The glass walls of Samarra (Iraq): Ninth-century Abbasid glass production and imports

15 Pages·2017·10.96 MB·English

RESEARCHARTICLE
The glass walls of Samarra (Iraq): Ninth-
century Abbasid glass production and imports
NadineSchibille1*,AndrewMeek2,MarkT.Wypyski3,JensKro¨ger4,MariamRosser-
Owen5,RosalindWadeHaddon6
1 IRAMAT-CEB,UMR5060,CNRS/Universite´d’Orle´ans,Orle´ans,France,2 DepartmentofScientific
Research,TheBritishMuseum,London,UnitedKingdom,3 DepartmentofScientificResearch,The
MetropolitanMuseumofArt,NewYork,NY,UnitedStatesofAmerica,4 Museumfu¨rIslamischeKunst,
StaatlicheMuseenzuBerlin,Berlin,Germany,5 MiddleEasternSection,VictoriaandAlbertMuseum,
London,UnitedKingdom,6 IndependentScholar,London,UnitedKingdom
a1111111111
a1111111111 *[email protected]
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a1111111111 Abstract
CapitaloftheAbbasidCaliphatebetween836and892CE,thepalace-cityofSamarraoffers
aprecisewindowintoearlyIslamicartandarchitecture.Excavationsconductedmorethan
100yearsagoareseenasthebeginningsofscientificIslamicarchaeology,andhaveyielded
OPENACCESS
anexceptionalarrayoffindsincludingawealthofglassartefacts.Thechemicalcomposition
Citation:SchibilleN,MeekA,WypyskiMT,Kro¨ger
ofglassreflectsthenatureoftherawmaterialsandtheirgeologicalprovenanceandcan
J,Rosser-OwenM,WadeHaddonR(2018)The
glasswallsofSamarra(Iraq):Ninth-century thereforerevealpasttechnologiesandeconomicandculturalinteractions.Throughhigh-res-
Abbasidglassproductionandimports.PLoSONE olutionanalysisofacomprehensiveglassassemblagefromSamarrawehavenewevidence
13(8):e0201749.https://doi.org/10.1371/journal.
thatpointstotheexistenceofanadvancedAbbasidglassindustry,aswellastheimportof
pone.0201749
specificglassobjectsforthethrivingnewcapitalcity.Quantitativeanalyticaldataof58ele-
Editor:DongHoonShin,SeoulNationalUniversity
mentsbylaserablationinductivelycoupledplasmamassspectrometry(LA-ICP-MS)showa
CollegeofMedicine,REPUBLICOFKOREA
strikingcorrelationbetweenobjecttypesandglasscompositions.Thecompositionalprofiles
Received:February2,2018
oftworelatedplantashgroupsofarchitecturalglasspointtoalocalproduction,destinedfor
Accepted:July20,2018 thedecorationofthefamedglasswallsofAbbasidpalaces.Theselectiveuseofobjects,
Published:August22,2018 materialsandcolourstocreatereflectiveandluminousglasswallsareindicativeofthegreat
culturalandeconomicvalueofglassduringtheAbbasidperiod.Ourfindingsthusconfirmthe
Copyright:©2018Schibilleetal.Thisisanopen
accessarticledistributedunderthetermsofthe veracityofwrittensourcesthatstipulatetheproductionofglassinthevicinityofSamarra,as
CreativeCommonsAttributionLicense,which wellastheimportofselectedartefactssuchasByzantinemosaictesserae.
permitsunrestricteduse,distribution,and
reproductioninanymedium,providedtheoriginal
authorandsourcearecredited.
DataAvailabilityStatement:Allrelevantdataare
withinthepaperanditsSupportingInformation
Introduction
files(S1andS2Tables).
Funding:PreliminaryEPMAanalyseswere TheArabconquestofEgyptandSyria-PalestineinthetwodecadesfollowingMuhammad’s
supportedbyaBritishAcademySmallResearch deathin632CEdoesnotappeartohavehadanimmediateimpactontheprimaryproduction
Grant(grantno.SG090427toNS).Thisprojecthas ofglassinthatregion.Traditionallarge-scaleRoman-typemineralsodabasedglassmaking
furthermorereceivedfundingfromtheEuropean
continuedwellintotheearlyIslamicperiod[1–4].Newrawglasscompositionsreflectingthe
ResearchCouncil(ERC)undertheEuropean
useofsoda-richplantashinsteadofmineralsoda(natron)emergedonlytowardstheendof
Union’sHorizon2020researchandinnovation
theeighthandearlyninthcenturybothinEgyptandtheLevant[2,5–7].Thereasonsunderly-
programme(grantagreementno.647315toNS).
Thefundingorganizationshadnoinfluenceinthe ingthesechangesremainunknown.However,theycoincidedwithanincreasingcultural
PLOSONE|https://doi.org/10.1371/journal.pone.0201749 August22,2018 1/15
Ninth-centuryAbbasidglassproduction
studydesign,datacollectionandanalysis,decision eastwardshiftthatbeganduringthelaterUmayyadperiodandacceleratedwiththeriseofthe
topublish,orpreparationofthemanuscript. AbbasidsandthefoundationofBaghdadasthenewcapitalin762CE[4,8].Whilenothing
Competinginterests:Theauthorshavedeclared remainsoftheroundcityofBaghdad,thecityofSamarrasome125kilometresnorthofBagh-
thatnocompetinginterestsexist. dadpreservestheartandarchaeologyoftheearlyAbbasidperiod[9].Samarrawasfoundedin
836CEbytheeighthAbbasidcaliphal-Mu‘tasim(r.833–842)ontheeasternbankoftheTigris
asavastpalatialcomplexknownasDāral-Khilāfa[10].Thecityunderwentfurtherdevelop-
mentandexpansionunderhissuccessors,mostnotablyhisnephewal-Mutawakkil(r.847–
861),whocommissionedtheGreatMosqueofSamarraandadditionalpalacessuchasal-
MutawakkiliyyaandBalkuwārā[11].Samarraservedastheadministrativecentreofthe
AbbasidEmpireuntil892CE,whentheAbbasidcourtreturnedtoBaghdad,markingtheend
ofSamarra’scaliphalperiod[9].
ArchaeologicalexcavationsconductedatSamarrabyaGermanexpeditionunderErnst
Herzfeldin1911and1912–13revealedextensivearchitecturalornamentationsofthepalaces,
includinglargenumbersofglassartefacts[12].Circumstantialevidencesuggeststhatglass
mighthavebeenworkedorevenmadeinSamarra.Al-Ya’qubireportsinhisKitabal-Buldan,
theprincipalcontemporarydescriptionofthefoundationofSamarra,thatthecaliphal-Mu‘ta-
sim‘broughtfromal-Basrapeoplewhomakeglass’[9],whilenearbyal-Qadisiyyawasknown
inthethirteenth-centuryastheglassworks(ma‘malal-zujaj)andas‘thelargevillage...where
glassismade’[9].Sitesurveysoftheareahaveyieldedsubstantialdebrisofapossibleglass
industry,andceramicfindstestifytoacontinuousoccupationofal-QadisiyyafromtheSasa-
nianperiodthroughtothethirteenthorfourteenthcentury[9].Anotherpieceofevidence
comesfromtheglassassemblageitself.GlasswasputtoinnovativeusesatSamarrainawide
rangeofarchitecturalornamentationsuchasmosaics,hollowdiamondshaped,triangular,
roundorovalinlaysofcolourlesstransparentglassandshinypurpleormillefioritiles.The
throneroomofal-Mu‘tasim’sDāral-Khilāfaranksamongtheearliestandmostimportant
examplesofarchitecturalglassdecorations(836–842CE)[13],andwrittensourcesindicate
thatthelavishglasswallsweresymbolicallycharged[9,14].Theproliferationofdecorative
architecturalglassisoneofthemostdistinguishingfeaturesinthearchaeologicalrecordof
Samarraandthistypeofglassisoftensupposedtohavebeenmadelocally[12,13].
Toreconstructthenetworksofsupplyandexchange,weconductedhigh-resolutionanaly-
sisofmajor,minorandtraceelementcompositionsofastatisticallysignificantnumberof
well-datedglasssamplesfromSamarrabyLA-ICP-MS.Thesenewcompositionaldataaredis-
cussedinrelationtopublishedearlyIslamicglassassemblages,andidentifiedboththeselective
importofspecificobjecttypesaswellastheregionalproductionofglassatSamarra.Ourfind-
ingsbringaboutaradicalreinterpretationofthescaleandsophisticationofproductionand
interregionaltradeofglassduringtheninthcentury.Theevidenceisfurthermoreconcurrent
withearlyIslamictextualsources.Thisisproofoftheaccuracyoftheseaccountsthatexpressa
culturalidentityandtheimportanceofglassinearlyIslamicsocieties.Thepresentworkthere-
foreconsiderablyexpandsearlieranalyticalstudiesofIslamicglassassemblages[15–18]by
relatingthecompositionaldatatoartefacttypeandopticalpropertiestoassesstheculturaland
economicvalueofvitreousmaterialsduringtheearlyIslamicperiod.
Materialsandmethods
Glasssamples
TheglassfindsfromSamarrahavebeenpublishedaspartoftheexcavationreportsin1928by
CarlJohanLamm[12],representingthefirstextensivepublicationonIslamicglassandIslamic
archaeologymoregenerally[19].Amongalmost400cataloguedglassfindsnowhousedinvari-
ousmuseumacrosstheworldarealargenumberofrelativelysimpleblownandundecorated
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Ninth-centuryAbbasidglassproduction
vessels,includingbowls/plates,bottles,jugsandlamps,andmouldblownvesselsmostlyof
weaklycolouredorcobaltblueglass,aswellasmoresophisticatedvesselswithpinched,
stamped,engravedorlinearwheel-cutdecorationsandafewexampleswithappliedandpainted
surfacedecorations[12].Numerousarchitecturalglassesmakeupalargepartoftheassemblage,
andincludewindows,monochromeormillefioritiles,mosaictesserae,drawntesseraeaswellas
hollowinlaysofcolourlesstransparentglassthatarediamondshaped,triangular,roundoroval.
Forthepresentstudy,265oftheSamarraglassfindsfromthepubliccollectionsoftheMuseum
fu¨rislamischeKunstinBerlin(Germany),theDepartmentoftheMiddleEastintheBritish
MuseumandtheMiddleEasternSectionintheVictoriaandAlbertMuseuminLondon(UK)
wereselectedforanalysis.Nopermitswererequiredforthedescribedstudy,whichcomplied
withallrelevantregulations.Thesampleswerechosensoastoincludeallvesseltypes(bottles,
bowls,plates),opticalproperties(transparent,opaque,differentcolours)anddecorativetech-
niques(reliefcut,engraved,painted,mouldblown),andincludeallthearchitecturalglasses(Fig
1andS1Table).Mostoftheglassesforwhichthearchaeologicalcontextisknownwereeither
foundinoneofthepalaces(Dāral-Khilāfa,Balkuwara)orintheGreatMosque,andtheoriginal
cataloguenumbersfromLammarecross-referencedwherepossible(S1Table).
Fig1.GlassartefactsfromSamarrarepresentingthedifferentcompositionalgroups.(A)Regularlyshapedmosaictesseraeofnatron
typebaseglass(V&AA.58-1922);(B)glassinlaysofplantashgroup1(Sam806.2,I.9325.1,I.9325.2,I.9325.4;photosC.Krug,Museum
fu¨rislamischeKunst/StaatlicheMuseenzuBerlin);(C)fragmentofmillefioriglasstileofplantashgroup2(V&AC.743-1922);(D)
cobaltblueflaskneck(V&AC.750-1922);(E)rimfragmentofpaintedglassbowlbelongingtothemiscellaneoussamples(SamKat273;
photoM.Wypyski,Museumfu¨rislamischeKunst/StaatlicheMuseenzuBerlin).ImagesA,CandDfromtheVictoriaandAlbert
Museum,London[http://collections.vam.ac.uk];imagesBandEfromtheMuseumfu¨rislamischeKunst/StaatlicheMuseenzuBerlin
[www.smb-digital.de/eMuseumPlus].
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Ninth-centuryAbbasidglassproduction
Analyticalmethods
Smallfragmentsofglasswereremovedfromtheselectedartefacts,setinepoxyresinandpol-
ishedtoremoveanysurfacecontamination.Laserablationinductivelycoupledplasmamass
spectrometry(LA-ICP-MS)analyseswereperformedonthepolishedsamplesatIRAMAT-CEB
inOrle´ans(France),usingaThermofisherElementXRmassspectrometerandaResoneticUV
lasermicroprobeequippedwitha193nmExcimerlaser[20–22].Thelaserwassetata100μm
spotsizethatwasoccasionallyreducedwhenmanganesesaturationoccurredandanalyseswere
carriedoutat5mJwithafrequencyof10Hz,apre-ablationtimeof20secondsfollowedby30
secondsanalyticaltime.Quantitativewt%andppmconcentrationsofthefifty-eightelements
measuredwerecalculated,using28Siasinternalstandardandarangeofwell-characterisedglass
referencematerials(NistSRM610,CorningB,CandD,APL1).Toestablishprecisionandaccu-
racy,glassstandardsNISTSRM612andCorningAwererunatregularintervals(S2Table).The
detectionlimitsvarybetween0.1and0.01%formajorelementsandbetween20and500ppb
fortraceelements.
Results
Glassgroupprovenance
ThechemicalfingerprintsobtainedbyLA-ICP-MSclassifyallsamplesfromSamarraassoda-
lime-silicaglassestypicaloflateByzantineandearlyIslamicassemblagesfromtheeastern
MediterraneanandMesopotamia(S1Table).Themajorityofthesampleshaverelativelyhigh
levelsofpotash(K O>1.5%)andmagnesia(MgO>2%),indicatingthatasoda-richplant-
2
ashservedasthesourceofthealkali[23].Asmallgroupofsampleshaslowpotassiumand
magnesiumconcentrations(<1.5%)characteristicofglassmadewithsodaderivedfroma
mineralsource.Theyarenatron-typeglassescharacteristicofglassproducedpriortotheninth
centuryCE.Theemeraldgreendecorationofonevesselfragment(BerlinSam014)with
approximately70%leadoxidecorrespondstoahighleadsilicaglass(S1Table).Similarcom-
positionshavebeenidentifiedamongearlyIslamicglasses,forexample,fromtheSerc¸iLimani
shipwreck[24]aswellasNishapur[25].Thissamplewillnotbediscussedfurther.
Theanalysesidentifiedthreedifferentsourcesofsupply:thereuseofoldernatron-type
glass,importsofcontemporaryplantashglassfromtheLevantand/orEgypt,andMesopota-
mianglassproduction(Table1andFig2).Remarkably,theanalyticalresultsshowaclosecor-
respondencebetweenthecompositionalgroupsandobjecttypes.Almostalltraditional
Table1. Meansandrelativestandarderrorofthemean(RSEM)ofthefiveglassgroupsidentifiedatSamarra. Data[wt%]werereducedtotheshownmajor,minor
andtraceelementoxidesandnormalisedto100%.
NaO MgO AlO SiO PO Cl K O CaO TiO MnO FeO Sr Zr
2 2 3 2 2 5 2 2 2 3
[ppm] [ppm]
Natronglass tesserae(n=33) 16.9 0.72 2.72 69.1 0.15 1.02 0.65 7.00 0.14 0.57 1.04 411 72.9
RSEM[%] 2.10 6.20 3.47 0.59 11.7 3.98 6.67 6.35 8.55 22.7 16.8 7.35 8.90
Mesopotamian group1(n=72) 13.2 5.09 0.81 70.0 0.12 0.64 3.22 6.39 0.03 0.29 0.23 398 31.0
RSEM[%] 0.58 0.93 2.10 0.17 1.91 1.48 1.76 0.83 3.48 3.78 4.03 0.93 3.58
group2(n=136) 15.2 5.55 1.37 67.0 0.12 0.61 2.71 5.43 0.08 1.28 0.65 438 106
RSEM[%] 0.73 0.85 1.08 0.25 1.75 0.78 0.87 0.94 1.37 4.13 4.63 0.86 1.87
Imports Coflasks(n=23) 14.3 2.83 2.10 67.6 0.19 0.71 2.26 6.66 0.13 0.59 2.56 332 95.7
RSEM[%] 0.81 2.83 2.63 0.40 3.59 2.98 2.42 2.28 2.46 10.4 4.25 1.55 4.56
misc(n=15) 14.4 3.25 2.60 65.9 0.31 0.61 3.07 7.13 0.15 1.48 1.06 453 93.5
RSEM[%] 3.25 3.17 13.7 1.48 4.49 5.34 3.38 5.88 11.3 21.1 13.1 7.92 10.9
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Ninth-centuryAbbasidglassproduction
Fig2.DifferentfluxingagentsoftheSamarraglasses.KOversusMgOconcentrationsidentifydifferencesbetween
2
natron-typeglasses,plantashglassesfromtheSyro-PalestinianIslamictraditionandplantashglassesof
Mesopotamianprovenance(sub-divisionsareindicatedbydashedlines).
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regularlyshapedtesserae(Fig1A)andonecolourlessbeakerwithlinearwheelcutdecorations
(BerlinSam038MW)wereproducedfromnatron-typeglassessourcedfromtheLevantine
coastorEgypt.TheplantashglassescanbefurthersubdividedintoMediterraneanandMeso-
potamianglassgroups(Fig2)thatcorrespondroughlywithartefacttypesand/orcoloursand
byextensionthequalityoftheglass.Thecolourlessdiamondshapedandroundwallinlaysas
wellassomedelicatelydecoratedvesselsaremadeofaparticularlyhighqualityglassofMeso-
potamianorigin,whilethestronglycolouredarchitecturalglassesandthemajorityofaquacol-
ouredvesselsderivedfromasilicasourceofMesopotamianprovenancethatwasofalower
purity.Athirdplant-ashgroup(henceforthreferredtoasmiscellaneous)isrelativelyheteroge-
neousbothintermsofitscompositionaltraitsaswellasthetypologyanddecorativetech-
niquesemployed,suggestingtheimportofindividualobjectstoSamarra(S1Table).Finally,a
chemicallyrelativelytightclusterofcobaltblueflasksexhibitsclearMediterraneancharacteris-
tics.Witheachcompositionalgroupbeingrepresentedbyatleast15samples(>5%ofthe
assemblage),therelativestandarderrorsofthemean(RSEM)aregenerally(cid:20)5%forallbase
glasselements(Table1).Thisshowsthatourgroupassignmentisrobust,whichallowsusto
drawstatisticallyandarchaeologicallysignificantconclusionsfromourdata.
ReusedtesseraeofLevantineandEgyptianorigins
Uponcloserinspection,thenatron-typemosaictesseraedonotformahomogeneousgroup
butshowsignificantvariationsintheiraluminium,calciumandheavyelementconcentrations
reflectingdifferentsilicasourcesratherthansecondaryadditivesandthusdifferentorigins
(Fig3).Aboutonethirdofthetesseraearecharacterisedbymoderatesodalevels,aluminacon-
tentsofabout3%,limebetween8%and10.5%andlowheavymineralimpuritiesconsistent
withLevantineIglass(Fig3A).Thistypeofglasshasbeenidentifiedamongfourth-toeighth-
centuryglassassemblagesinIsrael(Apollonia,BetShean,DorandJalame)[2,26,27].Asec-
ondgroupexhibitssignsofrecycling.Itresemblesso-calledFoy-2[28]onaccountofelevated
heavyelements,relativelyhighsodalevelsandhigherstrontiumtocalciumratios(S1Table).
Foy-2iswidespreadamongsixth-andseventh-centuryCEglassassemblagesfromdiverse
MediterraneanandEuropeansites[22,29–31].Theremainingsampleshavesubstantially
lowercalciumlevels,hightitanium,zirconiumandhafnium,andvaryingtitaniumtoalumin-
iumratios(Fig3A).ThesefeaturespointtoanEgyptianorigin.Thesub-groupwiththelowest
limeandhighestaluminaconcentrationsresemblethecharacteristicsofEgyptI,aprimary
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Ninth-centuryAbbasidglassproduction
Fig3.Analysisofthesilicasourcesofthenatron-typeglassesfromSamarra.(A)CaOversustheratioofTiO /
2
AlO;(B)CeO/ZrO versusYO/ZrO;(C)TiO/AlO versusAlO/SiO.
2 3 2 2 2 3 2 2 2 3 2 3 2
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productiongroupdatingtotheeighthcenturyCEbasedonastudyofIslamicglassweights
[32].Asinglevesselfragmentwithlowaluminium,titaniumandrelativelyhighcalciumoxide
concentrationsandnotabletracesofmanganeseandantimonycorrespondstorecycled
Romanglass[33].
Correlationsbetweenthegeographicaloriginofnatron-typeglassesandtheratiosof
yttriumtozirconiumversusceriumtozirconium,andtitaniumtoaluminiumversusalumin-
iumtosilicahavebeenobservedelsewhere[6,31].GlassesfromEgypthavetypicallylower
Y O /ZrO andCeO /ZrO ratios(<0.08and<0.18,respectively)comparedtoglassesfrom
2 3 2 2 2
theLevant(>0.1and>0.24).Duetothehigheraluminalevelsrelativetothesilicaandtita-
niumcontents,LevantineIhassimultaneouslyhigherAl O /SiO (>0.04)andlowerTiO /
2 3 2 2
Al O ratios(<0.04)thanEgyptianglassgroups.Usingthesemodels,theinitialassignments
2 3
regardingtheprovenanceoftheSamarratesseraearereinforced(Fig3Band3C).Thetesserae
thathadbeenattributedtotheLevantineItypehavehighY O /ZrO andCeO /ZrO ratios
2 3 2 2 2
aswellashighaluminarelativetosilicaandlowtitaniumtoaluminaratios(Fig3Band3C).
TheEgyptiangroupshaveinversecharacteristics.TheprovenanceofFoy-2islessclear,asthe
samplesoccupyanintermediatepositionasregardstheirY O /ZrO andCeO /ZrO ratios
2 3 2 2 2
thatsuggestssomedegreeofmixingand/orrecycling[6].Recyclingisconfirmedbytheele-
vatedphosphorusandantimonycontentsofthesesamples(S1Table).
ThecollectionofmosaictesseraeretrievedfromSamarraisevidentlyaneclecticmixtureof
differentnatron-typebaseglasses,allofwhichpre-datethefoundationofSamarraintheninth
century.Thetesseraewereclearlynotproducedtoasinglerecipeorasinglecommission.
Theymightinsteadhavebeenscavengedfrombuildingsnolongerinuseandimportedto
SamarrafromthewesternregionsoftheAbbasidCaliphate,fromSyria-Palestineand/or
Egypt.Whatallthetesseraehaveincommonthoughisthefactthattheyarecompositionally
distinctfromtheoutputoftheglassmakingindustrythatsuppliedSamarrawiththebulkof
soda-richplantashglasses.
Regionalproductionofplantashglassesandimports
EarlierstudieshaveshownthatIslamicsoda-richplantashglassesfromtheLevantandEgypt
canbedistinguishedfromMesopotamianplantashglassesbasedonthepotashandmagnesia
concentrations[34].Applyingathresholdof6.5%forthesumofmagnesiumpluspotassium
oxide,theassemblagefromSamarracomprisespredominantlyMesopotamianplantash
glasses,whereasthecobaltblueflasksappeartorepresentaSyro-PalestinianorEgyptianpro-
duction(Fig2).Themiscellaneousplantashgrouphasdiversefeaturesandconsistsofamix
ofMediterraneanandMesopotamiansamples.TheglassesofMesopotamiangroup1and2
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Ninth-centuryAbbasidglassproduction
arefurthermorecharacterisedbyhighermagnesiumtocalciumratiosandexceptionallylow
phosphoruscontents(P O <0.15%)comparedtotheotherplantashtypes(Fig4A).Thelith-
2 5
iumandboronlevelsrelativetothesodaconcentrationsseparatetheplantashgroupseven
moreclearly(Fig4B).Thecobaltblueflasksformaneatclusterwithhigherboronlevelsthan
anyoftheotherplantashgroups,whilethemiscellaneousplantashgrouphasthelowestlith-
iumvaluesrelativetosoda,whichmayberelatedtoitsoveralllowermagnesiumcontents(Fig
2).Groups1and2resembleeachothercloselyintermsofmostash-relatedelementssuchas
potassium,magnesium,lithiumandboronbutdifferintheirabsolutecalciumcontents.
Group1hasonaveragehighercalciumoxidelevels((cid:25)6.4%)comparedtogroup2((cid:25)5.4%)
andaccordinglysomewhatlowermagnesiatolimeratios(Table1andFig4A).Thesecomposi-
tionalfeaturesindicatedifferencesintheplantsthatwereusedand/orinthepreparationofthe
ash.Soda-richplantashwascombinedeitherwithquartz-richsandorwithcrushedquartz
pebbles,providingadditionalelementstoseparatethegroupsandtracetheirlikelyorigins.
Highqualityarchitecturalglass—theuseofquartzpebbles(group1). Plant-ashgroup
1comprisesalmostallcolourlessglasses,particularlyallthediamondshapedandroundwall
inlays(Fig1B)andseveralfinelydecoratedvesselssuchasacold-painted,gildedandengraved
roundbottle(BerlinSam042)andareliefcutbowldecoratedwithpalmetteandanimalmotifs
thatissaidtorepresentthehigheststandardsofearlyIslamicglass(BerlinSam018)[35].The
samplesofgroup1containonaverageonly0.3%manganeseoxide,toolowtoactefficientlyas
adecolourant.Allsampleshavelowsilica-relatedimpuritiessuchasverylowtitaniumandzir-
coniumlevels(Fig5A),anaveragealuminacontentofonly0.8%(Fig5BandTable1)and
overallverylowtraceelements(Fig5C).Thesecompositionalcharacteristicsaretheresultof
theuseofaverycleansilicasource(seediscussion).
Aneconomicalvariant—theuseofquartz-richsand(group2). Group2representsthe
bulkoftheSamarraglassassemblageandincludesafewcolourlessandaquacolouredvessels,
allofthescratch-engravedsamples(e.g.BMSamarra146–153),lampsandwindows,aswellas
allofthecolouredarchitecturalglassessuchasdarkpurpleandgreendrawntesserae,chunky
goldleaftesseraeandmillefioritiles(Fig1CandS1Table).Thealumina,iron,titaniumand
zirconiumcontentsaswellasallothertraceandrareearthelementsareonaveragehigher
thaningroup1(Fig5).Thisprovidesevidencethatthesilicasourceunderlyinggroup2was
notaspureastheoneusedforgroup1.
Thepurityoftherawmaterialswasapparentlynotasdecisiveinthecaseofgroup2given
thefactthatcolouringorde-colouringagentswereaddedtotherawmaterial.Allthegoldleaf
tesserae,forexample,aremadefromtransparentglasswithagreenishtingeandcontainsignif-
icantamountsofmanganeseoxide(1%-2%)asadecolourant.Thedrawntesseraeareeither
Fig4.Variationsintheplantashcomponentofthefourplantashgroups.(A)phosphoruscomparedto
magnesiumtocalciumoxideratiosconfirmdifferentgeographicalorigins;(B)boronandlithiumconcentrations(both
normalisedtothesodaconcentrations)identifydifferentplantashcomponentsandadditives.
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Ninth-centuryAbbasidglassproduction
Fig5.AnalysisofthesilicasourcesoftheplantashgroupsfromSamarra.(A)differentialzirconiumandtitanium
correlationsclearlydistinguishthecobaltblueflasksfromtheotherplantashgroups;(B)aluminiumandiron
concentrationsindicatedifferentdegreesofsilicarelatedimpurities;(C)selectedtraceelements,normalisedtothe
uppercontinentalcrustcompositions[36],highlightingtheverylowcontaminantsinplantashgroup1.
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darkgreen,colouredbyacombinationofcopperandlead,ordarkpurpleduetomanganese.
Themillefioritilescomprisemanganeseblack,cobaltblue,copper-ironred,lead-tinyellow
andtinwhitesegments(S1Table).Giventhatthebaseglassofgroup2waseithercoloured
and/ordestinedforobjectsoflowerprestigesuchassmallinkbottles(BerlinSam024MW),a
moreeconomicalglassmakingrecipeandtechnologywereevidentlychosen.Thefactthat
groups1and2arecompositionallycloselyrelatedandofacommonMesopotamianorigin
demonstratesthediversityoftheAbbasidglassindustryandtheexistenceofdeliberatepro-
ductionstrategies.
Commoditybranding—thecobaltblueflasks. The23cobaltblueflasksformaverydis-
tinctivegroup(Fig1D).Theplantashcomponentresemblesthatofplantashglassproduced
inSyria-PalestineorEgyptwithmoderatepotassiumandmagnesiumlevels(Fig2).Thebase
glassofthesebottlesisderivedfromasilicasourcedefinedbyhighertitaniumtozirconium
ratiosaswellashigheralumina,traceandrareearthelementsthanplantashgroups1and2
(Fig5).Thecobaltblueflasksexhibitalsoadistinctboronsignature(Fig4B).Judgingfromlit-
erarytreatisesthatdescribetheprocessingofthecobaltbluepigmentforglazedtiles[37],the
elevatedboronmightinfactbetheresultoftheadditionofboraxduringthepre-treatmentof
thecobaltore.Cobaltisstronglycorrelatedwithironandcopper,andzinclevelsarelikewise
increased.Zinc-richcobaltisconsidereddiagnosticofearlyIslamicglassmaking[38–41]and
wasidentifiedprimarilyamongassemblagesofsupposedlyMesopotamianoriginfromCtesi-
phonandal-Raqqa[15],Nishapur[25,42]aswellasIslamicglassbeadsfromninth-ortenth-
centurytombsinAlbania[43].However,therelativezinclevelsinthecobaltblueflasksfrom
SamarraaremarkedlylowerthanthoseoftheMesopotamiancobalt-zincglasseswherezinc
contentstypicallyequalorexceedthecobaltconcentrations.Incontrast,acobalttozincratio
similartothatoftheSamarraflasks(*1.5x)wasrecentlyfoundinasetofalmostidentical
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Ninth-centuryAbbasidglassproduction
cobaltbluebottlesfromRamladatingfromtheninthtotheeleventhcentury[18]aswellasin
twofragmentsassociatedwiththetenth-centurytankfurnace2intheglassmakingcomplexof
Tyre[44].Consequently,thecobaltcolorantemployedfortheblueflasksfromSamarrais
morecloselyassociatedwitheasternMediterraneanglassmaking,andtheglassesaretherefore
not,ashasbeenpreviouslyproposed,ofMesopotamianprovenance[18].Theoriginofthe
cobaltmineralitselfremainselusive.ThecobaltmineofQamsar,Kāshān(Iran)thathasoften
beencitedasthesourceoftheIslamiccobaltpigmentdoesnotappeartocontainsufficient
zinctobeasuitablecobaltore[45].
AccordingtoLamm[12],theHerzfeldexcavationsyieldedfragmentsofabout170ofthese
elongatedtubularflaskswithshortnarrownecksandoftenroundedbases,madeofwafer-thin
cobaltblueglass(Fig1D).Thebottlesthemselvesbearsignsofmassproductionasthenecks
werecrudelysawnoff.Someofthenecksarestillclosedbyeithercottonwoolorpapyrus.
Thesetypesofelongatedcobaltbluebottlesappeartohavebeenverywidespreadduringthe
ninthtoeleventhcenturyCE.ThousandsoffragmentshavebeenfoundalloverNishapur[19]
andfurtherexamplesareknownfromEgyptatFaiyum,FustatandRaya,fromPalestineat
Tiberias,RamlaandCaesarea,fromTunisiaatSabraal-Mansuriya,aswellasfromtheArabian
Peninsula,ChinaandKenya[18,46].Thepeculiarshapeoftheseflasksisveryconspicuous,
whichmighthaveservedthepurposeofcommoditybrandingtofacilitateproductrecognition
ofapresumablyvaluableandlight-sensitivecontent[46].Theirsupplytothepalacesof
Samarra,wherelargenumberswerediscoveredintheso-called‘harim’southofthethrone
roomoftheDāral-Khilāfa[12],suggeststhattheyservedsomesortofcosmeticpurpose.
Miscellaneousvesselsandimports. Themiscellaneousgroupisastypologicallydiverse
asitiscompositionallyvaried.Itencompassesvesselsrepresentingdifferentdecorativestyles
suchasareliefcutvesselwithanimaldecoration(BerlinSam052MW),mouldblownbowls
withverticalribs(BerlinSam033MW),vesselswithappliedtrails(BerlinSam054MW),
paintedvesselfragments(Fig1E),asmallinkbottle(BerlinSam068MW)andtwomarvered
bowls(BerlinSam060&062MW),adecorativetechniquethatiscommonlyattributedtoan
EgyptianorSyrianprovenance[47].
Compositionally,thesamplesofthisgroupspanawiderangeofimpuritiesintroducedwith
thesilicasource.Thealuminalevels,forexample,varyfromabout0.7%toover6%(Fig5B),
whiletheheavyandrareearthelementsareonaveragehighercomparedtotheotherplantash
groupsfromSamarrawithapositiveeuropiumanomaly,probablyduetothefeldspathicfrac-
tionwithinthesilicasource(Fig5C).Ithastobestressed,however,thattheindividualsamples
ofthisgrouparehighlyvariable(Fig5Aand5B).Thehighimpuritiesandvarianceareindica-
tiveoftheuseofdifferentsandsasthestartingmaterial.Forinstance,thesampleswiththe
highestaluminacontentsandlowmagnesiumtocalciumratios(Fig6)bearallthecomposi-
tionalattributesofcolouredglassesfromNishapur,suggestingacentralAsianglassproduction
[18].Giventhattheglassesofthisgroupareneithercompositionallynortypologicallyuni-
form,itislikelythatfinishedglassobjectsarrivedindividuallyatSamarrasuchas,forexample,
asmallinkbottle(BerlinSam068MW)thatmayhavebeenthepersonalpossessionofoneof
themanypoetsthatwereactiveduringSamarra’scaliphalperiod.Thisisincontrasttothe
blueflasksthatappeartohavebeenimportedinlargenumbers,implyingamorecentrally
organisedtradeofthesespecialisedobjects.
Discussion
Thetraceelementanalysisincombinationwiththetypologicalaffiliationssupportsamodel
wherebyglasswasproducedatandselectivelyimportedtoSamarra.Allthenatron-typetes-
seraepredatethefoundationofSamarraandwereimportsfromSyria-Palestineand/orEgypt.
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Ninth-centuryAbbasidglassproduction
Fig6.TheSamarraplantashgroupscomparedtocontemporaryglassassemblages.Aluminaversusmagnesiato
limeratiosofpublisheddataofglassesfromRamla[18],Tyre[44],Nishapur(Wypyski,inpreparation)andVeh
Ardasir[48,49],indicatingtheproposedseparationlinesbetweenearlyIslamicglassesfromtheeasternMediterranean
withlowmagnesiatolimeratios,theSamarraglasswithhighmagnesiatolimeratiosandlowaluminalevels,and
MesopotamianglasseswithhigheraluminaconcentrationstypicalofcentralAsianproductions(lay-outofgraph
adaptedfrom[18]).
https://doi.org/10.1371/journal.pone.0201749.g006
Thecompositionalfeaturesofthecobaltblueflasksandsomemiscellaneoussamplespoint
likewisetoaneasternMediterraneanprovenance.Plantashgroups1and2ontheotherhand
areundoubtedlytheoutputofMesopotamianglassmakingeventhoughtheyrepresentdiffer-
entproductionstrategiesandrawingredients.Theanalyticaldataprovedthatthesilicasource
underlyingplantashgroup1wasexceptionallypure,implyingtheuseofquartzpebblesora
verycleanquartz-richsand.Theheavyelementlevels(Zr,Hf,Th)ofSamarragroup1only
slightlyexceedthoseofsomeIronAgeplantashglassesthatarebelievedtohavebeenmade
fromquartzpebbles[50].Nonetheless,itisnearimpossibletodistinguishbetweentheuseof
quartzpebblesandquartzrichsandsatthisstage.Whatiscertain,isthattheglassmakersdelib-
eratelychoseapuresilicasourcethatallowedthemtocloselycontroltheingredientsandthe
productionprocesses.Byexploitingrawmaterialsofhighpurity,theglassworkerswereableto
obtaintrulycolourlessglasswithnodiscernibletingeoraquacolourthatisusuallyimparted
toglassbytheironimpuritiesinsand.Theeffortandspecialcarethatwastakentoproduce
thesecolourlessglassesareindicativeoftheirculturalvalueandhighprestige.Thesilicasource
ofplantashgroup2wasricherinimpuritiesandpossiblyprovidedacheaperand/orless
work-intensivealternativetothehigh-qualitygroup1glasses.
CompositionalfeaturescomparabletothoseofSamarragroups1and2havepreviously
beenattributedtoMesopotamianglassmakingassimilarglasseshavebeenfoundatninth-to
tenth-centuryNishapur,earlyIslamicRamlaandRaqqa,aswellasamongthefourth-tofifth-
centurySasanianglassesfromVehArdaˇsīrsouthofBaghdad(Fig6)[2,7,15,25,48,49].Our
analyticalresultsprovidethefirstindicationoftheprimaryproductionofglassatSamarra
itself.ThemanufactureofglassinthevicinityofSamarraseemshighlylikelygiventherelative
homogeneityofthebulkoftheanalysedassemblage(groups1and2)compared,forinstance,
totheglassfindsfromNishapurorVehArdaˇsīr(Fig6).InviewofthevariabilityoftheseMes-
opotamianglasses,thetightclusteringofSamarragroup1,themajorityofwhichpertainstoa
specialtypeofarchitecturalglass,anditscompositionalresemblancetogroup2pointto
closelyrelatedprimaryproductions.Aregionalproductionofglasswouldhaveascertainedthe
readysupplyofvitreousmaterialsfortheornamentationofthenewlybuiltpalace-cityof
Samarra.ApossiblecandidateofanearlyIslamicglassproductionsiteisal-Qadisiyyaabout25
kmsouthofSamarra,knownfromhistoricalsourcesasthesitewhereglasswasproduced
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