Volume 41 2014

SERIES

Igneous Rock Associations 12.A Geologist’s Look atArchaeological Ceramicsand Glass

J. Victor OwenDepartment of GeologySaint Mary’s UniversityHalifax, NS, Canada, B3H 3C3E-mai: victor.owen@smu.ca

John D. GreenoughDepartment of Earth and Environmental SciencesUniversity of British Columbia, Okanagan3333 University WayKelowna, BC, Canada, V1V 1V7

SUMMARYCeramics and glass represent syntheticmetamorphic rocks and obsidian,respectively. Consequently, it is not sur-prising that many archaeologists havecollaborated with geologists on proj-ects dealing not only with lithic arti-facts, but with ceramic and glassobjects as well. This paper presents anoverview of these latter two materialsfrom a geological perspective, consid-

ering in turn how they are character-ized and classified, their ages con-strained, provenance and in someinstances use determined, and howthey were made.

SOMMAIRELa céramique et le verre représententrespectivement des roches métamor-phiques et l’obsidien synthétiques. Enconséquence, il n’est pas étonnant quebeaucoup d’archéologues aient colla-boré avec des géologues sur des projetstraitant non seulement des objetsfaçonnés lithiques, mais également desobjets en céramique et en verre. Cedocument présente un aperçu de cesderniers deux matériaux d’une perspec-tive géologique, considérant commentils sont caractérisés et classifiés, leursâges, provenances et emploies parfoisdéterminés, et comment elles ont étéfaites.

INTRODUCTIONIn the course of their education, geol-ogists become well versed in the tradi-tional mainstays of their discipline –mineralogy, stratigraphy, petrology,geochemistry, paleontology,geochronology, structural geology, tec-tonics – along with a smattering ofsupporting sciences. Among otherthings, we learn to place time andspace in a broader context than mostother scientists aside fromastronomers. Archaeologists are notmuch different, although their appreci-ation of time is on a much morerestricted scale – typically centuries andmillennia rather than mega- and giga-annum. Moreover, they usually map ona much more detailed scale than fieldgeologists, and the third dimension of

their maps is correspondingly shallow.Like geologists, however, archaeolo-gists have an acute appreciation ofstratigraphy, again on a minute scale,and, given the importance of lithicartifacts in their discipline, archaeolo-gists must be familiar with the types ofrocks exploited by ancient civilizations.

Dacite, the most commonlyused rock for stone tool manufacturein British Columbia and in some otherparts of western North America, is agood example, although it is common-ly referred to by archaeologists as‘glassy basalt’. Studies of this materialreveal the utility of geochemical analy-ses for fingerprinting lithic artifacts butalso illustrate some of the challenges.Mallory-Greenough et al. (2002; whole-rock study) and Greenough et al.(2004; mineral chemistry fingerprint-ing) showed that dacite used in toolmanufacture tends to be of local deri-vation, perhaps because the benefits ofusing non-optimal but readily-availablematerials outweigh the cost of trans-porting high-quality dacite because ofits weight (Beck et al. 2002; Greenoughet al. 2004). Still, the scope of theArrowstone Hills (Cache Creek) quarrysite in central British Columbia, whereartifacts occur over ≥ 4 km2 and arelocally buried over 2 m deep (Ball1997; Mallory-Greenough et al. 2002;Greenough et al. 2004), implies long-term utilization and potential use intrade. Curiously, widespread identifica-tion of Cache Creek-derived artifactshas yet to be established, but this maylargely reflect the limited application offingerprinting to studies of habitationsite artifacts.

As important as lithic artifactsare to many archaeologists, ceramics

Geoscience Canada, v. 41, http://dx.doi.org/10.12789/geocanj.2014.41.034 © 2014 GAC/AGC®

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are of equal significance, at least withrespect to Neolithic (and possibly Pale-olithic; Watzmann 2009) and youngercivilizations, and artifacts made fromsynthetic glass (as opposed to natural-ly-occurring glass, e.g. obsidian) candate back thousands of years as well.Given the overlap in some of thematerials that interest us both (e.g.rocks) and in the techniques we use tobetter understand them in terms ofrelative age (stratigraphy), absolute age(geochronology) and composition(geochemistry and petrology), it is notsurprising that some geologists includearchaeological artifacts among themedia that they investigate. This con-tribution is a geologist’s look at archae-ological ceramics and glass.

WHAT ARE CERAMICS?Ceramics are non-metallic solid materi-als prepared by heating and cooling of(generally) clay-bearing mixtures.Excluding some modern types of spe-cialized wares, some of which borderon glass, ceramics can be broadly sub-divided into low-temperature earthen-ware (pottery) that is porous (typically≥5 vol.% pores) unless glazed, andhigh-temperature (kiln T generally≥1200ºC) stoneware that commonlybut not invariably has a relatively lowporosity (Hamer and Hamer 1997; Fig.1).

Few ceramics are made entire-ly of clay. They usually require theaddition of aplastic material (e.g. sandparticles, crushed rock and/or driedplant material [temper]) to improveworkability if the ceramic paste is to bethrown on a potter’s wheel. Some typesof pottery, notably aboriginal wares,can include distinctive ingredients suchas shells. Temper grains can be usefulin sourcing the raw materials used inthe manufacture of archaeologicalceramics, as can mineralogical impuri-ties in the clay itself.

There are, not surprisingly, dif-ferent types of earthenware, includingred-coloured earthenware (‘redware’),yellow-coloured earthenware (‘yellow-ware’), fine earthenware (such ascreamware and pearlware), and faience.They can be subdivided according toglaze colour (e.g. pearlware versuscreamware), or body colour (e.g. red-ware, yellowware), or grain size (e.g.fine versus coarse earthenware). Sometypes of porcelain (‘soft-paste’ or ‘arti-ficial’ porcelain) are also considered tobe earthenware even though they arepartly vitrified, because they tend to berelatively porous, so are glazed after aninitial, relatively high-temperature fir-ing. So-called ‘true’ porcelains are arefined type of stoneware. They tradi-tionally were made from pastes con-taining kaolin and hydrothermally-

altered peraluminous granite (‘chinastone’ [‘petunse’]). In some instances, asource of lime was also included. Con-sequently, these wares can be modelledin the SiO2–Al2O3–K2O and SiO2–Al2O3–CaO systems, respectively.Unless decorated with over-glazeenamel colours, they are usually firedonly once, in a high-temperature bis-cuit kiln, so that the glaze bonds withthe vitrified and translucent body ofthe ware, creating an integrated body-glaze layer. It is this latter feature(rather than porosity) that most clearlydistinguishes stoneware from earthen-ware, which is usually fired twice, firstin a biscuit kiln, and then, after glazing,at lower temperature in a glost kiln.The glazes on earthenware, includingartificial porcelain, were traditionallylead-rich, unlike those on true porce-lains. These low-temperature glazes failto fuse with the substrate, sitting on itas a separate layer.

Porcelains are translucentowing in large part to the partial fillingof pore spaces by a glassy melt phase.Unlike true porcelain, most otherstonewares are opaque, in part becausethey tend to be thickly potted. Theyare made with lower-quality but rela-tively plastic refractory clays (stonewareclays) that can contain relatively highconcentrations of mineralogical impu-rities (e.g. heavy minerals). In contrast,

Figure 1. Schematic subdivision of ceramics based on firing schedule (one [stoneware] versus two [earthenware] firings, exclud-ing over-glaze decoration), paste and glaze colour (e.g. different types of earthenware), and compositions (e.g. fine earthenwareversus faience). This chart is broadly based on information appearing in Hamer and Hamer (1997); not all archaeologists orceramists necessarily endorse it, and there are many exceptions to these categories.

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true porcelain is made with relativelyhigh-quality clays that generally containfewer impurities. Some of these clays(notably kaolin) have low plasticity, soporcelain objects are commonly slipcast in plaster of Paris molds ratherthan thrown on a potter’s wheel.

Artificial porcelains wereinvented in the mid-18th century byEuropeans seeking to reproduce thetrue porcelain that originated in Asiaroughly 1000 years ago (and reinventedin Germany in the early 18th century(Roentgen 1996), and in the UK laterin the 18th century). Unlike the chemi-cally less-complex true porcelains, arti-ficial porcelain spans a wide range ofcompositions that reflects the varietyof starting materials used in their man-ufacture. In addition to clay and quartzor flint, these can include crushed glass(‘frit’), calcined bone ash, soapstone,barite, and/or gypsum. Bone china is ahybrid ware formed by blending ingre-dients used in true and artificial porce-lains (more specifically, those contain-ing bone ash). Regardless of whichancillary ingredients are used in artifi-cial porcelain pastes, the amount ofclay used in the manufacture of thesewares can be surprisingly small (oftenas little as ~10% clay by weight). InMg-rich British porcelains, steatitefrom ophiolites on the Lizard Peninsu-la replaced most of the clay used in theporcelain paste (cf. Sandon 1989). Themineralogy of these wares reflectstheir bulk compositions and the condi-tions at which they were fired. Owingto the purity of many of their ingredi-ents, solid solution minerals tend toapproach end-member compositions.They can include, in different types ofartificial porcelains, various combina-tions of enstatite, (pseudo)wollastonite,diopside and/or anorthite.

WHAT IS GLASS?Glass is an amorphous solid thatundergoes a transition between a hardand molten state. The glass transition ischaracterized by a dramatic change inviscosity, and by changes in heat capac-ity and thermal expansivity (Ojovanand Lee 2006). Glass in the transitionhas a rubbery character; below thetransition, it is hard and solid, abovethe transition, it is relatively fluid.Although the widely held view (basedon the observation that some ancient

window panes are thicker at their basethan top) that glass is a highly viscousfluid has been refuted (e.g. Zanotto1998), the character and structure ofglass remain controversial topics inmaterials science.

Just when and where glass wasfirst made are open questions. Pliny theElder claimed that glass was accidental-ly discovered by Phoenician merchantswhile preparing a meal on the shore ofthe Belus River in Palestine. Unable tofind rocks to support their cookingpots, they reputedly used blocks ofnatron (principally Na2CO3·10H2O; aflux) instead. The heat from the firecaused the beach sand and natron tomelt together. The melt cooled quickly,forming glass. This anecdote has longbeen discredited by scholars on thebasis that the temperatures producedby campfires are insufficient to causebeach sand to melt, even in the pres-ence of a flux such as natron.Instead, it has been proposed that thediscovery of glass originated with themanufacture of early glazed objectssuch as Badarian steatite beads (Short-land et al. 2006) and faience (Bowman1991). Glazes, after all, consist of glass.

The successful manufacture ofglass requires the addition of fluxes(usually alkalis, lead and/or borates)and stabilizers (e.g. lime, to render theglass strong and relatively insoluble) toglass-grade (silica-rich) sand.Colourants (to tint the glass) anddecolourants (to reduce the greencolour caused by the presence of iron)can also be used. This mixture ofingredients constitutes a glass batch,the material intended to be melted toform glass. Broken scrap glass (cullet)is often added to these batches to pro-mote melting. Glass batches are meltedin large, ceramic firing pots (refractoryclay crucibles) that must be free of anyflaws to minimize the risk of cata-strophic breakage in the furnace. Likethe temper grains used in the manufac-ture of some pottery, impurities (e.g.heavy minerals) in the sand used byglass manufacturers can be used tosource this raw material, even though itwas completely melted during firing ofthe glass batch in which it was used.This is possible because the trace ele-ment signatures of these impurities areinherited by the glass.

INVESTIGATION OF ARCHAEOLOGI-CAL CERAMICS AND GLASSArchaeologists who investigate histori-cal and ancient ceramics and glass usu-ally focus their attention on one ormore of five aspects of particular arti-facts: (1) what is it?, (2) how old is it?,(3) where was it made?, (4) how was itmade?, and (5) for what was it used?These points will be considered sepa-rately.

What Is It?This question is somewhat ambiguous:does it address the item as a formedobject or as a material? Small sherdsexcavated from archaeological sites cansometimes be pieced back together, sothe form of the original object can beidentified. More specifically, however,this question also refers to the type ofmaterial, be it ceramic or glass, ofwhich the artifact is made. Ceramicsare routinely subdivided into broad cat-egories based on aesthetic criteria suchas colour, grain size and degree oftranslucency (e.g. Hamer and Hamer1997). This allows the distinctionbetween, for example, coarse redwareand porcelain. In the case of visually-similar wares (e.g. the fine earthen-wares), chemical and petrographicanalysis (e.g. ‘fabric analysis’ sensu Pea-cock 1967 and Moody et al. 2003,among many others) may be requiredto make this distinction. For example,it was only after chemical analysis ofsamples originally described as eitherpearlware or protoporcelain (South2007) that the porcelaneous characterof sherds from the Cain Hoy, SouthCarolina site of John Bartlam’s pot-works was recognized (Owen 2007a).Bartlam is now recognized as havingproduced the first porcelain in what isnow the United States. The decorativefeatures of these porcelain sherds weresubsequently recognized on someteabowls that hitherto had been attrib-uted to London manufacturers (e.g.Isleworth). Strong evidence for theirAmerican provenance raised their com-mercial value in the antiques market ahundredfold (one such teabowl sold atauction for US$146,500 in January2013). Their historical significance isinestimable.

Although some experts candistinguish different types of glass (e.g.soda-lime versus potash-lead glass) sim-

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ply on the basis of its character (e.g.clarity, density, relative refractive index,and ‘ring’ when struck) or using a UVlamp, this distinction can require sometype of chemical test, if not a com-plete chemical analysis. The advent ofinexpensive analytical tools (e.g.portable XRF) that can analyse a vari-ety of media quickly and non-destruc-tively has the potential to make thecompositional classification of glass(and some types of high-fired ceram-ics; e.g. Owen 2007b) routine. Not allceramics are so readily distinguished.For example, creamware and trueporcelain can have overlapping compo-sitions (Owen 2011), although theirmineralogy, microstructure and porosi-ty are quite different.

How Old Is It?Archaeologists have recognized tempo-ral changes in the form, design, style ofdecoration and means of productionof ceramic and glass artifacts since theearly days of the discipline. Thisrequired careful documentation of arti-facts with good archaeological (asopposed to mixed or disturbed) con-text, and the comparison of theseobjects with those from older andyounger sites related to the same cul-ture. From this, a relative chronologyof artifact forms and decorationsemerged for particular regions and cul-tures over broad and sometimes over-lapping periods of time (e.g. EasternWoodland aboriginal pottery, ca. 2500BCE – 1000CE; see Peterson andSanger 1991, and Lattanzi 2009 andreferences therein). Absolute ages forsome ceramic wares can be establishedby dating the objects themselves orrelated material from the same archae-ological context. Examples includedosimetry dating (thermoluminescenceand optically-stimulated luminescence;e.g. Godfrey-Smith and Casey 2003),paleomagnetic and archaeomagneticdating (e.g. Hagstrum et al. 2005), and14C dating of charcoal associated withpotsherds. Direct dating of carbonresiding in or on potsherds, however,has proven problematic (e.g. Hedges etal. 1992; Stott et al. 2001). In the caseof historical fine ceramics produced byparticular factories, compositional cri-teria can be used to constrain the agesof wares made during particular peri-ods of that factory’s history (e.g. Owen

2003). If authentic, factory marks cir-cumvent more recondite means ofassigning wares to particular enterpris-es.

A major limitation to usingaesthetic criteria to estimate the age ofartifacts is the longevity of forms anddesigns. Likewise, widespread copyingof commercially successful objects(and their marks) can hamper deter-mining the provenance of these arti-facts.

Where Was It Made?Determining the provenance (i.e.‘sourcing’) of objects is an importantobjective in many archaeological stud-ies. Although this can sometimes beachieved by simply using aesthetic cri-teria, archaeologists increasingly rely onanalytical data for the objects them-selves (i.e. their bulk chemical compo-sitions) and/or, in the case of ceram-ics, their mineralogical constituents, tosource these artifacts. In this regard,provenance studies on ceramics mirrorthose for lithic artifacts. There is a vastliterature on sourcing both types ofmedia. What these studies have incommon is the requirement that databe available both for the artifactsthemselves and material from suspect-ed source areas. Even the compositionof glass, which preserves none of theraw materials used in its manufacture,can be sourced in this way, since evenglass-grade (high silica) sands invariablycontain heavy minerals that control thesignatures of many trace elements.These trace element profiles can bepassed down to the glass (and ceramic)artifacts manufactured from these rawmaterials. For example, glass-grade sili-ca sand (≥ 98% SiO2) occurring insouthern New Jersey and western Mas-sachusetts is known to have been usedby the Boston and Sandwich Glass Co.(Sandwich, Massachusetts, ca. 1826–1888). The sand from these two locali-ties have differing Nb concentrations, afeature that can be recognized in theSandwich glass each was used to make(Owen et al. 2005).

Ancient glass presents an addi-tional problem because some majorglassmaking centers exported ingots ofthis material to be shaped into usefulobjects elsewhere (e.g. Walton et al.2009); indeed, the wrecks of ships car-rying raw glass cargo have been discov-

ered in the Mediterranean (Foy andJezegou 2004). In this instance, theorigins of the glass itself and of thefinished objects differ, and require sep-arate sourcing. Strontium isotopes haveproved useful in determining theprovenance of some of the glass (andthe character and identity of its rawmaterials) from this area (Freestone etal. 2003), and both Sr and Pb isotopeshave been used to identify the recyclingof glass at ancient glassworks (Degryseet al. 2006). Recently, the isotopic com-positions of Sb from various stibnitedeposits have been determined to facil-itate sourcing ancient Sb-bearing glassartifacts from the Middle East (Loboet al. 2013).

Ceramic and glass provenancestudies mirror sedimentological sourc-ing in terms of design and implemen-tation. Some of the minerals common-ly used to identify the terranes fromwhich sedimentary detritus was shedare also used to constrain the prove-nance of historical and ancient ceram-ics and their raw materials. Among thetitania polymorphs, rutile has foundwide use in sedimentary provenancestudies (e.g. Triebold et al. 2012). Caremust be taken to correctly identify theparticular titania polymorph(s) inceramic artifacts, since these mineralspartition trace elements differently.Moreover, distinction must be madebetween bona fide detrital rutile and thatformed diagenetically (e.g. Pe-Piper etal. 2005). Notwithstanding thesepotential problems, heavy mineralshave been used to constrain the originof particular ceramic artifacts. Forexample, porcelain sauceboats excavat-ed in Philadelphia contain heavy min-erals (e.g. anatase) that compositionallyresemble those found in kaolin from atributary of the Delaware River that isknown from historical documents tohave been the source of at least someof the clay used by America’s secondporcelain manufacturer (Bonnin andMorris, Philadelphia, ca. 1770–73;Owen and Hunter 2009; Owen et al.2011a).

In some instances, heavy min-erals in ancient pottery can be tracedto specific rock units. Eastern Wood-land period Mi’kmaq pottery from theAnnapolis Basin/Bear River area, NovaScotia is a good example. These pot-sherds are loosely dated by their

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incised/impressed decorative motifs(e.g. Peterson and Sanger 1991) to asfar back as 2 ka. Some contain large(mm-scale), (sub)idiomorphic biotiteclasts that analysis suggests werederived from granodiorite of the SouthMountain batholith cropping outinland. This inference is substantiatedby the composition of monazite inclu-sions in biotite both in the granodioriteand the pottery, and by the composi-tion of the biotite itself (Fig. 2; J.V.Owen, D. Forfa, and J.D. Greenoughpers. comm. 2013).

Not surprisingly, the source ofheavy minerals found in ancient pot-tery cannot always be traced to a spe-cific locality or rock unit, but at leastsome specific sources of this materialcan usually be excluded. For example,the ancient Egyptians quarried basaltfor use in open vessels and pavingstones. Comparison of the trace ele-ment composition of pyroxenes andplagioclase (determined by laser probe)have helped link specific artifacts toquarry sites, but not the basalt temperfound in some ancient Egyptian pot-tery (Mallory-Greenough et al. 1999,2000). Bulk compositional data are alsouseful in characterizing archaeologicalearthenwares, and are widely used inprovenance studies (e.g. Weaver et al.2013 and references therein). They caneven hint at climate change-inducedvariations in the sediment used in theirmanufacture (e.g. Mallory-Greenoughand Greenough 1998). Statistical analy-sis (e.g. principal component analysis;multi-dimensional scaling [MDS]) iscommonly applied to analyticaldatasets to help identify compositionalgroupings of wares and relate them toother archaeological artifacts and/orsuspected sediment sources. An exam-ple showing regional variations in thecomposition of late Neolithic Ghana-ian pottery and its sediment sources isshown in Figure 3. This MDS diagramplot shows that ancient Ghanaian pot-ters gathered the raw materials fortheir trade from a wide area (Owen etal. 2013).

How Was It Made?The investigation of excavated objectsand of the sites where they were madehas revealed much about the produc-tion of ancient and historical ceramicsand glass, and it is beyond the scope of

this short review article to delve intothese topics. Geologists, however, cancontribute to a better understanding ofthe conditions (T, fO2) in the furnacesin which some of these objects weremade, provided that the minerals inthese samples formed and/or equili-brated during firing. For example,Philpotts and Wilson (1994) used ironoxide mineralogy to reconstruct thepeak temperature and oxygen fugacity

conditions at which aboriginal potterywas made. Aboriginal pottery aside,kiln-fired pottery was sometimes over-fired, and so suffered body distortion(sagging). Red-coloured earthenwarefrom the Eby pottery (Conestogo,Ontario, late 19th century) can showthis effect. These distorted sherds con-tain a melt phase that reacted withdiopside temper grains to form low-Capyroxene (enstatite) coronas, and from

Figure 2. Backscattered electron images of compositionally-similar monazite inclu-sions (‘M’) in biotite in (A) ancient Mi’kmaq pottery from the Annapolis Basin(Nova Scotia) and in (B) their suspected source rock, biotite granodiorite of theSouth Mountain batholith that crops out inland of the archaeological site wherethe pottery was found (J.V. Owen, D. Forfa, and J.D. Greenough pers. comm.2013). Monazite analyses and the Mg/(Mg+Fe) ratio (XMg) of biotite in both sam-ples are shown.

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which plagioclase crystallized (Fig. 4).Peak firing temperatures exceeded1100ºC (Owen and Dostal 2006).

Although the brief firingschedule (typically ~24–48 h) of high-T, vitrified wares (stoneware, porcelain)generally precludes the attainment ofequilibrium among crystalline and meltphases, phase diagrams neverthelesscan be used to constrain their firingtemperatures. Melts, however, rarelyplot on or near eutectics and cotecticson these diagrams, probably because ofthe influence of contiguous phases ontheir compositions (i.e. equilibrium isapproached only on a domainal scale;Iqbar et al. 2000; Owen et al. 2011b).It is, however, the simplicity of thechemical systems on which these dia-grams are based more than evidencefor lack of equilibrium that can limitthe usefulness of phase diagrams indetermining firing temperatures. Com-monly, one or more fluxes (e.g. Na, K,Pb) present in the ware is lacking onpertinent phase diagrams, so estimatedkiln temperatures for vitrified ceramicswill be maxima. In principle, analogueexperiments for these wares can be

undertaken to better constrain the fir-ing conditions of early ceramic pastes,but it is rarely the case that the resultsof this work faithfully replicate theartifacts themselves in terms of miner-alogy and microstructure.

In rare instances, porcelainsamples can contain microstructuralfeatures reminiscent of high-grademetamorphic rocks, and these can helpconstrain aspects of the firing historyof these wares. For example, a mid-18th

century teapot lid excavated in Londoncontains double coronas that separaterelict glass frit particles frommetakaolin in the matrix. The innercoronas correspond to a quenchedmelt phase. They are enclosed by felds-pathic (labradorite; An62) coronas thatextend into the clayey matrix (Fig. 5).The glass frit, now devitrified to a silicapolymorph – diopside – pseudowollas-tonite symplectite, served as a sourceof alkalis that fluxed the kaolin, pro-moting anatexis (Owen 2012). Extrac-tion of Ca from the inner coronas bythe developing labradorite rinds dis-placed the composition of the meltphase away from the 1345ºC eutectic

Figure 3. Multi-dimensional scalingplots showing the compositional clus-tering of (A) sediments (lateritic clayfrom Bulunga and Dabila, stream sedi-ment, and sediment from atop theGambaga Escarpment) used in making(B) late Neolithic Ghanaian pottery,subdivided into four groupings (I toIV) with one outlier (sample G6). (C)Overlapping compositions of pot-sherds and different types of sedimentfrom northern Ghana. Modified afterOwen et al. (2013).

Figure 4. Backscattered electron image of a diopside temper grain embayed andenclosed by a low-Ca pyroxene (enstatite; En) corona in the melt phase (glass) ofan overfired sherd of Eby pottery (Conestogo, Ontario, late 19th century). Notethe dark symplectic intergrowths (a silica polymorph?) in the inner part of thecorona, and quenched morphology of plagioclase microlites in the adjacent meltphase. Modified after Owen and Dostal (2006).

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on the SiO2–CaO–Al2O3 phase dia-gram (Fig. 6). The composition of theinner coronas is co-linear with the1345ºC eutectic and the CaO apex ofthe diagram, confirming that meltingoccurred at the thermal minimum. Theextent to which the presence of alkalissuppressed this eutectic, however, isnot known. The feldspathic coronasare porous, so must be a subsolidusfeature (i.e. formed during cooling).

Archaeological glass rarely ifever preserves unmelted batch ingredi-ents. Soda-lime waste glass from glass-works sites commonly contains (pseu-do?)wollastonite, but this evidentlyformed as the molten glass cooled.Distinguishing glass produced at theseglassworks from cullet added to glassbatches can be problematic. Since cul-let commonly included exotic brokenglass as well as waste glass produced atthe factory itself, this material can mis-lead as easily as inform archaeologistsseeking to characterize the wares pro-duced at particular glassworks. So toocan melted pieces of the furnaces inwhich glass batches were fired. Forexample, black glass rinds (Fig. 7A) onwhat are inferred to be parts of therock base of a furnace at CaledoniaSprings, Ontario, the location of oneof Canada’s first known glassworksand one that advertised for workersfamiliar with blowing black glass,proved to have compositions coincid-ing with the rock itself. The rockshown in Figure 7B is fused to a brick,and contains silicate and oxide minerals(calcic plagioclase, (pseudo)wollas-tonite, diopside, Fe-olivine andpleonaste; Fig. 8); it compositionallycorresponds to a calcareous mudstonewith subordinate, disrupted, darkmetapelite layers (Fig. 7B). Unlike anymetamorphic rock formed naturally, itis vesicular (Fig. 7A, B). It is essentiallya man-made migmatite, a product ofpyrometamorphism related to kiln-fir-ing. Based on plagioclase-melt andOl–Cpx thermometry, it evidentlyformed at T >1000ºC at 1 bar pressure(Owen and Culhane 2005). As usual,caution should be exercised whenapplying mineral- and mineral-meltthermometers to ceramics and relatedmaterials (in this case, a rock from thekiln), given evidence for disequilibrium(e.g. quench crystals; Fig. 8) in thesewares. A minimum temperature of

Figure 5. Backscattered electron image of double corona structures separatingdevitrified glass frit (now symplectites) from metakaolin (Kln) in mid-18th centuryBritish porcelain (1st patent Bow teapot lid). The coronas consist of an inner, glasslayer and an outer, feldspathic layer from which plagioclase microlites (Pl) extendinto matrix metakaolin. Note the concentration of pores (black spots) in the outercoronas, and presence of partly resorbed silica polymorphs (probably alpha quartz,Q) in the matrix. Modified after Owen (2012).

Figure 6. Displacement of melt compositions (see grey arrow) away from the1345ºC eutectic on the SiO2–CaO–Al2O3 phase diagram (Osborn and Muan 1960).The melt phase, represented by inner (glass) coronas in this 1st patent (‘A-marked’)Bow porcelain teapot lid, changed to more Ca-poor compositions as labradorite inthe surrounding (outer) corona (Fig. 5) formed. Modified after Owen (2012).

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~1000ºC is nonetheless consistentboth with the extensive melting of thisspecimen, and with what is knownabout the firing conditions that wereroutinely achieved in historical kilns.Until they were analysed, the blackglass rinds on these rocks and brickswere considered to be bona fide Caledo-nia Springs glass, intact examples ofwhich still elude identification.

For What Was It Used?The purpose for which particularceramic and glass artifacts were origi-nally used can often be surmised sim-ply based on the form of (reconstruct-ed) objects. It may not be obvious,however, what specific materials indi-vidual vessels once held. However,residues of these materials are some-times preserved on sherds of thesevessels, and their compositions canhelp identify them. Various types oforganic residue have been recoveredfrom ancient ceramic vessels, includingamino acids, waxes, cholesterol andfatty acids. Attention has focused onthe latter because of their stability(although they are still subject todecomposition), ease of extractionfrom the artifact, and the widespreadavailability of instruments required fortheir analysis (Eerkins 2007). Analysisof fatty acids has led to the identifica-tion of the original contents of vessels,and this in turn places constraints onthe diet of ancient peoples, and helpsto identify the character of some ofthe other materials they used in theirdaily lives (e.g. glue; Mitkidou et al.2008). These specialized analyses haverevolutionized the investigation ofancient ceramics, and by shedding lighton aspects of the day-to-day activitiesof our early ancestors, address one ofthe main objectives of archaeologists.

CONCLUSIONSGeologists have long contributed toarchaeological studies of lithic artifacts,particularly with respect to the sourceof raw materials used in their produc-tion. Increasingly, archaeologists areconsulting geologists (and other scien-tists) about ceramics and glass, whichcan be likened to synthetic rocks(metamorphic rocks in the case ofceramics; obsidian in the case of glass).High-fired wares can be objectivelyclassified based on their chemical com-

Figure 7. Photographs of a partly melted, vesicular, calcareous metamudstonefrom a 19th century glass furnace at Caledonia Springs, Ontario, showing (A) thepartial glass rind coating (especially conspicuous at arrow) the surface of grey cal-careous metamudstone from Caledonia Springs, and (B) the same sample in crosssection, revealing a broken brick fused to the grey rock by a narrow layer of plagio-clase (Pl)-bearing glass, the thicker black glass rind on the rock, disrupted blackrock (metapelite) layer, and vesicles (V) in the rock. Modified after Owen and Cul-hane (2005).

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position in much the same way as vol-canic rocks, which, like vitrified ceram-ics, can contain both crystalline andglass phases.

The expertise that we, as geol-ogists, acquire during the course of our

careers provides us with a unique per-spective on these media, as well as theknowledge and means to analyse them.We are not trained to properly excavatearchaeological artifacts, and shouldn’t,but we can work on them once they

arrive at our lab. The burgeoning col-laboration between archaeologists andgeologists is but one example of theincreasingly cross-disciplinary aspect ofscientific research. For academics fac-ing diminishing research funding, oneof the appealing aspects of investigat-ing archaeological artifacts is that it canbe done in a cost-effective manner.Archaeologists do the field work; wecan do some of the analyses, and inmany instances, the analysis of a single,informative artifact can produce signif-icant, publishable results. Perhaps moreimportantly, collaboration with archae-ologists not only broadens our hori-zons, it provides an opportunity toexpand the application of the skills wehave honed while working on moretraditional geological projects, and pro-vides additional information on arti-facts that otherwise would not havebeen discovered.

ACKNOWLEDGEMENTSOur work on archaeological artifactshas been funded by NSERC (JVO andJD), SSHRC (JVO), Saint Mary’s Uni-versity Faculty of Graduate Studiesand Research (JVO), the ChipstoneFoundation (JVO), and the CummingCeramic Research Foundation (JVO).The manuscript benefitted from com-ments by two Journal referees.

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Received July 2013Accepted November 2013First published on the web January 2014

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