusionoreffusionbypolymerforconsolidationandsealing.Backscatter
electronimageswereacquiredusinganFEIXL40EnvironmentalScanningElectron
Microscope(ESEM)equippedwithanEDAXenergydispersivex-rayanalyzer
system(EDS).TheESEMwasoperatinginlowvacuummodeatapressureof0.3
Torrwatervapor.Acceleratingvoltagewas15kVataworkingdistanceof10mm.
Credit:GeraldAnzaloneandStephanieTankersley,MichiganTechnological
University.
24
3.2.PolymerImpregnation
WhenitbecameclearthatSC-CO2extractionremovedwaterandothervolatilesin
samples,leavingtheporesopen,theteambegantoexperimentwithimpregnations
ofpolymersassealantsthatwouldcreateaneffectivemoisturebarrier.Theteam
reviewedpolymersforthepurpose,butasthiswassuchapreliminarystudy,they
optedforknownandtrustedconservationtreatments,including
Acryloid/ParaloidTMB-72andPolyurethane.
Theteamconductedexperimentsinwhichtheyleftsomecontrolsamples
untreated,thesoakedsamplesineachpolymer,andthentheyranonesetof
samplesthroughaSC-CO2cyclewherethenailssatinsolutionwithinthebomb.The
teamdidnotsealuntreatednails,sincesuchtreatmentiswellknowntoexacerbate
corrosion.
3.2.1Assessment:Scanningvs.SEMEBSImaging
SEMSecondaryImagingcouldnotspecificallyidentifyanypolymerinthesamples
withoutaddingsometypeoftracerdyetotheacetonesolution.TheSEM
BackscatterImagingdidrevealpolymer,however,whichispicturedinFigure8.
Brightestpartsofimagesaremostdense(iron),corrosionandslagaregrey,andthe
polymerappearsasverydarkspots.TheSEMconfirmedtheseattributionsby
examiningtheelementalcompositionofthedifferentmaterials.Themetalwas
primarilyiron,phosphorousandsilicon.Slaghadhighsiliconcontent,butlessiron.
Thetreatedsamplealsohasverydarkspotsthatcontainchlorine,potassium,
calcium,andsilicon(Acryloid/ParaloidTMB-72).
Thepenetrationwascomplete,reachingseveralhundredmicronsintothe
sample.Precisemeasurementwasnotpossiblebecausethecrackingmethodusedto
preparethesamplesmayhavedamagedthembyshatteringthepolymer.Infuture
studies,theteamwilladoptmoreefficientandquantitativetechniquesfor
measuringthepolymerpenetrationintosamples.Quantitativecalculationscouldbe
possibleifweaddatracerordyetothepolymersolutionusingtwopossible
methods.Inordertoeffectivelyuseopticalmicroscopy,acompatiblefluorescent
staincouldbeaddedtothepolymerandadifferentfluorescentstainintheslide
epoxy.Onecouldthenuseepifluorescentilluminationforopticalmicroscopicvoid
analysisandquantifytheareaofpolymerfilledporesandcracksvs.thosethatwere
filledbyepoxybecausetheyremainedemptyvoidsfollowingSC-CO2treatment.This
techniquewouldhavetheaddedadvantageofcuttingandpolishingsamples
stabilizedinepoxy,whichwouldreducetheshattereffectofthecrackingtechnique
usedinthisstudy.WecouldalternatelycontinuetouseSEMEBSifauniquetracer
wereaddedtothepolymer,suchasvaporstainlikeRutheniumTetroxide(RuO4).
Thatwouldallowmeasurementofthatisotopeoverthecrosssectionofthesample,
includingacontinuousreadingindicatingthedropoffofRutheniumasthebeam
movedtowardthecenterofthesample’scrosssection.
25
Figure8.TwoSEMbackscatterImagesofsampleESS002C,afterSC-CO2Extraction
andsubsequentSC-CO2impregnationwithAcryloid/ParaloidTMB-72(10%solution
inacetone).Thebrightestmaterialinthebackscatterimagesareiron(“Fe”,orange
arrows),whilecorrosionandslagappeardarkergrey(“C”,greenarrows).The
polymerappearsasverydarkspots(“P”,bluearrows)thathavecompositionsof
chlorine,potassium,calciumandsilicon.Polymerwasobservedfillingvoidsdeep
intothesample,surpassing300micronsindepth.Backscatterelectronimageswere
acquiredusinganFEIXL40EnvironmentalScanningElectronMicroscope(ESEM)
equippedwithanEDAXenergydispersivex-rayanalyzersystem(EDS).TheESEM
wasoperatinginlowvacuummodeatapressureof0.3Torrwatervapor.
Acceleratingvoltagewas15kVataworkingdistanceof10mm.Theseconditions
permittedobservationofthespecimenswithoutcoatingsoastomaximizethe
chanceofdetectingpolymer.Theseimagesofwroughtironsampleswerecoated
withplatinum-palladium.Credit:GeraldAnzaloneandStephanieTankersley,
MichiganTechnologicalUniversity.
26
3.3Assessment:Weathering
ThetreatmentoutcomeleftobjectswithaverythinfilmofAcryloid/ParaloidTMB72.
Thissurfacecoatwasthinandgenerallydidnotdistractfromaestheticeffectofthe
testedobjects,consideringtexture,color,andfinish.Thiscoastingcouldprobablybe
thickerwithnolosstotheseoutcomes,sofutureexperimentswillhavetoadd
comparativetrialswith20%and30%solutions,orperhapsuseabrush-appliedtop
coatoveranimpregnatedsampletoassesschanges.Astheresearchteam
consideredtheconditionsofstorageanddisplayofobjectsatlocalheritagesitesand
museums,theydecidedthatsomeadditionaltestingofthecoatingswaswarranted.
Industrialheritage’srustyobjectsareoftendisplayedinsmall,community-ownedor
operatedmuseumslikelocalhistoricalsocieties.Asaresult,ironobjectsareoften
storedandexhibitedintheopen,evenoutdoors.Thisraisesconcernsabouthowthe
polymertreatmentswouldreactordegradewhenexposedtosunlightforlong
periods.
Becauseofconcernforpolymerweatheringanddegradationduetosolar
exposure,theteamtestedeightsamplestreatedwithAcryloid/ParaloidTMB72and
Polyurethanetoseehowtheywouldageifobjectswiththesetreatmentswere
storedinamannerthatexposedthemtodirectorfilteredsunlight.Amongtheeight
steelwashersprepared,twoeachweresoakedin10%Acryloid/ParaloidTMB72,
20%Acryloid/ParaloidTMB-72,10%polyurethane.Oncedry,thesampleswerethen
cured4hourswithaDymaxBlueWave200UVLightCuringSystem.Thecoated
washerswerephotographedat2hourand4hourmarksduringtesting.Dymaxstaff
estimatedthatUVexposureinNewEnglandonasunnysummerdaycouldbe
around3mW/cm2,orabout0.3mW/cm2perhourofdaylight.TheBlueWave200
providesabout20,000mW/cm2perhour3.Eachhourofexposureunderthelampis
roughlyequivalentto6,667days,orabout18yearsofUVexposure.Thepolymers
werephotographedafter2and4hoursofexposure,simulatingapproximately36
and72yearsrespectively.Whilethepolyurethanesamplesyellowedduringtesting,
theAcryloid/ParaloidTMB72showednosignificantchangesincolor.Theteam
exposedsomesamplestreatedwithB72for24hourswithoutsignificant
discoloration.Fromthesetests,weconcludedthatUVexposurefromopenstorage
nearwindowswillnotappreciablydegradethevisualeffectofthe
Acryloid/ParaloidTMB72coating.
Becausemanycorrodedferrousmetalshavedarksurfaces(“black”and
“brown”rusts),anydegradationofthepolymerswouldnotadverselyeffectthe
aestheticappearanceoftreatedartifacts.Thoseobjectswithbrightlycoloredrust,
“Yellow”and“Red”rusts,aregenerallyhydratedrusts.Thepolymercoatingdarkens
thesesurfacesslightly,butanysealerwillhavethiseffect.Itisworthnotingthatthe
slightdiscoloringinthesecasescomesnowherenearthedarkeningeffectoftannic
acidtreatmentscommonlyusedtocreatestablepatinaonferrousmetals.TheSCCO2processwilldehydratethesestructures,however,whichwillalsochangecolor
insomesituations.Thisisdiscussedfurtherbelow,butmoresystematictestsare
3DymaxstaffdonotrecommendusingtheBlueWave200forweatheringtesting
becausethisequipmentisdesignedforusecuringfunctionalpolymers.
27
necessaryusingthesespecificcorrosionproductsinordertoassessthispotential
problem.
Figure9.SteelwasherscoatedwithParaloidTMB72(left)andPolyurethane(right)
after4hoursexposurewithaDymaxBlueWave200UVLightCuringSystem.The
PolyurethaneshowssignificantdiscolorationcomparedwiththeParaloidTM.
3.4ComparativeTraditionalConservationPractices
Concurrentwiththeexplorationofnoveltechniquestodewater,consolidate,and
sealferrousmetalobjectswithSC-CO2treatment,studentresearchersalsoapplied
establishedconservationtechniquestoarchaeologicalartifacts(Table2).Team
membersconservedtwentydifferentobjectsusingamixtureoftechniquesranging
fromsoakingforchlorideextraction,dewateringthroughacetonebath,electrolytic
removalofcorrosionproducts,tannicacidtreatmentforpatina,andsealingwith
B72orarchivalwaxblends.Thegeneralpracticeduringthesetreatmentswas
describedinDeeganandScarlett(2008)andisnotrepeatedhereindetail.These
projectsservedtoprovidestudentswithexperienceintechnicalpracticethatthey
couldusewhenevaluatingtheeffectiveness,usefulness,andethicalimplicationsof
theirexperimentaloutcomeswithSC-CO2.
28
Table2.Traditionalconservationplansexecutedduringtheproject.
Object
Provenience
Collection
Description
Information
Horseshoewith 38S14W,Feature27, DavenportPotterysite,Parowan,
intactnails
Level15,bag153
Utah,UtahPotteryProjectCollection
Brassshell
44S16W,Level2,Bag DavenportPotterysite,Parowan,
casing
203
Utah,UtahPotteryProjectCollection
CuffLink
38S14W,Level22,
DavenportPotterysite,Parowan,
Bag203
Utah,UtahPotteryProjectCollection
Brassshell
38S14W,Level19,
DavenportPotterysite,Parowan,
casing
Bag175
Utah,UtahPotteryProjectCollection
Smallbronzepin 38S14W,Feature27, DavenportPotterysite,Parowan,
orpeg
Level9,Bag125
Utah,UtahPotteryProjectCollection
Castiron
38S14W,Feature27, DavenportPotterysite,Parowan,
shotgunshell
Level20,Bag191
Utah,UtahPotteryProjectCollection
crimper
Castironbell39S12W,Level1,Bag DavenportPotterysite,Parowan,
shapedobject
208
Utah,UtahPotteryProjectCollection
Castironlatch
28S14W,Level18,
DavenportPotterysite,Parowan,
assembly
Bag173
Utah,UtahPotteryProjectCollection
Hole-in-topiron 38S14W,Feature27, DavenportPotterysite,Parowan,
canfragment
Level17,Bag163
Utah,UtahPotteryProjectCollection
Fragmentsiron
38S14W,Level18,
DavenportPotterysite,Parowan,
canwithlid
Bag173-2
Utah,UtahPotteryProjectCollection
Wroughtiron
38S18W,Feature28, DavenportPotterysite,Parowan,
cylinderandpin Level2,Bag137
Utah,UtahPotteryProjectCollection
Castiron
07172013-01
LakeManganeseRavine,Keweenaw
industrialjack
County,Michigan
Wroughtiron
QuadD,Level5,Bag CliftonInteryard,CliffMine
hook
38.
ArchaeologicalProjectCollection.
CastIronstove
QuadE,Level6,Bag CliftonInteryard,CliffMine
fragment
206
ArchaeologicalProjectCollection.
Castironstove
QuadA,Level6,Bag CliftonInteryard,CliffMine
burnercover
37
ArchaeologicalProjectCollection.
Wroughtiron
Bag120
CliftonInteryard,CliffMine
compositechain
ArchaeologicalProjectCollection.
Ironhinge
TrenchA,Level1+S, CliftonInteryard,CliffMine
QuadIV,Bag4
ArchaeologicalProjectCollection.
Smalliron
TrenchA,Level2,
CliftonInteryard,CliffMine
padlock
QuadIV,Bag83
ArchaeologicalProjectCollection.
Ironstamp
SM101.T12,Level2. CliffMineStampMill,CliffMine
mortarscreen
ArchaeologicalProjectCollection.
Largecastiron
50N19E-50N24E,
CliftonInteryard,CliffMine
stovedoor
Level7.
ArchaeologicalProjectCollection.
29
Figure10.TheseartifactsperfectlyrepresentthepotentialofSC-CO2conservation
forindustrialheritage.ThisladleandboltweredonatedtoMichiganTechby
someonewhohadcollectedthematalocalsmeltersite.Theobjectsarebadly
corroded.Thebolt’ssurfacedetailsareentirelyfossilizedinthecorrosionproduct.
Thewroughtironladle,bycontrast,containsnosubstantialironcoreandisentirely
corrosionproduct.Italsolikelycontainshazardouscompounds.SC-CO2extraction
couldremovetheremainingwater,effuseParaloidTMB72throughouttheobjectsas
aconsolidantandbarriertowatervapor,removingsometoxics(suchasmercury)
andencapsulatingotherparticulatematter(suchasasbestos)currentlyencrusted
withinthecorrosionproduct.
Table2summarizestheartifactsthatweretreatedinthesestudies.The
objectswerefromtheCliffMineArchaeologicalProjectandUtahPotteryProject
collectionscuratedinMichiganTechnologicalUniversity’sIndustrialHeritageand
Archaeologyfacility.Treatmentsvariedaccordingtothenatureandconditionof
eachobject,rangingfromgentlecleaningwithnylonbristlebrushes(brassobjects)
tode-concretionwithsmallsteelchisels.Someartifactswereentirelycomposedof
corrosionproductwithnointactmetalcore,suchaspartsofthepadlockandthe
fragmentsofironcans.Theindustrialjack,bycontrast,wasrobustwithonlyathin
surfaceofcorrosion(althoughtheinternalsurfacesoftheformerly-movingparts
couldnotbeassessed).Allobjectsweretestedforchloridecontaminationusing
testingstripswhilesoakinginadeionizedwaterbath.ObjectsfromUtahthattested
30
positiveweresoakeduntilthechlorideswerenolongerdetected.Objectslikethe
castironstovepartscouldallbeeffectivelycleanedwithelectrolytictreatments.
Thesetreatmentstookfromtwotosixmonthsinthetanks,however,andnotall
artifactsrespondedfavorablytoelectrolyticcleaning.Thepunchedscreenfromthe
stampmortarboxwasparticularlyresistanttocleaninginthismanner.Following
treatments,theobjectswereallreturnedtostorageintheappropriatecollection.
4.ResultsandDiscussion
Asdiscussedabove,thetestsdemonstratethatsupercriticalcarbondioxide
treatmentsarehighlyeffectivetreatmentsfordewatering,consolidating,andsealing
corrodedironartifacts.Theprocedureissafe,effective,anddependinguponthe
choiceofpolymer,reversible.Labscouldapplytheproceduretobatchloadsof
objects,includingcorrodedmassesofobjectsandsomeobjectsofcomposite
materials.Theobjectscouldbetreatedastheyarerecoveredandbeforeorafter
usingotherconservationtechniques.Theprocedurecanbebotheffectivelyand
ethicallyapplied.Whilethereremainanumberofunansweredquestionsdiscussed
below,wewillseekadditionalresourcestooperationalizethetechnique.
Asthesamplesremainedinstorageinambientroomconditions,repeated
observationsinthemonthsfollowingSC-CO2treatmentrevealeddifferencesinthe
samplegroups.Noneofthesearesurprisingresults:
1. Asexpected,untreatedsamples(group“a”)begandelaminatingintheirbags
withinafewdaysorweeks.Thisprocesswasrapidatfirstandcontinuedata
slowerrateovertime.Whencorrodedironartifactsfromterrestrialsitesare
allowedtodry,delaminationisalmostalwaysobserved.Oncetheartifacts
dryoutafterremovalfromtheirmoistsoilcontext,ferrouscompoundsand
amorphousmaterialsundergorapidaerialoxidation,changingintoferric
oxyhydroxides(Selwyn,Sirois,andArgyropoulos1999;GilbergandSeeley
1982).Thesechangesweakenthestructuralbondsbetweencorrosion
productsandtheironcore.Theeffectissimilartothemannerinwhichthe
evolutionofhydrogencanflakeoffcorrosionduringelectrolytictreatment.
2. ThosesamplesthatweredriedwithSC-CO2extraction,butnotsealed,
quicklybeganrehydratingwithatmosphericmoisturewhilestoredin
polyethelenebags.Aftersixmonths,thisledtodelaminatingbehaviorsimilar
to(butlessseverethan)theuntreatedsamples.
3. DelaminationwasnotobservedonanySC-CO2driedandsealedsamplesat
sixmonths,whichweinterprettomeanthattheobjectswerefreeofactive
decay.Oneofthesamplessoakedintheconsolidantdidbeginexfoliating
smallgrainsofcorrosionproductbetweensixmonthsandoneyear.The
archivedwroughtironboltsample(04-04-04-F50)hadabitofcorrosion
dustinitsbagafteroneyear.Thecauseofthiswasunclear.Thesamplebags
weremovedrepeatedlyduringthattimeandmechanicalstressmayhave
causedsmall,impregnatedpiecesofcorrosiontodetach.Itisalsopossible
thatvaporwatermayhavefilledirregularitiesinthesurfaceofthepolymer
coating.Thismaybeaddressedinfutureexperimentsbyusinga15%-20%
B72solutioninacetoneduringtreatmentandbybrushingobjectswitha
31
higher-concentrationB72coatinguponthecompletionofSC-CO2treatment.
Thisshouldimpartamorerobustsurfacetotheconsolidant.
4. Thecomparativeconservationtreatmentplansproducedgoodoutcomes,but
alsounsurprisingly,generallytookmuch,muchlonger.Thecastironstove
doorwascleanedusingelectrolyticprocessinganditspentmanyweeksin
thefirstbath,thenrequiredretreatmentagainwhenchemicaldrying
revealedthatstubborncorrosionproductremainedinsomecavities.This
doorwasanotheridealexampleofhowSCFprocessescouldaid
conservation.Ifthedoorcouldhavebeenlightlycleanedwithelectrolytic
processes,butthenasupercriticaltreatmentusedtoextractallremaining
waterfromthebody,morecorrosioncouldhavebeenleftintactonthe
object’ssurface(respectingtheobject’slifehistory),preventingthepitting
andsomewhatunevenplatingoftheconservedsurface,andprovidingfora
completemoisturebarriersealthroughouttheareasofrivetedrepair(which
arenowweakenedandatgreaterriskofdevelopingnewgalvaniccells).
ThereisnoindicationofactivecorrosionontheSC-CO2impregnated
artifacts,butthisisdifficulttomeasureonobjectsthathavenotbeendeconcreted
orelectrolyticallycleaned.Inthefuture,massmeasurementsmayberequiredto
genuinelytestfortheabsenceofdecayovertimeinsealedartifacts.Watkinson
(1983)demonstratedthatvolume:massratiocanbeaneffectivetooltomeasurean
object’smineralization,whichcouldbeappliedwiththesematerialstotrack
changesoveralongperiodoftime.Noweepinghasbeenobservedonanyartifacts,
confirmingexpectationthatnoIron(II)chlorideorIron(III)chloride(ferrousor
ferricchlorides)arepresent.Furtherdiscussionofthismonitoringproblem
continuesbelowin4.f.
SCFextractionandimpregnationhavetremendouspromiseasabasicbatch
treatmentforarchaeologicalandheritagecollections,savinggreatdealofmoneyin
longtermcurationcosts.Theimplicationsofoursmallstudyrequireadditional
discussionwithinthecontextofotherexistingliterature,whichlendgreater
significancetoourfindings.FacilitieswithoutadequateHVACcouldusethese
treatmentsasamethodtostabilizeferrousmetalartifacts,andtheliteraturepoints
tocompositeartifactsandthoseofothermaterialsaswell,wheretheycouldbe
safelystoredatawiderrangeofRHandtemperatureswithoutriskofactivating
corrosionprocesses.Butmanymoreapplicationscouldbedevelopedtoenhancethe
valueofSCFtreatmentplans,fornewandlegacycollectionsinawiderangeofareas.
4.1.Assessmentoflong-termeffectivenessandpolymerselection
Therearethousandsofpolymersfromwhichtochooseforconservationof
particularobjects.Acryloid/ParaloidTMB-72isverystable,whichwefoundinour
ownsimulationsandreinforcedintheliterature.Conservatorshavesomeconcern
thatthispolymerexhibitssolventretention(studiescitedinDavidsonandBrown
2012,100),whereunderambientconditions,therapidevaporationofsolventnear
thesurfacepermitsthinfilmstoformwhichthenacttoretainremainingsolventin
trappedbubbles(Li2006,286).Becauseoftherapidevaporationofacetonein
ambientconditions,samplesalsosometimesexhibit“reversemigration”wherethe
32
polymerisdrawnbackouttothesamplesurfaceasitdries(DavidsonandBrown
2012,102-103).Inestablishedexperimentswithvaporandvacuumchambers,this
problemcanbemitigatedeffectively.Asthesamplesinthisstudywereimpregnated
usingSCF,thepolymereffusedevenlythroughouttheobjectsandsamplesappear
thoroughlyimpregnated.Withthetechniquesdescribedaboveforquantitative
measurementofimpregnation,moretestswillrevealwhetherornotreverse
migrationoccursduringdecompressionandthevolume(ifany)ofuncuredpolymer
orresidualsolventtrappedinsidetheobject.
Li(2006)demonstratedthatundercertainconditions,ParaloidTMB-72can
becomeaculturemediumforbiologicalgrowthleadingtosubsequent
biodeterioration.Mostformsofcarbonwillserveasfoodformicroorganisms,such
asthecarboninresidualacetonesolventtrappedinsidethemicrostructuredecayed
ferrousmetalartifacts.Otherformsofcorrosionareessentiallybiodeterioration
processes,wherebacteriafacilitatedecaybycreatingchemicalexchangesbetweena
biofilmandametalsurface.ExamplesofthisincludewhenSulfateReducing
Bacteria(SRB)formthinfilmsoversurfaces,convertingsulfateionsintoiron
sulfides.IronReducingBacteria(IRB)dothesameandsignificantlycontributeto
GreenRustformation,particularlyforferrousobjectsinmoistsoilsoverlongtime
spans(Neff2012;McNeilandLittle1999).
ItisnoteworthythatSC-CO2treatmentenhancesanyefforttocontrol
bacterialandfungalgrowth.Luo(2002)citedstudiesthatC02SCFeffectivelykilled
bacteria.EventreatmentsundertwohoursindurationwillkillEscherichiacoli,
Staphylococcusaureus,andAspergillusneomycin,andwilldecreaseby99%the
proportionofBacillusbacteriainasample.Targetedresearchisnecessaryinthis
areatoassesstheeffectsofSCFexposure,withvariousco-solvents,uponIRB,SRB,
andotherbiologicalagentsofconcern.
Finally,becausetheB72canbe“reactivated”andsoftenedforremovalor
reapplication,thetreatmentisconsideredreversible.Thisissignificant,becausenot
allpolymertreatmentsarereversibleandthuspolymerizationisacontroversial
practicewithintheconservationcommunity.Theclearestexampleofthis
controversyistheapplicationofsiliconeoilstoarchaeologicalartifacts,particularly
organicmaterials(moststronglyadvocatedbySmith2003).Yeteveninthese
studies,Smithdemonstratedthatexposuretopolymersincompositeobjectshadno
negativeeffectuponlaterattemptstoconservecastironshotwithtraditional
electrolytictechniques.
WhilemanyotherpolymersmaybeappliedinSCFtreatment,thosesoluble
inacetone,likeParaloidTMB72,haveadistinctadvantageinuse.Summarizing
pointsputforthbyCrettéetal(2012,3),SC-CO2isapoorSCFsolventforwaterby
itself.SC-CO2isagoodsolventfornon-polarmoleculesandsomelowweightpolar
molecules.BecauseSC-CO2hasaquadrupolemomentinsteadofapermanentdipole
moment,polarmoleculeslikewaterdonotdissolveasreadilyintothesolution,soa
greatervolumeofCO2isrequiredtodiffusethesamequantityofwaterandthe
processtakesmoretime.Teshirogietal.(2001)demonstratedthatamphiphilic
solvents(suchasacetone,methanol,andethanol)thathavegoodaffinityforboth
waterandcarbondioxidecangreatlyincreasetheefficiencyofmasstransferin
diffusion.Theco-solventsreducethevolumeofcarbondioxiderequired.While
33
Teshirogietal.foundethanoltobethemosteffectivesolvent,acetonealsoworked
verywell.Bycontrast,Sousaetal.(2007)foundco-solventcombinationsof
isopropanolandwaterinSC-CO2tobemosteffectiveatextractingdirtfromtextile
samples.
SeveralstudieshaveexperimentedwithPolyethyleneGlycol(PEG)intheir
workwithwoodbecauseconservatorshavepreferredthispolyethercompoundto
stabilizeandimpregnatewaterloggedwoodformanyyears.Conservatorshaveused
PEGfordecadesandhaveconservedhundredsofthousandsofobjectsofwood,
paper,leather,andotherorganicmaterials,includingPEGsolutionswithcorrosion
inhibitorsforwaterloggedcompositeiron-woodartifacts(Bobichonetal.2000).
Mostofthesetreatmentsproducedexcellentresults.AsWayneSmithsaidinhis
review,however,manyofthoseobjectswereconservedbeforetechnicians
understoodtheadvantagesofpreparingartifactsbyfreeze-drying,orbefore
advancesinPEGchemistryledtoblendedmolecularweighttreatments,orwhere
theconservators’effortssimplyresultedinpooroutcomes.Badtreatmentresulted
inorganicmaterialswithbadlyalteredcolor,“wet”surfaces,andlargevolumeof
“weeping”unboundPEGwhichdamagesartifactsandmakesthemdifficultto
exhibitorstore(Smith2006,30-31).
Teshirogietal(2001)workedwithPEG#4000intheirstudy.Chaumatetal.
(1998,1999)experimentedwithvariousformulationsofPEGsolutionsintheir
attemptstoexplosivelyinflatewoodthatshrankduringpreviousdryingand
conservationtreatments.Kaye,Cole-Hamilton,andMorphet(2000)foundthatusing
aSCFcontainingmethanol,PEGcouldbeextractedfromconservedobjects,leaving
verylittleremainingintheobjects.Theimplicationsofthisarethatconservators
canuseSC-CO2fluidwithmethanolasaco-solventforextractionstoremovePEG
frompoorlydonepastconsolidationsofwoodartifacts,whileusingwaterSCF
solutionstoimpregnatewithnewPEGwhenappropriate.Authorsfoundtheystill
neededtoapplyconsolidantsaftertreatment,findingsuccesswithParaloidTMB72
dissolvedinp-xylene(dimethylbenzene)yieldingthebestresult.
Finally,Teshirogietal(2001)alsoshowedthatasinglestep,two-stage
processgaveverysatisfactoryresult,extractingthesolventandwaterwhile
diffusingpolymersintosamplepores.Aswefoundinourstudy,usingasingle
treatmentwoulddramaticallyincreasetheefficiencyoftheSCFprocessforartifact
conservation.Thiswillrequirenewinstrumentationsetupinourlabthatwillallow
ustoadjustthecompositionoftheSCFduringtheextraction,addingorremoving
co-solventsand/orchleatingagents.Wewillalsoneedtobeabletodrawoffand
characterizetheSCFduringtreatment,measuringtheextractionratesofchlorides,
mercury,orwhatevertargetedcompoundsarebeingremoved.
4.2.StructuralStabilityandComparativeStudies
ThemajorityofstudiesapplyingSC-CO2extractiontoarchaeologicalmaterialshas
centeredonorganicmaterials,particularlyvarioustypesofwaterloggedwood.
Woodisparticularlychallengingtoconservators,becausethedecayoforganic
structureleavesthematerialfragileandsubjecttoshrinkage,warping,and
distortion.Thishappensbecausewaterreplaces60%-80%ofthebulkoforganic
structuralmaterialinthecellsofwoodymaterials.Ifthewatermassisthen
34
removed,thelossofbulkresultsinshrinkage.Bycontrast,warpingoccursinwood
samplesbecauseofsurfacetension.Thecohesivesurfacetensionofwater(orliquid
consolidant)ineachcellorvoidformsameniscusresistanttocapillaryaction,while
atthesametimephasetransitions(mostlyasevaporationfromsurfaces)cause
osmoticpressuretomovefluidthroughtheobject’scellularstructure(Luo2002).
Thetensionsbetweenosmoticpressureandsurfacetensioncauseswarp.Thisis
exacerbatedwhenthematerial,likecork,includeshydrophobicproperties.Because
bothdecayandwaterreplacementofbulkisalwaysirregularthroughoutanobject,
thesechangescauseincreasesintensionbetweensurfacesthatleadtocrackingand
differentiallydistributedwarpdependingupontheorientationofthegrain,location
andextentofdecay,andotherstructures.Traditionaltreatmentofwaterlogged
wood,alongwithmostotherorganicmaterials,involvebathstowhichslowly
replacewaterwithasolventlikemethanol,whilealsoextractingsaltsand
introducingbiocideorfungicidewhenneeded,thenreplacingthesolventwitha
bulkstabilizer.Asdiscussedabove,PEGhasbecomethemostpopulargroupof
stabilizers.Whilethesestudiesdonotprovideperfectmodelsfortheapplications
underconsiderationhere,thenumberofstudiesapplyingSCFextractionandtheir
variedsuccessmakestheirreviewuseful.
Conservators’pursedexperimentswithSC-CO2applicationstoaddresstwo
mainconcerns:fasterprocessingtimesandincreasedpenetrationofPEGinto
objects.TheyhaveassessedoutcomesbyexaminingthequantityofPEG
impregnatingobjectsorsamples,coloralterations,strainsandstressescreated
duringtreatment(assessedbydeformationoutcomes),andweightchangeto
objects.BecauseSCFshavenosignificantsurfacetension,thetreatmentshouldbe
moregentleonwaterloggedwoodthanothertypesofdrying,despitethechangesin
overallpressure.
ThesestudiesgenerallyrevealedthatusingSC-CO2toonlyextractwater
fromobjectsyieldsresultsthatfallbetweenfreezeandopenairdrying,including
goodcolorandtexturepreservation,butmoderateshrinking,cracking,andwarping.
Schindelholzetal.(2007)andSchindelholzetal.(2007)presentedacomparison
betweensimpleairdrying,freezedryingwithPEGimpregnation,andsupercritical
drying.Theirfindingswereunsurprising.Air-driedsamplessufferedworstcracking,
warping,andshrinkage.Bothfreeze-driedandsupercritical-driedsamplesexhibited
somecrackingandwarping,andattimes,thesupercriticalsamplesshrankmore
thanfreezedriedsamples.Airandsupercriticaldryingpreservedsurfacecolor,
whilethePEGimpregnationdarkenedsurfaces.Investigatorscouldnotdetermineif
thatshrinkagehadoccurredduringthemethanolexchangetreatmentorduring
supercriticaltreatment.Moretothepoint,theseresearchersmadeacomparison
betweenonetechniquethatincludedreplacementwithabulkingagent(freeze
dryingwithPEG)andtwotechniquesthatdidnot(airandsupercriticaldrying).Ina
similarstudy,Crettéetal.(2012)appliedSC-CO2dryingwithmethanolasacosolventtodryartifactsofcork,whichisnotoriouslydifficulttoconserve.Theydid
notusePEGoranotherbulkingagentandalsofound2%-4%shrinkageinall
directions,alongwithashifttolightercolorsindriedsurfaces.
WhenSC-CO2processeshavebeenusedtoextractwaterandimpregnate
sampleswithabulkingagent,however,theresultsaregenerallyexcellent,
35
comparabletootherbestpractices,anddramaticallyfaster.Studiesfindthat12-24
hoursofSCFtreatmentroughlyequateto18ormoremonthsinbathtreatmentsat
ambientconditions(Coueré1998).Teshirogietal(2001)found∼1%tangential
shrinkageusingPEG4000.Coeuréetal.(1998)testedPEG400andPEG4000and
foundnosignificantshrinkage.KayeandCole-Hamilton(1994)andKay,ColeHamilton,andMorphet(2000)testedcork,bone,bark,antler,pinecones,rope,and
compositeartifacts(includingferrousmetalwithwoodand/orbone).Theirtested
woodsamplesexperiencedshrinkageratesbetween3%-6%withameanof3.3%
andcrackingoccurredon21%ofwoodspecimens.Significantly,supercriticaldried
artifactsshowednoadditionalshrinkageorchangeindimension,evenwhenstored
forfiveyearsinuncontrolledenvironmentalconditionsinalab.
Somestudieshavepursuedotherquestions,suchaChamutetal.(1999)
attempttoswellwoodsamplesthathadpreviouslylostvolumewhendrying.They
foundpromisingresultswhentheyusedrapiddecompressionof15-minuteSC-CO2
treatments.D’Andreaetal.(2003)gotgoodresultsusingthetechniquetodry“hard”
and“soft”waterloggedbonematerialfromanItaliansitefromtheNeolithic,using
SC-CO2withmethanolasaco-solvent.Whiletheweightchangeduetowaterloss
wasexpectedandtheauthorsmentionednowarping,delaminating,ornoticeable
shrinkage,theystatedthatthetechniquediddeformorobscuretracesofuse-wear
onthebones,particularlywhatwepresumetobemicroscopictraitsofcutmarks.
Theauthorsdidnotelaborateonthisbeyondabriefmentionintheirconclusions.
4.3.Changestomineralogicalstructuresupondehydration
OnearearequiringmoreexaminationistheeffectofSCFtreatmentstothe
microstructureandothercharacteristicsofiron,steel,ferrousmetalandcomposite
objects.Thesemetallurgicalcharacteristicsareafunctionoftemperature,pressure,
andtime.Ifthistechniqueistobedeterminedethicalasamethodtostabilize,
consolidate,andsealcorrodedobjects,thenresearchmustdemonstratethat
exposuretothesehighpressuresdoesnotchangethemicrostructureorproperties
ofthemetal,oratleastshowthatthechangescanbeidentifiedanddetermined
acceptabletrade-offsforeffectiveconservation.Themicrostructureiskeytothe
propertiesoftheobject,suchasitshardness,andcanrevealcluesastomanufacture
anduseinanobject’slifehistory,soitspreservation(whenpossible)isessential.A
mineralformlikemartensite,forexample,isanindicationofquenchingincarbon
steel.GerhardEggertexpressedconcernthatthisstructuremaybedisruptedby
heattreatmentsinconservation,withtemperaturesaslowas180°C.Hisconcern
wasechoedbyothersindiscussion(Selwyn2011,50)andisadistinctthreadin
conservationliteraturegenerally(Schmidt-OttandBoissonnas2002).TheSC-CO2
extractionsconductedinthisstudyweredoneatleast40°Cbelowthemost
conservativecriticaltemperaturepointsidentifiedintheliterature.
Thesubcriticalstudiesdiscussedbelowinsection4.4demonstratedthat
thosetreatmentscausedirreversiblechangesinthemineralizationstructureson
corrodedironartifacts.deViviésetal.(2007)showedthatsubcriticalfluid
treatmentactivatedchangesinthestructureofcorrosionmineralizations,where
akaganéiteandlepidocrocitereformedintohematite.Afterobservingcolorchanges
36
inthecorrosionproducts,shiftingfromlightochretonestoredtonesduring
subcriticaltreatment,González-Pereyraetal.(2010,50)undertookaMicroRaman
Spectroscopyanalysisofsubcriticaltreatedartifactsandrevealedonlygoethite,
hematiteormagnetite.Thissuggestedthatifanylepidocrocite,akaganéite,orother
lessstablephasesofcorrosionproducthadbeenpresent,theyweretransformedby
subcriticaltreatmentintomorestablephases,consistentwithdeViviésetal.(2007).
Otherstudiesapplyingsubcriticaltreatmentshavealsoobservedthesechanges,
includingchangesfromlepidocrociteandmagnetiteobservedaslocalizedcolor
changefromorangetoblack(Näsänenetal.2011).
Neff(2012,41-42)arguedthatthesubcriticaldechlorinationprocesses
developedbyClemson’sWLCCteamispromising,butscientistsneedtounderstand
theactualeffectsonthechemistry,structures,andmorphologyofthecorrosion
productsbeforethetechniquecanbeappliedmorewidely.Thechemicaland
structuralchangesofthecorrosionproducts,suchasthephasetransformations,
mustbemapped(asinthisstudy)andunderstoodifconservatorsaretounderstand
theparameterscontrollingthekineticsofdechlorinationandapplythetechnique
widely.NeffhasformedacollaborationwithresearchersatPMEandA-Corrosto
determinetheparametersofmineralogicalchanges,optimizethesubcritical
treatmentprocess,andfurtherclarifytheevolutionofthecorrosionproductsso
thatthetechniquecanbeappliedonanindustrialscale.Thatworkisongoing.
Wang(2005)experimentedSCFextractionwithhydratedinorganic
compounds,includingferrouscompounds,measuringmasslossduringextraction.
Hiskeyfindingwasthatalmostallhydratedmineralspecieslosetheirwaterduring
SC-CO2extractionandresultssuggestedthatalmostallphysicallyorchemically
bondedwatercanbeextractedwithsupercriticalCO2.Theauthorwasableto
demonstratesignificantmasslossfromthesecompounds:FeCl2•4H2O;
Fe(NH4)2(SO4)2•12H2O;FeSO4•7H2O;Fe(NO3)3•9H2O;Fe(NH4)3(SO4)2•6H2O.In
addition,Wangshowedthatferroussulfatedehydratesinsupercriticaltreatment,
whileironheptahydrate/ferroussulfate(FeSO4•7H2O)dehydratedmoreeffectively
bySCFextractionthanthermaltreatments,undertheproperconditions.
Sincewatercanbeextractedfromironhydroxidesandoxyhydroxideswith
SCF,dothesedehydratedcompoundsthenformstableanhydrousferricoxides,like
alpha-phasehematite,gamma-phasemaghemite,ormagnetite?Thesechangesare
similartothemineralogicalshiftsinsubcriticaltreatments.Experimentswillbe
requiredtoassessthesechanges,aswellastoassesstheeffectsofthesechangeson
artifacts,includingmassloss,increasesinporosityinthecorrosionlayers,changes
tothestructureofthecorrosionproduct,andanyresultingincreaseordecreasein
thestrengthofadhesionamongcorrosionlayersandremainingiron.
4.4.Theproblemofextractingchloridesandsalts
Theapplicationofsubcriticalandsupercriticalfluidtreatmentsholdgreatpromise
asbatchtreatmentsthatcanacceleratetheextractionofchloridesaltsfromferrous
metalartifacts.Asdiscussedabove,thekeystoslowingorstoppingdecayofferrous
metalartifactsaredehydration(removalofliquidorvaporwater,orweaklybonded
waterthatcanbecomeavailableforreaction),stoppingbiologicalgrowth,retarding
themovementofelectrons(toinhibitreactions),andremovingorinactivating
37
highlyreactivefreeCl-ionsandboundchlorides.Thusfar,ourworkhascenteredon
thefirstandthusthirdissues.Whileourworkwasnotintentionallydirectedat
inhibitingbacteriologicalorfungalactivity,SCFtreatmentsfundamentallyinhibit
thoseprocesses.Chloridespresentthefinal,andfrankly,themostdaunting
challenge.
NorthandPearsonalsopioneeredtraditionalmethodsforextracting
chlorine.Since1975,thepracticehasinvolvedusingsodiumhydroxidesolution(0.5
mol/L,pH13.7)containing0.5mol/Lsodiumsulphite,heatedupto70°Cand
stirred.Aftertreatment,thechemicalagentsarewashedoutthoroughlyand
residuesareprecipitatedwith0.1mol/Lbariumhydroxide.Dryingisachievedusing
acetone.Thetechniqueisslowandinvolveslotsofcausticchemicals(Schmutzler
andEbinger-Rist2008).
BuildingonthepioneeringworkofN.A.NorthandC.Pearsoncitedabove,
LyndsieSelwynandhercolleagueshaveestablishedmoredetailedunderstandingof
thechaoticdiffusionofchloridesintoandoutofarchaeologicaliron.(Selwyn2004,
294-306;Selwyn,etal.2001,109-120;Selwyn,etal.1999,217-232).Theextraction
ofchloridesreliesupondifferentconcentrationgradientsdrawingofchloridesto
diffuseintothebathsolution.Thiscanbeacceleratedwhenhydroxideions(OH-)are
alsointhebathsolution,sinceCl-willexchangewiththe(OH)-inmolecular
arrangements.Butforthistohappen,anexcessofhydroxidemustdiffuseintothe
object,activatethereaction,thentheliberatedchloridemustdiffuseout,oftenalong
“tortuous”pathsthroughporesandintersticesamongcorrosionlayers,metal,and
concretions,manyofwhichcanactasmembrane-likebarriersthatinterruptthe
diffusionprocessbycohesivesurfacetensiondiscussedabove(Näsänenetal.2011,
2).
Overthepastdecade,WarrenLaschConservationCenter(WLCC)
researchershavepublishedaseriesofexperimentsextractingchloridesusing
subcriticalfluidtreatmentsonhundredsofdiverseartifactsandtestspecimens.This
teamhasbeenusingalkalinesolutionsraisedtotemperaturesabove100°Cand
pressuresabove30bar,aregionwherewaterisaboveitsnormalboilingpoint,but
remainsaliquidinsteadofgasorvapor.Byworkingatsubcriticalcombinationsof
pressureandtemperature(forwater),theWLCCcollaboratorscan“tune”the
propertiesofwatertousenewthermodynamicandkineticoptionsandchemical
pathsunavailableinsimplediffusiontreatmentsinambient(a.k.a.“atmospheric”
conditions.Inthesubcriticalregion,waterandwatersolutionsexhibitlower
viscosity,surfacetension,andwhenusingthesetemperatures,density.These
changesimprovewater’sbehaviorasasolventandincreaseit’sabilitytopenetrate
poresandinterstices.Thechangesenabletheresearcherstouselower
concentrationsofNa(OH)intheirsolution.(Näsänenetal.2011).Theirresultsalso
suggestthatthepressureandtemperaturetreatmentsconvertakagéniteand
lepidocrociteintohematite,whichismorestableandlessreactive(deViviésetal.
2007;González-Pereyraetal.2010).
TheWLCCstaffhaveusedsubcriticaltreatmentstoshortenthesalt
extractiontimetoanaverageof15days(González-Pereyraetal.2010,39),where
traditionaltreatmentswouldhavetakenasmanyweeksormonths.Theirtests
showconsistentremovalofalldetectiblechloridefromobjects,includinginterior
38
pores,wheretheydetectednochloridesbutidentifiedsodiumresiduefromthe
Na(OH)solution,whichtheyarguedrepresentedproofthatthesolutionand
extractionhadentirelypenetratedtheartifacts.Asaconsequenceofthetreatment,
theobjectsshowednoactivecorrosionsignsaftertwoyearsofopenstorage.This
remainedtrue,notably,evenwithoutprotectivecoatingspreventingmoisture
reabsorption(González-Pereyraetal.2010,45-50).
Perhapsmostimportantly,themanystudiesundertakenbyWLCC
researchersclearlyindicatethatsubcriticaltreatmentsallowsafeandrapidchloride
extractionsfromconcretedartifacts,removingthesaltsandchemicallystabilizing
entirenodules.Thisiscompletebeforeanyothercleaningorconservationworkis
undertaken,withoutnegativeimpactsuponthecohesionofthelayersofcorrosion
productsortheadhesionofthoselayerstotheremainingmetalcore(GonzálezPereyraetal.2010,39-40).Irreversiblechangestothecolorofthecorrosion
productsisperhapstheonlynegativeeffect,andthisisinconsequentialasaneffect
ofstabilization.Thisdrawbackisfaroutweighedbythebenefitsderivedfromposttreatmentdeconcretionandmechanicalcleaningofthecorrosionmatrix,whichcan
bedoneataslowerpacewithadryobject(Näsänenetal.2011).
TheWLCCstaffarecurrentlyexperimentingwithcombinationsofsubcritical
andsupercriticaltreatmentstoseeiftheprocesscouldincludetheapplicationof
tailoredcorrosioninhibitorsandconsolidantsinasinglesteptreatment(Näsänenet
al.2011,20-11;González-Pereyraetal.2010,49),essentiallywhatwehavedonein
thepresentstudy.
Whilethiswasnotpartofourexperimentstothispoint,supercriticalfluid
treatmentscouldbetailoredtoextractchlorides,atleastbytestingNa(OH)aspart
oftheSCFsolution.Moreimportantly,withaspecificallydesignedSCF,allchlorides
couldberemoved.Wang(2005)assertedoxyhydroxidescanbedehydroxylated
withSC-CO2,presumablyincludingthechloridehostingmineralakagenéite(βFeOOH).Akagenéniteoftenformsatthecorrosioninterfacebetweenexisting
productsandtheremainingironsurfaceandcanformveryquicklyifsaturatediron
samplescontainingchloridesareallowedtoairdry4(CookandPeterson2005).The
chloridesheldwithinitscrystallatticearenotoriouslydifficulttoextractby
establishedandethicalmechanisms,andthemineralreactivatesanywhereabove
15%RH.Currenttheorystatesthatthesurfacehydroxylgroupsonβ-FeOOHwill
chemisorborphysisorbatmosphericwaterthatthendissolvesthesurface-adsorbed
chloridetoproduceanelectrolyte,thusleadingtoadditionalcorrosionofironin
contactwiththeakaganéiteinthematerial(WatkinsonandLewis2005).As
mentionedabove,thedehydroxylatedmineralwillpresumablyformintoamore
stableferricoxide,butthismustbedetermined.
ASCFcouldbeoptimallyconfiguredtoextractchlorides,includingfreeions,
thoseheldwithincrystalstructures,andperhapseventhosebondedtoironasferric
chloride(FeCl3).Purecarbondioxideisaweaksolventformetals,however,sono
4Itisworthnotingthatasrecentlyas1994,manyarchaeologistsstilldebated
whetherwetironartifactsrecoveredfromthefieldshouldbedriedbeforepacking
themtobeshippedtothelab(Sease1994:66).
39
metalsareextractedinpureSC-CO2.Byaddingcomplexagents,theSCFbecomesa
muchmoreefficientsolventformetals.InhissimulationsofMartiansoils,for
example,Wang(2005,87)usedasmallamountofwaterwiththeco-solvent
chelate5asthefluidtoattemptSC-CO2extractionofmetals.WangshowedthatMg2+
couldbeextractedfromMgCl2.Similarexperimentscouldbeconductedwithferrous
andferricchloridepowders,usinganappropriateSCF.
SC-CO2(oralternateSCF)extractionmayoffersomecriticaladvantageover
sub-ornear-criticaltreatments.Ourextractionswereconductedat60°C,below
thoseusedinthesubcriticalextractionsatWLCC.Withlongersoakingtimeatpeak
pressure,thetemperaturecouldevenberunatroomtemperature,remainingbelow
temperaturethresholdsthatmightpromptchangestothemetallurgicalstructureor
hardnessoftheartifacts.
4.5.Extractionofhazardousandvolatilecompounds
Industrialarchaeologists,conservators,andthemanagersofindustrialheritage
shouldnotethatSCFextractionalsoholdsgreatpromiseasamethodtoremove
hazardouscompoundsfromartifacts.Inourproposalforthiswork,wewrotethat
wehopedtoextractwaterfromcorrodedironobjectswhileleavingdirtandgrime
intact.Amongthereasonsforthis,oilstainsandgreaseareoftenimportanttothe
lifehistoryoftools,machineparts,andindustrialheritageobjectsmadeofferrous
metals.Toacertainextent,thistechniquecanbetunedtominimizeormaximizethe
extractionofhydrocarbons.Ourresearchintocomparativestudiesofthesolubility
ofmachineoilsandgreaseusingSCFshowedthatthetechniquecanalsobeusedto
removetoxiccompoundsfromindustrialandotherculturalheritagematerials,
whereenvironmentalpollutionorcontaminantsthreatenhumanhealthandsafety.
Becauseoftheconglomeratingtendencyofindustrialproduction,heritage
scientistsknowthatsitesaresometimescontaminatedwithhazardousortoxic
residues.Thesecompoundscanalsomigrateingroundwateramongnearbysites
withinawatershed,spreadingpost-industrialcontaminationtositesfromalltime
periodsinanarea.Thecontaminationoftenpresentsethicalchallengesto
archaeologistsandheritageprofessionalsbecauseremediationcomesintoconflict
withpreservation.Theremediationoftoxinspreemptsheritagepreservation.
SCFtreatmentscanextract,neutralize,and/orencapsulatemanyofthese
contaminants.Contaminatedartifactsmustoftenbecatalogedonsiteduring
remediationandthendisposedof,alongwiththeothercontaminatedsoilsand
debris.Objectsthatarepartofexistinglegacycollectionsaresometimesfoundtobe
contaminatedandmustthenbedisposedofashazardouswaste.Curating
contaminatedobjectswouldpresenttoogreatarisktocurrentpopulationswhen
balancedagainsttheirculturalsignificance.Amongcontaminatedcollections,most
pre-andnon-industrialheritageartifactsabsorbcontaminantsfromtheirpostdepositionalcontext.Fortheobjectsofindustrialheritage,however,theresiduesof
contaminationcanoftenbepartofthelifehistoryoftheartifactsthatconservators
5Chelateasaring-structurecompoundwhereligandmoleculesbondwithametal
ionontwoormoresides,inthiscase,ahighperformanceperfluoropolyether.
40
wouldotherwisefindtheywereethicallyobligatedtopreserveinsitu.TheSCF
techniqueshouldbeconsideredanimportanttoolinmakingthesedecisions.
SCFextractioncanbetunedtoremoveawiderangeofcompounds.Whilenot
conductingexperimentsrelatedtoconservationscience,Wang(2005,12))
summarizedsomeindustrialapplications,whereSCFofCO2,Propane,andBenzene
havebeenusedtoextractpetroleum,oil,asphaltum,andnicotineresidues;beanand
sunflowerlipids;essencesofblackpepper,hops,andpineapple;aswellaslemon,
corn,fennel,cinnamon,andalmondoils.Givenindustrialapplications,itisnot
surprisingtonotethatconservatorshaveshownthatSC-CO2extractioncanbe
tunedtoremoveawiderangeofmoleculesfromtheirdiffusedstatewithinan
object’smicrostructure.Roweetal.(2013)showedtheycouldextracta
polyglycerol-basedpolymerandfattyacidmoieties(methylesterderivatives)that
mayhaveformedinareactionbetweenmummy-embalmingmaterialsandtheirSCfluidmixture,inadditiontobeeswax,coconutoil,frankincense,glycerol,andhumic
acidsinvaryingamounts.
Kaye,Cole-Hamilton,andMorphet(2000)examinedextractedsolutionsfrom
maritimeartifactsusingGasChromatograpy-MassSpectroscopy(GC-MS)to
measuretheextractedcompounds.Theresultsshowedthatextractscontained
vanillin,syringaldehyde,beta-sitosterol(awoodsteroid),numerouslong-chainfatty
acidswithupto18carbonatoms,andterpenedehydro-abieticacid.
Scientistsandengineershavehighlydevelopedtechniquestoextractorganic
molecules,butbycomparison,theextractionofinorganicandmetalliccompoundsis
comparativelynew.Yetsincethe1990s,theintroductionofcomplexchelating
agentsinSCFhasinspiredaflurryofresearchinthisarea.Scientistshaveextracted
organicandinorganicmercuryfromsoilsandsolidmaterials(Waietal.1993;
Issaro,Abi-Ghanem,andBermond2009).Theyhavealsodecontaminatedsoils
containingNapthalene(Smythetal.1999)andPCBs(Yaketal.1999).Many
publicationsinenvironmentalchemistrydiscussextractionofaliphaticcompounds,
carbolicandotheracids,severalpesticidesandherbicides,fertilizers,andvarious
phenols.SCFprocesscanalreadybedeployedonanindustrialscaleforthe
remediationofcontaminatedsoils(Cocero,Alonso,andLucas2000).
SomeconservatorshaveusedSCFtoextracttoxicmaterialsfromcollections,
wherecompoundshadbeenaddedtopreserveorganicmaterialsunderolder
museumpractice.Theselegacycollectionspresentproblemsbecausenowthedust
fromtheseobjectspresentsunacceptablethreattohumanhealthformuseumstaff
andvisitors.Kang,Unger,andMorrell(2004)ranexperimentstoshowthatDDT
(dichlorodiphenyltrichloroethane)couldbeextractedfromwoodentestblockswith
variouscoatings,resultinginveryslightvariationsinsurfacecolor.Consistent
findingswerereportedbyTello,Jelen,andUnger(2005),whousedcontaminated
collectionsfromtheEthnologicalMuseumofBerlin,samplingleather,fur,hair,hide,
skins,feathers,hair,woventextilesofwoolandcotton.Theymeasuredtheircontent
ofarsenic,mercury,DDT,lindane(g-hexachlorocyclohexane),andPCP
(pentachlorophenol),andperformedSCFextraction,usingSC-CO2withethanolasa
co-solvent(alsosometimeswithtrimercaptotriazine(TMT)asachelating
compound).Theoutcomeswereexcellent,wheretheyremoved70%-90%ofthe
mercury,80%-100%oftheDDT,50%ofthearsenic,and60%-90%ofthelindane.
41
ConservatorscanapplySCFextractiontocontaminatedculturalheritage,but
theapproachcannotbestandardized.Eachsiteorcollectionwillpresentuniqueand
specificcontaminantsrequiringequallyuniqueandspecificSCFextraction.
Technicianswillchoosetheirfluid,solventsandco-solvents,chelatingcompounds,
temperature,pressure,andextractiontimesaccordingtotheneedsofeachcase.
HeritagemanagersmustbecomeawarethatSCFextractionisonepotentialsolution
totoxicssothattheywillnotassumethatdisposalordestructionremaintheonly
optionsduringremediationprojects.
4.6.Corrosionassessment
Professionalcuratorsandheritagemanagerswillneedsomenon-destructivetesting
optionstomonitortheeffectivenessoftreatmentsovertime,perhapswithout
removingthecorrosionnodulesorlayers.Insection3.2.2above,weoutlinedtwo
methodsfordestructiveanalysesyieldingquantitativecalculationstoassessthe
penetrationofpolymerintosamples,usingeitheropticalmicroscopywith
epifluorescentilluminationofvoidanalysisorSEMEBSusingavaporstainlike
RutheniumTetroxide(RuO4).Intheirstudyassessingtheoutcomesofconservation
practicesfromWinchester,England,KeeneandOrton(1985,30)remarkedthatit
wouldhavebeenusefuliftheycouldhavesystematicallycomparedradiograph
imagesofcorrosionnodulesandmineralizedobjectswithironcores.Radiographs
beforeandaftertreatmentandthenrepeatedaftertheobjectshadspentdecadesin
storagewouldhaveallowedthemtosystematicallyexaminetheeffectivenessof
treatmentsattemptingtostabilizecorrosionlayersoveranironcorewhilealso
preservingthecorrosionlayerinplace.Thistypeofassessmentwillbeessentialas
thetechniquesdiscussedaboveareappliedtostabilizecorrodednodulesfrom
maritimeandchloride-bearingterrestrialenvironments.
Establishedtechnologiesnowallowforthedirectassessmentidentifiedby
KeenandOrton,whichmightfollowthestructuredlightscanningor3Dmodeling
donebyotherresearchers,butalsomappingobject’sinternallayers.Researchers
willneedtoestablishamethodtomapchangestoartifactstructuresusing
somethinglikemultiscaleX-rayComputerTomographic(CT)imagingtobuildhighresolution3Dvisualizationsofcorrodedironlayersthatallowidentificationof
changesovertime.Thistypeofworkisalreadyunderwayatlaboratorieslikethatof
theNewTechniquesforAncientMaterialsresearchgroupledbyProf.ClaireGervais
atSwitzerland’sBernUniversityoftheArts6(c.f.Jacot-Guillarmodetal.2015).
4.7.Scalinguptheprocess/workinthefield
WhileworkingintheMichiganTechlab,wewereverylimitedbythesizeofour
pressurebomb.Ifthistechniqueistobecomeanormalizedpracticedonewidelyin
thefield,theprocessmustbescaleduptoatruebatchoperationwithbothscaleand
mobility.WhilemanyquestionsremaintobeaddressedbeforeSCFextractioncan
besystematicallyapplied,existingliteraturealreadydemonstratesthatthe
techniquewillworktotreatlargeartifactsorbatchesofobjects.
6http://www.gervaislab.ch/research.html
42
Ourteamdiscussedhowtheprocesscouldbescaledup.Doestheincreased
energyandmaterialsdemandoflargerscaleprocessingmakethetechnique
unrealisticandcost-prohibitive?Iftheequipmentcouldbemademobile,anda
cooperativeeffortmadeamongalargenumberofinstitutions,couldthecostof
settingthesystemupbemeasuredagainstcostsavingsintreatmentcollections?
Thereareseveraloptions,including:
i.aninstitutionlikeMichiganTechortheWLCCcouldacquireanindustrial-scale
compositeextractionautoclaveandbeginreceivinglarge-scaleobjectsor
largecollectionsfortreatment,deliveredtoasitelikeHoughton.Industrial
autoclavesarereadilyavailable,butexpensive.
ii.acollaborativeorganizationorconsultingcompanycouldpurchaseamobile
compositeextractionautoclavethatcanbemovedfromfacilitytofacility.
Suchdevicesarealreadymanufacturedforbiohazardwastetreatment,
althoughtheymoreexpensivethanstationarymodels.
iii.designingasubterraneanorsemi-subterraneansystemthatcouldbecheaply
builtonasiteusingreinforcedconcrete.Expertscouldassemblethesystem
onthegroundsofamuseumoranarchaeologicalsite,thendemolishthe
chamberwhentheworkwascompletewhiletakingkeypartsandequipment
tothenextsite.Thiswouldremovetheneedtotransportalargepressure
tankbetweenjobsandcouldperhapsbelessexpensive.Developingthis
systemwouldrequiremuchmoreresearch.
MichiganTechnologicalUniversity’smultidisciplinaryeducationalprograms,
includingSeniorDesignandEnterprise,areideallysuitedtoexaminetheseoptions.
Intheseprograms,agroupofstudentsfromdifferentdepartmentscoulddesign
thesesystemsanddevelopcost-benefitanalysesforthemwhilesupervisedby
professionalengineersandfaculty.
5.Conclusions
SupercriticalFluidExtractionusingSC-CO2isausefulandeffectivewaytodewater
andstabilizebatchesofencrustedferrousartifactsfromindustrialheritagesites.
Thetechniqueisfasterthantraditionaltreatments—fasterbyseveralordersof
magnitude.Whereacannonmayspend4yearsintraditionaltreatments,awelltunedSCFextractionmightconcludein4days.Thestressesintroducedupon
objectsbythetreatmentarenomoredamagingthanongoing,activedecayorany
otherexistingtreatmentstostopcorrosion.Theprocedurecanbereversible,inthe
sensethatthetreatedobjectsarestableforlong-termcurationorthepolymercan
beremovedformoredetailedinterventionsatsomepointinthefuture.The
compoundsusedareallquitebenignandwhiletheoperationofequipmentat
supercriticalpressuresrequiresspecialtraining,theprocessisgreenandsafeto
humans.Whilemuchworkremainstobedonetodetermineoptimalpolymers,
selectionsofsolventsandco-solvents,andextractionofchloridesandtoxic
compounds,theSCFtechniqueisverypromising.
Teammembersdesignedandexecutedformalconservationtreatmentsfor
32objectsinallduringthisstudy.Twentyferrousmetalartifactswereconserved
usingtraditionaltechniques,whiletenartifactsweresubjectedtosystematicSCF
43
treatmentsandfollowupassessments.Twoadditionalartifactsweretreatedand
addedtothecomparativecollectioncuratedatMichiganTech,wheretheycanbe
examinedaspartofthediachronicstudyofoutcomes.Studentteamscompletedall
thework,researching,designing,andexecutingtreatmentplanswhilesupervised
byfacultymentors.Thestudentsandfacultywrotearesearchblogaboutthe
project,butafterspeakingwithconsultingconservators,wewereadvisedtofinish
thereportsandpublicationsbeforepostingtheblogentries.Thosepublicpostings
arepending.
Thisstudyestablishedarobustcollaborativerelationshipsurrounding
conservationandcorrosionscienceatMichiganTechnologicalUniversity.Agroupof
eightundergraduateandgraduatestudentsfromthedepartmentsofSocialScience,
MaterialsScienceandEngineering,andChemicalEngineeringengagedwiththis
projectoverthreeyears(2012/13-2014/15),learningaboutconservationscience
andethics,andpresentingtheirfindingsatcampuseventsandattheMidwest
ArchaeologyConference(Atkinsonetal.2014a)andtheMidwestHistorical
ArchaeologyConference(Atkinsonetal.2014b).Fromthatgroup,onestudentwent
ontodevelopanundergraduatethesisonmetalsconservation,won3rdplaceoverall
atthe2015UndergraduateResearchExpo,andhasmadeapplicationtograduate
schoolstobeginaMSprograminconservationscienceandarchaeology.Faculty
presentedfindingsattwomajorconservationmeetings,BigStuff2015(Scarlett
2015)andtheGrupoLatinoamericanodeRestauracióndeMetales(Scarlett2014).
Aversionofthosepresentationsisunderreviewforpublicationintheproceedings
oftheBigStuffmeetingandseveralothertargetedarticlesareinpreparationfor
industrialarchaeologyandindustrialheritagejournals.
Thefacultylearnedagreatdealaboutcollaborationsfromthisproject.We
havesinceextendedourcollaborationstoothersintheDepartmentofMaterials
ScienceandEngineeringandtheDepartmentofChemistry,mentoringstudent
investigationsofsiliconeoiltreatmentsofarchaeologicalleather,forexample.
Facultyhavealsoinitiateddiscussionsaboutcorefacilitiesimprovementsat
MichiganTech,identifyingwaystoexpandourcapacityforsupercriticalchemistry
andconservationbyaddinganindustrialscaleautoclavetoourfacilitiesand
seekingsupportforpost-docresearchersinconservationscience.
Asafinaloutcome,theresearchershavecontactwithprofessional
conservatorsandscientistsatA-Corros,TheWarrenLaschResearchCenteratthe
ClemsonUniversityRestorationInstitute,theArchaeologicalPreservationResearch
LaboratoryatTexasA&MUniversity,andtheGermanMiningMuseum.Welook
forwardtocollaborationswiththesescientistsasweekseekfundstosupportthe
ongoingworktodevelopSCFconservation,enhanceourowncorefacilities,mentor
studentsintograduateeducationandconservationcareers,andprovide
conservationservicestotheheritageinstitutionsinourregion.
6.Acknowledgments
Wewishtothanktheadvisorsforourexperiments,initiallyDr.StefanBrügerhoff
andC.WayneSmith,thenagroupofconservatorsattheVCongreso
LatinoamericanodeRestauracióndeMetal,includingJohnScott,SorayaAlcalá,
IreneDelaveris,andLuisEnriqueCastilloNarrea.Muchlaterinourwork,we
44
benefitedbyconversationswithPhilippedeViviésandPaulMardikianatBigStuff
2015.Ourexperimentsandourinsightsintoconservationscienceweregreatly
improvedasaresultoftheseinteractions.Asalways,anyerrorshereinareourown.
WearealsodeeplyindebtedtoMaryStriegelandthestaffattheNationalCenterfor
PreservationTechnologyandTraining,whowereunderstandingofdelaysinour
project,supportedthreeextensionsofthework,andcultivatedourfirstmajor
collaborationinconservationscience.
TimScarlettalsowishestothankthestaffatMichiganTechnologicalUniversity’s
SponsoredProgramsOfficeandSponsoredProgramsAccounting,particularlyLisa
Jukkala,DarleneSaari,andTracyLaPlante.GinaStevensprovidedessential
administrativesupportintheDepartmentofSocialSciences.Anumberofstaffat
MichiganTechprovidedassistancetrackingdowninternationalconference
proceedingsandnotoriouslydifficulttofindcopiesofconservationreports,
includingNoraAllredandLibraryAssistantsStephanieReed,KariBellinSloat,and
ElisabethDennis,amongotherswhohandleemailsfromIll@mtu.edu.
7.References
Ankersmit,B.,M.Griesser-Stermscheg,andS.Sutherland.2008.RustNeverSleeps:
recognizingmetalsandtheircorrosionproducts.Winnipeg:ParksCanada,
WesternandNorthernServiceCentre.
http://www.depotwijzer.be/sites/default/files/files/rust_never_sleeps.pdf
Atkinson,Alexander,BrendanPelto,EricPomber,andTimothyJamesScarlett.
2014a.“ExperimentsInCostEffectiveIronConservationTechniquesAt
MichiganTechnologicalUniversity.”Posterpresentedatthe2014Midwest
ArchaeologicalConference,Champlain,IL,3rd.October2014.
Atkinson,Alexander,BrendanPelto,EricPomber,TimothyJamesScarlett,Shubham
Barole,StephanieTankersley,andGerardCaneba.2014b.“Experimentsin
CostEffectiveIronConservationTechniquesatMichiganTechnological
University”Posterpresentedatthe10thAnnualMidwestHistorical
ArchaeologyConference,Niles,MI,27thSeptember,2014.
Bayle,M.,P.deViviés,J.B.Memet,E.Foy,P.Dillmann,andD.Neff.2015.“Corrosion
producttransformationsinalkalinebathsunderpressureandhigh
temperature:Thesub-criticalstabilisationofmarineironartefactsstored
underatmosphericconditions.”MaterialsandCorrosion,prepublication
online.
http://onlinelibrary.wiley.com/doi/10.1002/maco.201508257/abstract;jses
sionid=FDC35F4854308C78696DD7ECFCB2C4D7.f04t02
Bobichon,C.,C.Degrigny,F.Dalard,andQ.K.Tran.2000.“AnElectrochemicalStudy
ofIronCorrosionInhibitorsinAqueousPolyethyleneGlycolSolutions.”
StudiesinConservation45(3):145-153.
Brüggerhoff,S.,L.Koenig,C.Ochwat,andS.Seidel.2008.Preservationofindustrial
45
heritageobjectspresentedinacriticalclimateenvironment–the
demonstrationmineoftheGermanminingMuseum.in:Konference
konzervátorů-restáuratorů,Pribram2008,28-32.
Caneba,GTandY.L.Dar.2011.EmulsionFree-RadicalRetrograde-Precipitation
Polymerization.Heidelberg:Springer-Verlag.
Caneba,G.T.2011,Free-RadicalRetrograde-PrecipitationPolymerization(FRRPP):
NovelConcept,Processes,Materials,andEnergyAspects.Heidelberg:SpringerVerlag.
Chaumat,G.,Q.K.Tran,C.Perre,andG.Lumia.1999.“TrialsofShapeRecovering
fromCollapsedWaterloggedWoodbyTreatmentwithCO2Supercritical
Fluid.”inProceedingsofthe7thICOMGrouponWetOrganicArchaeological
MaterialsConference,Grenoble,France,1998,ed.C.Bonnot-Diconne,X.Hiron,
Q.K.Tran,andP.Hoffmann,137-142.Grenoble,France:ARC-NucléartCEA.
Cocero,M.J.,E.Alonso,andS.Lucas.2000.“PilotPlantforSoilRemediationwith
SupercriticalCO2underQuasi-IsobaricConditions.”IndustrialEngineering
andChemistryResearch39(12):4597–4602.
Coeuré,P.,G.Chaumat,Q.K.Tran,andC.Perre.1998.“DieKonservierungvon
Naßholz:VersuchemitPolyethyleneglykolinSuperkritischerKohlendioxid
Flüssigkeit”ArbeitsblätterfürRestauratoren31(1):262-265.
Conrads,M.2008.“TransparenteBeschichtungsstoffezumSchutzvon
Kulturgütern.”MaterialsandCorrosion59(3):232-238.
Cook,D.C.,andC.E.Peterson.2005.“CorrosionofSubmergedArtifactsandthe
ConservationoftheUSSMonitor.”inIndustrialApplicationsoftheMossbauer
Effect,AmericanInstituteofPhysicsConferenceProceedings,Vol765,ed.M.
Gracia,J.F.Marco,andF.Plazaola,91-96.Melville,NewYork:American
InstituteofPhysics.
Cretté,SA,N.G.González-Pereyra,B.Rennison,M.P.Scafuri,P.Mardikian,M.J.
Drews,andM.Carrier.2012.“ConservingWaterloggedArchaeologicalCorks
UsingSupercriticalCO2andMonitoringTheirShrinkageUsingStructuredLight3DScanning.”inProceedingsofthe11thICOM-CCGrouponWetOrganic
ArchaeologicalMaterialsConferenceinGreenville,2010,eds.E.Williamsand
K.Straetkvern,257-142.Bremerhaven,Germany:InternationalCouncilof
Museums(ICOM),CommitteeforConservationWorkingGrouponWet
OrganicArchaeologicalMaterials.http://www.lulu.com/shop/emilywilliams-and-kristiane-straetkvern/proceedings-of-the-11th-icom-cc-groupon-wet-organic-archaeological-materials-conference-greenville2010/paperback/product-20033707.html
Cretté,S.A.,N.G.González-Pereyra,andB.Rennison.2013.“Conservationand
TreatmentMonitoringofWaterloggedArcheologicalCorksUsing
SupercriticalCO2.”TheJournalofSupercriticalFluids79(July):299-313
http://issf2012.com/handouts/documents/224_001.pdf
46
Cronyn,J.M.1990.TheElementsofArchaeologicalConservation.NewYork:
Routledge.
D’Andrea,A.,S.Mariani,A.Aliboni,A.Tagliacozzo,andE.Cerilli.2003.“Supercritical
DryingProcessesinConservationofWaterloggedOsteologicalRemains.”In
Proceedingsofthe6thInternationalSymposiumonSupercriticalFluids,
Versailles,France,April28-30,2003.
http://www.isasf.net/fileadmin/files/Docs/Versailles/Papers/PMn1.pdf.
Dalard,F,Y.Gourbeyre,andC.Degrigny.2002.“ChlorideRemovalfrom
ArchaeologicalCastIronbyPulsatingCurrent.”StudiesinConservation
47(2):117-121.http://www.jstor.org/stable/1506851
deViviés,P.,D.Cook,M.Drews,N.Gonzales,P.Mardikian,andJ.B.Memet.2007.
“TransformationofAkaganeiteinArchaeologicalIronArtefactsUsing
SubcriticalTreatment.”inMetal07:interimmeetingoftheICOM-CCMetal
WG:Amsterdam,17-21September2007.Volume5:Protectionofmetal
artefacts,ed.C.Degrigny,R.vanLangh,I.Joosten,andB.Ankersmit,26–30.
Amsterdam:Rijksmuseum.
Deegan,M.,andT.J.Scarlett.2008.“TheConservationofFerrousMetalsfromthe
WestPointFoundrySite.”BulletinoftheNewYorkStateArchaeological
Association124:56-68.
Fraunhofer-InstitutfürSilicatforschungISC.(2008).‘Consist:Eco-friendly
conservationofIndustrialMonumentsmadeofIronandSteel.’Pp.26-27.In
AnnualReport2007,Electronicdocumentdownloadedfrom
http://www.isc.fraunhofer.de/uploads/media/annual_report_2007_01.pdf
onThursday,October13,2011
Gilberg,M.R.,andN.J.Seeley.1982.“TheAlkalineSodiumSulphiteReduction
ProcessforArchaeologicalIron:ACloserLook.”StudiesinConservation
27(4):180-184.
Gonzales,N.,D.Cook,P.DeViviés,M.DrewsandP.Mardikian.2007.“TheEffectsof
CathodicPolarization,SoakinginAlkalineSolutionsandSubcriticalWateron
CastIronCorrosionProducts.”inMetal07:interimmeetingoftheICOM-CC
MetalWG:Amsterdam,17-21September2007.Volume3:Useof
electrochemicaltechniquesinmetalconservation,ed.C.Degrigny,R.van
Langh,B.Ankersmit,andI.Joosten,32-37.Amsterdam:Rijksmuseum.
Gonzalez-Pereyra,N.,T.Brocard,S.Cretté,P.DeViviés,M.Drews,andP.Mardikian.
2011.“TheUseofSubcriticalFluidsfortheStabilizationofConcretedIron
Artefacts.”inMetal2010:ProceedingsoftheInterimMeetingoftheICOM-CC
MetalWorkingGroup.eds.P.Mardikian,C.Chemello,C.Watters,andP.Hull,
39-49.Charleston,SouthCarolina:ClemsonUniversity.
http://www.lulu.com/shop/clemson-university/metal-2010-proceedingsof-the-interim-meeting-of-the-icom-cc-metal-working-group-charlestonsouth-carolina-usa11-15-october-2010-digital-version/ebook/product18691539.html
47
González,NG,P.deViviés,M.J.Drews,andP.Mardikian.2004.“HuntingFreeand
BoundChlorideintheWroughtIronRivetsfromtheAmericanCivilWar
SubmarineH.L.Hunley.”JournaloftheAmericanInstituteforConservation
43(2):161-174.http://www.jstor.org/stable/4129651
González,NG,P.Mardikian,L.Näsänen,andM.J.Drews.2010.“Theuseof
subcriticalfluidsforthestabilisationofarchaeologicaliron:anoverview.”in
CorrosionandConservationofCulturalHeritageMetallicArtefacts,eds.P.
Dillmann,D.Watkinson,A.Angelini,andA.Adriaens,434-465.Philadelphia,
PA:WoodheadPublishingLimited.
http://www.sciencedirect.com/science/article/pii/B978178242154250020
7
GuilminotE.,N.Huet,D.Neff,P.Dillmann,C.Rémazeilles,P.Refait,S.Reguer,L.
Bertrand,F.Nicot,andF.Mielcarek.2008.“DechlorinationofArchaeological
IronArtefacts:DechlorinationEfficiencyAssessmentAssistedbyPhysicochemicalAnalyticalHigh-techMethods.”inICOMCommitteeforConservation,
15thTriennialConferenceinNewDelhi,Volume1,ed.J.Bridgland,435-443.
NewDelhi:AlliedPublishers.
Hamilton,DL.1999.MethodsofConservingArchaeologicalMaterialfrom
UnderwaterSites.CollegeStation:ConservationResearchLaboratory,Center
forMaritimeArchaeologyandConservation,TexasA&MUniversity.
Helm-HansenN.,J.VanLanchot,C.D.Szalkay,andS.Turgoose.1993.
“ElectrochemicalAssessmentandMonitoringofStabilisationofHeavily
CorrodedArchaeologicalIronArtefacts.”CorrosionScience35:767–774.
IntroteknikAB(n.d.).Non-DestructiveConservationofIndustrialHeritages.Self
publishedpamphlet,downloadedfrom
http://www.introteknik.se/eng/Media/Industrial%20Heritage.pdfon
Thursday,October13,2011.
Issaro,N.,C.Abi-Ghanem,andA.Bermond.2009.“Fractionationstudiesofmercury
insoilsandsediments:Areviewofthechemicalreagentsusedformercury
extraction.”AnalyticaChimicaActa631(1):1–12.
Jacot-Guillarmod,M.,O.Rozenbaum,V.L'Hostis,P.Dillmann,D.NeffandC.Gervais.
2015."Degradationmechanismsofreinforcingironrebarsinmonuments:
theroleofmultiscaleporosityintheformationofcorrosionproducts
investigatedbyX-raytomography."JournalofAnalyticalAtomicSpectrometry
30:580.
Kang,S.M,A.Unger,andJ.J.Morrell.2004.“TheEffectofSupercriticalCarbon
DioxideExtractiononColorRetentionandPesticideReductionofWooden
Artifacts.”JournaloftheAmericanInstituteforConservation43(2):151-160.
Kaye,BandD.J.Cole-Hamilton.1998.“SupercriticalDryingofWaterlogged
ArchaeologicalWood.”inHiddenDimensions:theculturalsignificanceof
wetlandarchaeology,ed.K.Gernick,329-339.Vancouver:UBCPress.
48
Kaye,B,andD.J.Cole-Hamilton.1994.“Novelapproachestotheconservationofwet
wood.”inACelebrationofWood:proceedingsofaconferenceheldbyYork
ArchaeologicalWoodCentreinYork,June1993,ed.J.A.Spriggs,35-48.York:
YorkArchaeologicalWoodCentre.
Kaye,B,D.J.Cole-Hamilton,andK.Morphet.2000.“SupercriticalDrying:ANew
MethodforConservingWaterloggedArchaeologicalMaterials.”Studiesin
Conservation45(4):233-252.http://www.jstor.org/pss/1506861
Keene,S.,andC.Orton.1985.“StabilityofTreatedArchaeologicalIron:An
Assessment.”StudiesinConservation30(3):136-142.
KergourlayF.,E.Guilminot,D.Neff,C.Remazeilles,S.Reguer,P.Refait,R.Mirambet,
E.FoyandP.Dillmann.2010.“InfluenceofCorrosionProductsNatureon
DechlorinationTreatment:CaseofWroughtIronArchaeologicalBarsStored
2YearsinAirBeforeNaOHTreatment.”CorrosionEngineeringScienceand
Technology45:407–13.
KiesenbergJ.1997.AusgewählteUntersuchungenzurÜberprüfungderWirksamkeit
vontransparentenBeschichtungssystemen-hierbesondersWachse-als
KorrosionsschutzaufkorrodierteIndustriedenkmälernausEisenundStahl.
Bochum:Diplomarbeit.
Li,C.2006.“BiodeteriorationofAcrylicPolymersParaloidB-72andB-44:Reporton
FieldTrials.”AnatolianArchaeologicalStudies15:283-290.
Liu,J,Y.Li,andM.Wu.2008.“ElectrochemicalMethodsforChlorideRemovalfrom
SimulatedCastIronArtefacts.”StudiesinConservation53(1):41-48.
http://www.jstor.org/stable/20619528
Luo,X.2002,“Theapplicationofsupercriticaldryingindehydratingwaterlogged
archaeologicalartifacts.”SciencesofConservationandArchaeology
14(supplement):101-107.
MardikianP.,N.Gonzalez,M.Drews,P.deViviés.2010.“NewPerspectives
RegardingtheStabilizationofTerrestrialandMarineArchaeologicalIron.”in
Iron,Steel&SteamshipArchaeology,Proceedingsofthe2ndAustralian
SeminarheldinPerth,Melbourne,andSydney,2009.ed.M.McCarthy,113118.Fremantle:WesternAustraliaMaritimeMuseum.
Mattheisen,H,L.R.Hilbert,andD.J.Gregory.2003.“SideriteasaCorrosionProduct
onArchaeologicalIronfromaWaterloggedEnvironment.”Studiesin
Conservation48(3):183-194.
http://www.jstor.org/stable/1506891?seq=1&cid=pdfreference#references_tab_contents
Matthiesen,H.,L.R.Hilbert,andD.J.Gregory.2003.“SideriteasaCorrosionProduct
onArchaeologicalIronfromaWaterloggedEnvironment.”Studiesin
Conservation48(3):183-194.
McNeil,M.B.andB.J.Little.1999.“TheUseofMineralogicalDatainInterpretation
ofLong-TermMicrobiologicalCorrosionProcesses:SulfidingReactions.”
49
JournaloftheAmericanInstituteforConservation38(2):186-199.
Memet,J.2008.“ConservationofUnderwaterCulturalHeritage:characteristicsand
newtechnologies.”MuseumInternational60(4):42-49.
Mottner,P,H.Roemich,M.Pilz,S.Brüggerhoff,andJ.Kiesenberg.2004.
“InvestigationsofTransparentCoatingsfortheConservationofIronand
SteelOutdoorIndustrialMonuments.”inMetal2001:proceedingsofthe
internationalconferenceonmetalsconservation,Santiago,Chile,2-6April
2001,ed.I.D.MacLeod,279-288.Welshpool,Australia:WesternAustralian
Museum.
Näsänen,L.M.E.,N.G.González-Pereyra,andS.A.Cretté.2011,‘TheSubcritical
Mass-TreatmentofaRangeofIronArtifactsfromVaryingContexts’,inThe
2011Asia-PacificRegionalConferenceonUnderwaterCulturalHeritage
Proceedings,Manila,Philippines,ed.MStaniforth,733-744.Bangkok:Asian
AcademyforHeritageManagement.AccessedOctober6,2015,TheMUA
Collection,http://www.themua.org/collections/items/show/1255
Näsänen,L.M.E.,N.G.González-Pereyra,S.A.Cretté,andP.DeViviés.2013.“The
applicabilityofsubcriticalfluidstotheconservationofactivelycorroding
ironartifactsofculturalsignificance.”TheJournalofSupercriticalFluids
79(July):289-298.
Navrotsky,A,L.Mazeina,andJ.Majzlan.2008.“Size-DrivenStructuraland
ThermodynamicComplexityinIronOxides”Science319(5870):1635-1638.
NeffD.,P.Dillmann,L.Bellot-Gurlet,andG.Beranger.2005.“CorrosionofIron
ArchaeologicalArtefactsinSoil:CharacterisationoftheCorrosionSystem.”
CorrosionScience47:515–35.
Neff,D.2012.“Dunucléaireàlaconservationdupatrimoine-Compréhensiondes
mécanismesdecorrosiondesalliagesferreuxsurlelongtermegrâceàla
caractérisationmulti-échelles.”Thesis,MaterialChemistry,UniversitéPierre
etMarieCurie.https://tel.archives-ouvertes.fr/tel-00741750/document
Newman,S.2002.“IronandIronAlloys:areviewofsometreatmentsforhistoric
artifacts.”inBacktoBasics:Selectedpapersfromaseriesofconferences
organizedbytheMetalsSectionofUKIC,1999-2000,ed.HMoody,43-48.
London:MetalsSectionPress,TheMetalsSectionoftheUnitedKingdom
InstituteforConservation.
North,N.1982.“CorrosionProductsonMarineIron.”StudiesinConservation
27(2):75-83.
North,N.A.,andC.Pearson.1977.“ThermalDecompositionofFeOClandMarine
CastIronCorrosionProducts.”StudiesinConservation22(3):146-157.
Pingitore,G.,T.Cerchiara,G.Chidichimo,M.Castriota,C.Gattuso,andD.Marino.
2015.“Structuralcharacterizationofcorrosionproductlayerson
archaeologicalironartifactsfromVignaNuova,Crotone(Italy).”Journalof
CulturalHeritage16(3):372-376.
50
RefaitP.,andJ.M.Genin.1997.“TheMechanismsofOxidationofFerrous
Hydroxychlorideβ-Fe2(OH)3ClinAqueousSolution:TheFormationof
AkaganeitevsGoethite.”CorrosionScience39:539–53.
Réguer,S.,F.Mirambet,C.Rémazeilles,D.Vantelon,F.Kergourlay,D.Neff,andD.
Dillmann.2015.“Ironcorrosioninarchaeologicalcontext:Structural
refinementoftheferroushydroxychlorideβ-Fe2(OH)3Cl.”CorrosionScience
100(November):589–598.
http://www.sciencedirect.com/science/article/pii/S0010938X15300676
Rogers,B.2004.TheArchaeologistsManualforConservation:aGuidetoNon-Toxic,
MinimalInterventionArtifactStablization,NewYork:Kluwer
Academic/PlenumPublishers.
Rowe,M.W.,J.Phomakay,J.O.Lay,O.Guevara,K.Srinivas,W.K.Hollis,K.L.
Steelman,T.Guilderson,T.W.StaffordJr.,S.L.Chapman,andJ.W.King.2013.
“Applicationofsupercriticalcarbondioxide–co-solventmixturesforremoval
oforganicmaterialfromarcheologicalartifactsforradiocarbondating.”The
JournalofSupercriticalFluids79(July):314–323.
Scarlett,T.J.2014.“EstabilizacióndeLotedeArtefactosFerrososdeMetalesque
usanCO2SupercríticoPolímeroImpregnación.”PaperpresentedattheV
CongresoLatinoamericanodeRestauracióndeMetal,Grupo
LatinoamericanodeRestauracióndeMetales(G.L.R.M.),Lima,Peru,5th
September2014.
Scarlett,T.J.2015.“BatchTreatmentsforIndustrialHeritageCollections:
SupercriticalCO2TreatmentstoStabilizeFerrousMetalArtifacts.”Paper
presentedatBigStuff2015,CentreHistoriqueMinier(CHM),Lewarde,
France,4thSeptember2015.
Schindelholz,E,R.Blanchette,D.Cook,M.Drews,J.Jurgens,S.Hand,B.Held,andB.
Seifert.2009.“AnevaluationofsupercriticaldryingandPEG/freezedryingof
waterloggedarcheologicalwood.”InProceedingsofthe10thICOMGroupon
WetOrganicArchaeologicalMaterials,Amsterdam,Netherlands,10-15
September2007,eds.K.Strœtkvern,D.J.Huisman,T.Grant.Amersfoort:
RijksdienstvoorArcheologie,CultuurlandschapenMonumenten.accessed
10/6/2015fromhttp://www.icomcc.org/161/Proceedings/#.VhV1gRNVikq
Schindelholz,E.2007,‘AnEvaluationofSupercriticalDryingandPEG/FreezeDrying
ofWaterloggedArchaeologicalWood’,ReporttotheNationalCenterfor
PreservationTechnologyandTraining,NationalParkService,Natchitoches,
viewedOctober13,2011,http://ncptt.nps.gov/wp-content/uploads/200704-optimized.pdf
Schmidt-Ott,K.,andV.Boissonnas.2002.Low-PressureHydrogenPlasma:An
AssessmentofItsApplicationonArchaeologicalIron.StudiesinConservation
47(2):81-87.
51
Schmutzler,B.,andN.Ebinger-Rist.2008.“Theconservationofironobjectsin
archaeologicalpreservation–Applicationandfurtherdevelopmentof
alkalinesulphitemethodforconservationoflargequantitiesofironfinds.”
MaterialsandCorrosion59(3):248-253.
Sease,C.1994.AConservationManualfortheFieldArchaeologist,3rdEdition.
ArchaeologicalResearchTools4.LosAngeles:InstituteforArchaeology,
UniversityofCalifornia.
Sederholm,B.,andM.Forssander.2002.“OvercoatingofStructurespaintedwith
lead-basedpaint:analternativestrategyincorrosionprotection
maintenance.”KIRapport2002:4.
Seipelt,B.,M.Pilz,andJ.Kiesenberg.1998.“TransparentCoatings—suitable
corrosionprotectionforindustrialheritagemadeofiron?”inMetals
98:ProceedingsoftheInternationalConferenceonMetalsConservation,eds.W
MoureyandL.Robbiola,291-296.London:James&JamesLtd.
Seipelt,B.,andS.Brüggerhoff.1997.“Protectionoftechnicalculturalheritage
againstatmosphericcorrosion.”inMetal95:ProceedingsoftheInternational
conferenceonmetalsconservation,Semur-en-Auxois,25-28September1995,
eds.I.MacLeod,S.L.Oennec,andL.Robbiola,228-232.London:James&
James,London.
Seipelt,B.,J.Kiesenberg,andM.Pilz.1998.“TransparenterKorrosionsschutzfür
IndustriedenkmälerausEisenundStahl.”Industrie-Kultur2:16-17.
Selwyn,LS,andV.Argyropoulos2005.“RemovalofChlorideandIronIonsfrom
ArchaeologicalWroughtIronwithSodiumHydroxideandEthylenediamine
Solutions.”StudiesinConservation50(2):81-100.
http://www.jstor.org/stable/25487727
Selwyn,L.S.2004.“OverviewofArchaeologicalIron:theCorrosionProblem,Key
FactorsAffectingTreatment,andGapsinCurrentKnowledge”inMetal04:
ProceedingsoftheInternationalConferenceonMetalConservation,ed,J.
AshtonandD.Hallam,294-306.Canberra:NationalMuseumofAustralia.
http://www.nma.gov.au/about_us/publications/metal_04_conference_proce
edings
Selwyn,L.S.2011,‘PanelDiscussion:TreatmentofArchaeologicalIron’,inMetal
2010:ProceedingsoftheInterimMeetingoftheICOM-CCMetalWorking
Group.eds.P.Mardikian,C.Chemello,C.Watters,andP.Hull,50-53.
Charleston,SouthCarolina:ClemsonUniversity.
http://www.lulu.com/shop/clemson-university/metal-2010-proceedingsof-the-interim-meeting-of-the-icom-cc-metal-working-group-charlestonsouth-carolina-usa11-15-october-2010-digital-version/ebook/product18691539.html
Selwyn,L.S.,P.J.Sirois,andV.Argyropoulos.1999.“TheCorrosionofExcavated
ArchaeologicalIronwithDetailsonWeepingandAkaganéite.”Studiesin
Conservation44(4):217-232.
52
Selwyn,L.S.,W.R.McKinnon,andV.Argyropoulos.2001.“ModelsForChlorideIon
DiffusionInArchaeologicalIron.”StudiesinConservation46(2):109-120.
Shashoua,Y.,M.Taube,andT.Holst.2009.“ProtectionofIronandSteelinIndustrial
HeritageObjects.”inIncredibleIndustry:Preservingtheevidenceofindustrial
society,eds.M.Ryhl-Svendsen,K.Borchersen,andW.Odder,175-186.
Copenhagen:NordicAssociationofConservators,Nationalmuseet.
Smith,C.W.2003.ArchaeologicalConservationusingPolymers:PracticalApplications
forOrganicArtifactStabilization.CollegeStation:TexasA&MUniversity
Press.
Smyth,T.J.,R.G.Zytner,andW.H.Syiver.1999.“InfluenceofWateronthe
SupercriticalFluidExtractionofNaphthalenefromSoil.”Journalof
HazardousMaterials66(2):183.
Sousa,M,M.J.Melo,T.Casimiro,andA.Aguilar-Ricardo.2007.“TheArtofCO2for
ArtConservation:AGreenApproachtoAntiqueTextileCleaning.”Green
Chemistry9(9):943-947.
http://pubs.rsc.org/en/Content/ArticleLanding/2007/GC/b617543k
http://pubs.rsc.org/en/Content/ArticleLanding/2007/GC/b617543k#!divA
bstract
Svishchev,I.M.,D.T.Kallikragas,andA.Y.Plugatyr.2013.“Moleculardynamics
simulationsofsupercriticalwaterattheironhydroxidesurface.”Journalof
SupercriticalFluids78:7–11.
http://www.sciencedirect.com/science/article/pii/S0896844613001149
Tello,H.,E.Jelen,andA.Unger.2005.“DecontaminationofEthnologicalCollections
usingSupercriticalCarbonDioxide.”CollectionForum19(1-2):45-48.
Teshirogi,M,E.Tahata,M.Kikuchi,H.Inomata,Y.Kohdzuma,T.Koezuka,andM.
Sawada.2002.“Conservationtreatmentofwater-loggedwoodwith
supercriticalcarbondioxide.”inProceedingsofthe8thICOMGrouponWet
OrganicArchaeologicalMaterialsConference,Stockholm,2001,eds.P.
Hoffman,J.Spriggs,T.Grant,C.Cook,andA.Recht,371-377.Bremerhaven:
DeutschesSchiffahrtsmuseum.
http://www.dsm.museum/service/publikationen/weitereschriften/proceedings-of-the-8th-icom-group-on-wet-organicarchaeological-materials.4475.de.html
Wai,C.M.,Y.Lin,R.Brauer,andS.Wang.1993.“Supercriticalfluidextractionof
organicandinorganicmercuryfromsolidmaterials.”Talanta40(9):1325–
1330.
Wang,Q.2007.“AnInvestigationofDeteriorationofArchaeologicalIron.”Studiesin
Conservation52(2):125-134.http://www.jstor.org/stable/20619492
Wang,T.2005,SupercriticalFluidExtractionofMineralsinMartianSoils.Ph.D.
Dissertation,ChemicalEngineering,VanderbiltUniversity.
http://etd.library.vanderbilt.edu/available/etd-11222005-
53
135311/unrestricted/TongWang-disertation.pdf
Watkinson,D,andM.T.Lewis.2005.“DesiccatedStorageofChloride-Contaminated
ArchaeologicalIronObjects.”StudiesinConservation50(4):241-252.
http://www.jstor.org/stable/25487756
Watkinson,D.1983.“DegreeofMineralization:ItsSignificancefortheStabilityand
TreatmentofExcavatedIronwork.”StudiesinConservation28(2):85-90.
Watkinson,D.2010.“PreservationofMetallicCulturalHeritage.”inShreir’s
Corrosion,1stEdition.ed.T.Richardson,3308-3340,Philadelphia:Elsevier
B.V.
Watson,J,V.Fell,andJ.Jones.2008,InvestigativeConservation.Swindon:English
HeritagePublishing.
Weber,J.2008.RostinKunstundAlltagdes20.Jahrhunderts[RustinArtand
EverydayLifeofthe20thCentury],Berlin:Reimer.
Weizhen,O,andX.Chunchun.2005.“StudiesonLocalizedCorrosionand
DesalinationTreatmentofSimulatedCastIronArtifacts.”Studiesin
Conservation50(2):101-108.
54