POROSITYDETERMINATIONWITHHELIUMPYCNOMETRYASAMETHODTOCHARACTERIZEWATERLOGGEDWOODSANDTHEEFFICACYOFTHECONSERVATIONTREATMENTS*I.D.DONATOandG.LAZZARADipartimentodiChimica‘StanislaoCannizzaro’,UniversitàdegliStudidiPalermo,VialedelleScienze—Edificio17,90128Palermo,ItalyTheheliumpycnometerallowsustomeasurethecell-walldensityofdrywoodsandthebasicdensityofwoodsamplessoakedwithwaterand/oraconsolidantsolutionifanon-volatilesolventisused.Theseparameterswerecorrelatedtotheporosity,whichfordegradedwater-loggedwoodisrelatedtothemaximumwatercontent.Moreover,thishasrevealedthepossibilityofinvestigating,bymeansofaccuratecell-walldensitydetermination,theefficacyofseveralconsolidantsinthetreatmentofwaterloggedwoods.KEYWORDS:HELIUMPYCNOMETRY,POROSITY,DENSITY,WATERLOGGEDWOOD,CONSOLIDATIONINTRODUCTIONManypropertiesofarchaeologicalwood,particularlythoseinvolvingdeterioration,sorption,penetrability,swellingandstrain-relatedphenomena,areprobablydependentontheporosityofthewood,which,inturn,dependsoncell-wallandbasicdensities(Caprettietal.2008).Thus,accuratemeasurementsofthesephysicalparametersareessentialforacorrectdeterminationofsuchproperties.Severalconventionalmethodshavebeensuccessfullyappliedfordeterminationofthephysicalandchemicalpropertiesofwaterloggedarchaeologicalwoods(Caprettietal.2008;Pizzoetal.2010).Theconceptofdensityisverysimple,andthemassandvolumeofabodywouldseemtobeamongtheeasiestphysicalparameterstomeasure.Actually,foraporous,hygroscopic,polymericmaterial,themeasurementofthevolumeisextremelycontroversial.Thisisparticularlytrueinthedeterminationofbasicandcell-walldensitiesofwood.Fordetermina-tionofthebasicdensity,thevolumeismeasuredbyeitheroftwotechniques.Thevolumeofsampleswithapropergeometricalshapeisobtainedfromthecharacteristicdimensionsmeasuredbymeansofacalliper,whereasforsampleswithanirregularshape,thevolumeisobtainedbymeansofaRegnaultpycnometer,employingmercuryasareferenceliquid.Themeasureofthecell-wallvolumeinordertodeterminethecell-walldensityismoredifficult.Gaspycnometry,basedontheBoyle–Mariottelawofthevolume–pressurerelationship,isaneffectivetooltodeterminethevolumeofporousmaterialsbecauseitdoesnotpossessthelimitationsofothertestingmethods(Tamari2004),suchastheproblemofairentrapment.Ingaspycnometrythemainapproximationintheestimationofthesamplevolumeisthenon–idealgasbehaviouranditsadsorptionontothesolidmaterial.Nevertheless,experimentalresults(WeberandBastick1968)andmodernsimulationsofgasadsorptiononsolids(NeimarkandRavikovitch1997;TaluandMyers2001)havedemonstratedthatheliumcanreasonablybeconsideredasanidealandnon-adsorbinggasatroomtemperature(300K)andlowpressure(<0.5MPa).Agas*Received12July2011;accepted7November2011bs_bs_bannerArchaeometry54,5(2012)906–915doi:10.1111/j.1475-4754.2011.00657.x©UniversityofOxford,2012pycnometer,usingpurehelium,wasemployedinthisworktomeasurethecell-walldensityofdegradedwaterloggedwoodafterdehydration,andofwoodfindspreservedwithenforcingmaterialsafterexsiccation.Moreover,heliumpycnometrycouldbeusedtomeasurethebasicdensitywhenawoodsamplesoakedwithwaterand/oraconsolidantsolutionofanon-volatilesolventisused.Theprimarypurposeofthisstudywasto:(a)correlatetheMaximumWaterContent(MWC%)(Florian1990,8)withtheporosityofdegradedwaterloggedwood;(b)testthemodificationsofthecell-walldensityofsoftwoodandhardwooddehydratedarchaeo-logicalfinds;and(c)s...
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