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Research paper
Technological fingerprints of Black-Gloss Ware from Motya(Western Sicily, Italy)
Caterina De Vito a,⁎, Laura Medeghini a, Silvano Mignardi a, Diletta Orlandi b, Lorenzo Nigro c,Federica Spagnoli c, Pier Paolo Lottici d, Danilo Bersani d
a Department of Earth Sciences, Sapienza University, P.le A. Moro 5, 00185 Rome, Italyb Applied Sciences for Cultural Heritage, Sapienza University, P.le A. Moro 5, 00185 Rome, Italyc Department of Sciences of Antiquities, Sapienza University, P.le A. Moro 5, 00185 Rome, Italyd Department of Physics and Earth Sciences, University of Parma, Parco Area delle Scienze 7/a, 43124 Parma, Italy
Article history:Received 14 December 2012Received in revised form 14 November 2013Accepted 18 December 2013Available online 19 January 2014
Keywords:FiringTechnologyμ-Raman spectroscopyMulti-analytical studyBlack-Gloss Ware
Black-Gloss Ware artifacts, unearthed at the Phoenician-Punic site of Motya (Sicily, Italy), dating back from theend of 6th to the early 4th century BC, have been investigated. Mineralogical, petrographical and chemicalcharacterization has been performed to elucidate the technological aspects, to distinguish locally manufacturedpottery from those imported and to provide information on raw materials used for their production. A multi-analytical approach based on optical microscopy, μ-Raman spectroscopy, scanning electron microscopy andX-ray diffraction investigations has been used. Results show that the internal body is composed of quartz,K-feldspar, plagioclase feldspar, pyroxene, mica, gehlenite and hematite. In addition, magnetite, hercynite andamorphous carbon occur in the black gloss. On the basis of these mineralogical assemblages, we can infer thatthe ceramic artifacts were exposed to similar firing temperatures and fO2, i.e. estimated T in the range950–1100 °C under oxidizing–reducing–oxidizing conditions.
It is generally known that the characterization of ancient pottery is acomplicated task due to the mineralogical and chemical heterogeneitiesof the starting raw materials (Andaloro et al., 2011; Bersani et al., 2010;Ionescu et al., 2011; Kramar et al., 2012; Medeghini et al., 2013a, b).They are a mixture of clays and temper consisting mainly of illite, smec-tite, kaolinite, quartz, K-feldspar, plagioclase feldspar and carbonateminerals.
The physical and chemical properties, and the structure of potteryare mainly influenced by raw material composition (Tschegg et al.,2009), amounts of inclusions in the ceramic fabric and firing conditionsindicative of the local technology available at that time (İssi et al., 2011;Rathossi and Pontikes, 2010; Riccardi et al., 1999). Consequently, thechallenge is to determine how such parameters drive and control theformation of this handmade material.
Attic Black-Gloss Ware (ABGW) was produced for several centuriesin Athens (i.e., 6th–4th centuries BC). These ceramics were highlyappreciated and extensively used in Greece, Italy and around theMediterranean world (McDonald, 1981, 1982). This wide distributionof ABGW makes it an ideal candidate for the study of technology,
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trade routes, and intercultural relations and for the reconstruction ofthe economic system of the Mediterranean area (Kingery, 1991).
Numerous studies (Gliozzo et al., 2004; Maggetti et al., 1981; Titeet al., 1982) on Black-GlossWare artifacts indicated a main mineralogi-cal composition including quartz, K-feldspar, plagioclase feldspar andhematite for the internal body; whereas for the black gloss magnetite,hercynite and rare maghemite.
Studies on the mineral composition of the black gloss and the ce-ramic body of ABGW, using high-resolution techniques (i.e., powderdiffraction using synchrotron X-rays and transmission electron micros-copy) determined the nature, the grain size of crystallites and latticeparameters of the minerals of the black gloss (Gliozzo et al., 2004;Tang et al., 2001). In particular, the grain sizes of crystallites of magne-tite, hematite and silicate minerals are b0.03 μm.
Concerning the provenance, as the pottery-maker usually trans-forms the original clays through cycles of selections, the original finger-prints can be modified. The problem increases when the quality ofceramic requires a high selection of raw material from which coarsesize fractions are removed by suspension as, for example, for theproduction of Black-Gloss Ware.
The objective of this work is to characterize a set of coeval Black-Gloss Ware fragments attributed to different productions in order todetermine the discriminating elements among different samples andthe potential differences in composition and/or technology.
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2. The archaeological context
Motya is a 45-ha island located 1 km from the coast of Sicily, be-tweenMarsala in the South and Trapani in the North (Fig. 1). The islandhosts one of the most important Phoenician sites of the WesternMediterranean since the first phases of Phoenician expansion to theWest dating back from 770 to 750 BC (Fig. 1).
Excavations by Rome “La Sapienza” Archaeological Expedition toMotya, which carried on since 2002 (Nigro, 2003; Nigro and Spagnoli,2012) brought about numerous specimens of Black-Gloss Ware ofAttic or other productions.
The ceramic fragments selected for this study were unearthed inthree main areas: Area C, on the south-western corner of the islandwhere a huge sacred compound including a main building, the Temple,connected to a sacred pool, the Kothon, and several shrines and cult in-stallations were found (Nigro, 2010; Nigro and Spagnoli, 2012); Area D,on the western slopes of the Acropolis, occupied by several patricianresidences; and Area F where the so called “Western Fortress”, border-ing theWest Gate and the adjacent city walls, was found (Nigro, 2010).
Morphologic and stylistic grounds of these Black-Gloss Waresallowed the archeologists to distinguish at least five different possibleproductions in use at Motya during the 6th–4th centuries BC, four ofwhich have been investigated in the present study [i.e., Attic BGW(ABGW), Punic BGW (PBGW), Greek Colonies in Sicily (SGC) and otherproductions imported probably from Etruria and Segeste (L)].
3. Materials and methods
3.1. Materials
Sixty-three small fragments of ceramic, attributed by archeologiststo four different productions have been studied. Representative samplesof the different productions are shown in Fig. 2.
Fig. 1. View of the archaeological site showing
3.2. Experimental
Micro-Raman data were obtained using a Jobin-Yvon HoribaLabRam apparatus, with the 632.8 nm line of He–Ne laser used for exci-tation and equipped with an Olympus microscope and a motorized x–ystage (Department of Physics, University of Parma, Italy). These datawere interpreted on the basis of existing databases (Burgio and Clark,2001) and the Parma University database (http://www.fis.unipr.it/phevix/ramandb.php, accessed July 15, 2012).
Petrographic thin-section analysis was performed by polarizing mi-croscopy according to Whitbread (1986, 1995) criteria with the aim toacquire details on the microstructure of the groundmass and inclusionsof the internal body and on the external black gloss.
The ceramic fragments were investigated by scanning electron mi-croscopy using a FEI-Quanta 400 (SEM-EDAX) instrument, operatingat 20 kV, equippedwith X-ray energy-dispersive spectroscopy (Depart-ment of Earth Sciences, Sapienza University, Rome, Italy). For eachpolished thin section, EDAX spectra were acquired. Moreover, SEMimaging was performed to evaluate the ceramic micro-morphologyand the vitrification degree of the samples (Tite and Maniatis, 1975).
X-ray powder diffraction (XRD) patterns were obtained using a Sei-fert diffractometer operating at 40 kV and 30 mA (Department of EarthSciences, Sapienza University, Rome, Italy). The XRD patterns were re-corded from 5° to 60° 2θ at a scan step of 0.02° and with a countingtime of 8 s per step using Cu Kα radiation. Part of the internal bodyand black gloss of each potsherd has been finely ground by hand inagate mortar and analyzed using a back-filled, randomly orientedmount.
In addition, to minimize the contamination of the black gloss by theinternal body, representative samples of the black gloss have also beenanalyzed using a laboratory parallel-beam Bruker AXS D8 Advance dif-fractometer operating in transmission in θ–θ geometry, whose experi-mental set up requires the preparation of very small amounts ofsample (b5 mg) as a capillary. The samples of the external black gloss
Fig. 2. Representative samples of the different productions; MC = samples from Motya zone C, MD = samples from Motya zone D, MF = samples from Motya zone F, see Fig. 1.
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have been prepared by gently scratching the thin film and the purity ofthe sample was checked under a microscope.
4. Results and discussion
4.1. Ceramic body
4.1.1. Color of the ceramicCommonly, the color of ceramic is considered as an indicator of firing
temperatures and fO2, i.e. oxidizing or reducing conditions. In addition,the chemical composition of the raw clays, the different forms of Fe inoxides and hydroxides and the presence of organic material contributeto the wide variability of the color (Maniatis, 2009). The color of the in-ternal body for the four productions fromMotya can be summarized asfollows according to the Munsell Soil Color Chart (1992): ABGW rangesfrom reddish yellow (5YR 6/6, 5YR 7/6, 7.5YR 7/6) to light red (2.5YR6/8); PBGW ranges from reddish yellow (7.5YR 7/6), light yellowishbrown (10YR 6/4), light red (2.5YR 6/8) to pinkish gray (7.5YR 7/2);SGC ranges from very pale brown (10YR 8/3), light brown (7.5YR 6/4),light red (10R 6/6), reddish yellow (7.5YR 8/6, 5YR 6/6, 5YR 6/8), red(2.5YR 5/8, 2.5YR 6/6) to light bluish gray (GLEY 2 7/1) and L rangesfrom greenish gray (GLEY 2 5/1, GLEY 1 6/1) to light reddish brown
(2.5YR 6/4). This variability is the result of different constraints suchas firing temperatures and/or fO2. Moreover, as the studied samples be-long to different productions, part of them imported, the chemical com-position of the different raw clays also plays a role in the color of thefired ceramic.
4.1.2. Petrographic analysisThe optical microscopy observations of representative samples,
i.e., 30 specimens, of the four productions indicate that only one fabricis present and themain features in termsofmicrostructure, groundmassand inclusions are described below (Fig. 3), according to the criteria ofWhitbread (1986, 1995).
The microstructure has a porosity which ranges from 5 to 10%,consisting mainly of meso-vescicles and rare macro-vughs with sub-parallel alignment to themargin of the sample. Unimodal grain-size dis-tribution is observed. The groundmass is homogeneous, with the colorranging from orange to brown-reddish; no optical activity is observed.The inclusions are mainly represented by subangular to subroundedgrains of quartz and iron oxides and fine grain size ranging from10 μm to 0.125 mm. Rare mica has been observed. Secondary calcitein vughs has also been identified.
Fig. 4. Micro-Raman spectrum of the ceramic body (sample n.1 MF.04.1273/32, SGC) re-veals the presence of quartz (Qtz), K-feldspars (Kfs), hematite (Hem) and magnetite(Mag). The featureless fluorescence backgroundwas subtracted for clarity (the acquisitiontime was 30 s with 3 accumulations).
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As concerning the size and shape of voids it is possible to infer differ-ent treatments of the paste during themanufacturing of vessels (Cuomodi Caprio, 2007). Indeed, roundedmeso-vescicles should be derived froman initial processing by hand; conversely, irregular, elongated-shapemacro-vughs, sub-parallel to the margin of the sample, are associatedwith a final wheel manufacturing, according to archaeological interpre-tations and literature data (Reedy, 2008).
The absence of optical activity of thematrix, related to high degree ofsintering and in turn to the maximum firing temperature, suggests thatthe fabric of ceramic samples fromMotya is the result of high firing tem-perature, at least 950 °C as previously reported (Reedy, 2008). Despitethemacroscopic differences (i.e., color, morphology and style) observedin samples belonging to different productions, the petrographic analy-ses did not permit the identification of different fabrics to be used asdiscriminating elements among the samples.
4.1.3. Micro-Raman spectroscopy analysisRaman spectra reveal the presence of quartz, calcite, plagioclase
feldspar, K-feldspar and iron oxides such as hematite and magnetite,as shown in Fig. 4.
The intense peak at 464 cm−1, characteristic of quartz, is due to theA1 symmetric stretching mode (Si–O–Si). The Raman spectra of the in-ternal body reveal hematite, with typical peaks at 299, 224, 411, 611,
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498 and 245 cm−1, together with magnetite, having a characteristicstrong band at about 660 cm−1, and calcite, showing a typical strongpeak at 1086 cm−1 due to the symmetric stretching mode of the car-bonate ion, in addition to other weak bands at 712 and 281 cm−1
(Bersani et al., 1999, 2000; De Faria and Lopez, 2007; De Faria et al.,1997).
In some samples, the typical doublet around 500 cm−1 characteristicof K-feldspar and plagioclase feldspar has been observed. Plagioclasefeldspar is characterized by a strong band between 500 and 510 cm−1
and a less intense band in the range 478–488 cm−1 whereas alkali feld-spar is identified by a strong band at 513–514 cm−1 and other twopeaksin the range 450–500 cm−1. On the basis of the Raman spectroscopy re-sults the followingmineralogical composition for the internal body, sim-ilar in all specimens, may be proposed: quartz + plagioclase feldspar +K-feldspar + magnetite + hematite.
These results suggest that the paste was prepared by adding to theclays some non plastic material to improve workability and influenceother characteristics of fired vessels (Ospitali et al., 2005). The sourcesof mineral tempering are normally sands, containing quartz plagioclasefeldspar and K-feldspar, which are used to increase the plasticity of thepaste.
The μ-Raman spectroscopy allows to clearly distinguish between dif-ferent iron oxides: hematite andmagnetite. The occurrence of these twoiron oxides is connected with the atmosphere during firing. Indeed,magnetite (dark in color) forms in reducing atmosphere, while the oxi-dized phase, hematite (red in color), is stabilized under oxidizing condi-tion. According to De Faria et al. (1997) and Hanesh (2009), theconcomitant presence of different Fe-oxides should indicate the alterna-tion of reducing/oxidizing conditions during the firing cycle.
4.1.4. X-ray diffraction analysisThe XRD patterns of the bulk mineral assemblage of the internal
body (Table 1) reveal that the most abundant phases are quartz, occur-ring as inclusions in the vitrified matrix, and secondary calcite in vughs,as previously observed in thin section. In addition, An-rich plagioclasefeldspar, K-feldspar, iron oxides (i.e., magnetite and hematite), pyrox-ene (diopside), mica and gehlenite are generally present in minoramounts (Fig. 5). Accessory minerals include zircon, apatite and rutile.As the presence or absence of specific mineral phases provides con-straints on the maximum firing temperature and fO2, the mineral as-semblages of Motya samples are compared to data obtained fromfiring experiments (Jordán et al., 1999; Maritan et al., 2006; Nagy
et al., 2000; Nodari et al., 2007; Rathossi and Pontikes, 2010; Riccardiet al., 1999). (See Table 2.)
A significant mineralogical feature of Motya ceramic is the absenceof clay minerals which suggests firing temperatures higher than900 °C; indeed, clays decompose by the removal of the hydroxyl groupsof the silicate lattice at temperatures between 450 and 900 °C (Jordánet al., 1999). Moreover, the absence of primary calcite suggests firingtemperatures above 850 °C. As reported in the literature (Rathossi andPontikes, 2010), calcite decomposes into CaO and CO2 at temperaturesranging from 750 °C to 850 °C, depending on the firing conditions(i.e., oxidizing or reducing atmosphere) of the system. In this thermalrange, free CaO reacts with free silica and alumina, derived from thebreakdownof clayminerals, to formgehlenite according to the reaction:
Al2O3 þ SiO2 þ 2CaCO3⇆Ca2Al2SiO7 þ 2CO2 ð1Þ
Other reactions involving mica and calcite, among others, can lead tothe formation of gehlenite and K-feldspar:
The occurrence of gehlenite suggests that claymaterials rich in calci-um carbonate and/or the addition of carbonates in the clayey materialswere used (Riccardi et al., 1999). Gehlenite, occurring in minor to traceamounts in the majority of the analyzed samples, is indicative of firingtemperatures in the range from 950 °C to 1100 °C. Moreover, its forma-tion depends primarily on the grain size of the starting raw material(Jordán et al., 1999; Maggetti et al., 1981; Maritan et al., 2006; Nagyet al., 2000; Nodari et al., 2007; Tschegg et al., 2009).
The presence of trace amounts ofmica,mainly sericite, which breaksdown in the range of 900–1000 °C (Aras, 2004), suggests that the firingtemperature of ceramic did not exceed this range. In fact, the abundanceof these minerals decreases when the temperature reaches the thresh-old of 1000 °C. Based on experimental results of Rathossi and Pontikes(2010) the ceramic samples from Motya, containing trace amounts ofsuch mineral, were fired at temperatures in the range 950–1100 °C.
Magnetite formed at the expense of the oxidized form of the addedFe-oxides in the paste, but can also be the result of chlorite breakdownwhich releases Fe2+ in the system (Nagy et al., 2000). This phase,magnetite, crystallizes in the reducing conditions of the firing cycleand remains under oxidizing conditions even if part of magnetite istransformed into hematite, due to the limited possibility of oxidizing
plagioclase; Cal = calcite; Kfs = K-feldspar; Mag = megnetic; Hem = hematite;Px =; Ap = apatite).
Fig. 5.XRDpatterns of the internal body and black gloss of samples n.21/MD.02.227/7 (ABGW), n.26/MF.04.1257/4 (PBGW) and n.34/MD.03.1056/47 (SGC) quartz (Qtz), secondary calcite(Cal), mica (Mca), plagioclase feldspar (Pl), magnetite (Mag), gehlenite (Gh), JCPDS, 2000.
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gasses or fluids to penetrate the ceramic body. In addition, hematite isformed after the dehydration of the added Fe-oxyhydroxide and Fe-oxide phases of the rawmaterials and/or after the dehydroxylation of il-lite (Rathossi and Pontikes, 2010). The co-occurrence in some samplesof both reduced and oxidized forms of Fe-oxides likemagnetite and he-matite suggests an early oxidizing atmosphere followed by reducingconditions. The formation of hematite during cooling could be ascribedto the re-oxidation of part of magnetite by fluids or gasses, indicatingthat the internal body of the ceramic does not behave as “an imperme-able structure”. The estimated firing temperatures of all pottery speci-mens, reconstructed on the basis of the XRD data, fall in the shadedarea of Fig. 6, in which the mineral phases are reported, considering
also the absence of illite/clay minerals and the presence of secondarycalcite.
The potential sources of clay used for ceramic production are diffi-cult to infer as themajority of ceramic artifacts were imported. Howev-er, hypothesis on the sources of rawmaterial for the PBGWand SGC canbe proposed. In particular, the characterization of clayey materialsfound in a working tank unearthed in the so-called “Officina deiVasai” (6th century BC, Alaimo et al., 1997, 1998) at Motya, showsthat the mineralogical assemblage of the clay minerals was made ofsmectite, illite and kaolinite mixed with grains of quartz, K-feldspar,plagioclase feldspar and calcareous materials. These materials wereprobably collected from exploited by the alluvial deposits proximal to
Table 2Representativemineralogical composition of the black gloss. (Legend:Qtz = quartz; Pl = plagioclase; Cal = calcite; Kfs = K-feldspar;Mag = magnetite; Hem = hematite; Hc = hercynite;Mu = mullite; px = pyroxene; Gh = gehlenite; Mca = mica).
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the older fan of the Birgi river situated on the opposite side of Motyaisland (Alaimo et al., 1998). To infer the provenance of raw materialfor SGC production is more complicated as numerous clay resources ofSicily have been used for ceramic production since 8th century BC inGreek and Phoenician colonies (Montana et al., 2011).
4.1.5. Morphological and microchemical analysisThe electron micrographs of Fig. 7 show that the internal body is
characterized by a high degree of sintering, whereas the BSE imagessuggest the presence of grains of crystalline inclusions (see Fig. 7c).The relatively low abundance of crystalline phases, displaying a veryfine grain size, suggests that the original clay minerals were notcompletely transformed into high-temperature phases but rather to amixture of amorphous to glassy phases, pointing to a short time of firingat high temperatures.
Fig. 6. Relationship between the mineralogical composition of Motya samples and esti-mated temperature range of firing (shaded area). Themineral phases detected are: quartz(Qtz), plagioclase feldspar (Pl), K-felspar (Kfs), gehlenite (Gh), hematite (Hem), Hercynite(Hc) and pyroxene (Px); secondary calcite (Cal) and illite (Ill), absent in our samples,wereadded for comparison.
EDAX spectra of the chemical composition of the internal body arereported in Fig. 7 b, c and d; however, some Fe-enriched areas are alsopresent (Fig. 7 c).
The crystalline inclusions, according to BSE images and EDAX data,are mainly represented by grains of quartz, K-feldspar, plagioclase feld-spar, mica and Fe-oxides, confirming the analytical results obtained. Inaddition, the samples of the PBGW and SGC productions contain grainsof zircon, Ti-oxides and apatite. This phosphate was not detected in thesamples of the ABGW and L productions.
Based on the SEM results, we can infer that the high degree ofsintering observed in Motya samples occurred at relatively high tem-peratures. Previous experimental results (McConville et al., 2000;Rathossi and Pontikes, 2010) pointed out that sintering stage in clay–calcite mixtures starts at about 950 °C, and it is controlled by viscousflow (Hajjaji and Kacim, 2004). The extreme sintering of ceramic isthe result of the melting of the clay minerals which collapse, duringthe cooling process, and produce a vitrified matrix less porous and inturn a harder ceramic body (McConville et al., 2000; Wattanasiriwechet al., 2009). The high degree of sintering is also controlled by the pres-ence of Fe2+ in the system, as it is a structuremodifier ion,which affectsthe polymerization of the silicate melt, and in turn reduces its viscosity(Rathossi and Pontikes, 2010).
Normally, the more plausible candidates asmelting agents are alkalifeldspars, carbonate and phosphate minerals. Our results provide evi-dence that the melting agents in the production of Motya ceramicwere alkali feldspar, carbonate and apatite. In particular, the presenceof apatite in samples of two of the four productions confirms the roleof phosphorus as a lowmelting agent as previously suggested in numer-ous petrological experiments (London, 1992). Moreover, apatite can beconsidered a tracer and discriminatingmineral among the black potteryassigned to different productions.
4.2. Black gloss
Themajority of studied samples present both surfaces covered by anopaque black gloss, while samples 1 (SGC, MF.04.1273/32) and 11(ABGW, MF.05.1287/24) show a metallic luster (Fig. 2).
Fig. 7. SEM image showing the high degree of vitrification of the internal body in samplen.21 (MD.02.227/7, ABGW) (a); BSE images and EDAX spectra of representative samplesof three productions: ABGW, sample n.13 MF.06.1324/2 (b); PBGW, sample n.24MD.06.1042/8 (c); SGC, sample n.34 MD.02.216/42 (d).
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4.2.1. Petrographic analysisAll the samples show internal and external black slips characterized
by a dark-brown color, with the exception of samples 15 (MC.04.939/21, ABGW) and 57 (MF.06.1324/2, ABGW) having a reddish color inplane polarized light (PPL), and always dark under crossed polarizedlight (XP) (with the only exception of sample 34/MD.03.1056/47,SGC). The absence of pleocroism under XPmay be ascribed to the glassynature of the layer. The black gloss often shows an irregular thin film ofcalcite, probably related to the post-firing process in the burial environ-ment due to circulation of fluids. This film does not correspond to acommon glaze applied on ancient ceramic to give a luster (Kingeryet al., 1976) but clearly shows a sign of carbonate precipitation underburial conditions.
4.2.2. Micro-Raman spectroscopy analysisRaman spectra, collected on different areas of the black gloss of all
samples, indicate the presence of magnetite (Fig. 8). In addition tomag-netite, in some samples belonging to PBGW and L productions, the twotypical broad bands at 1372 and 1590 cm−1 have been assigned toamorphous carbon (not shown). Moreover, calcite in all samples andrare gypsum, by the strong band at 1008 cm−1, have been identified(not shown).
As in the case of the internal body, Raman data revealed similarmin-eralogical composition,mainlymagnetite, in samples of the same groupand in samples of the different productions. According to the results ofOspitali et al. (2005), the occurrence of amorphous carbon, not homoge-neously arranged in the black gloss, could suggest a different distribu-tion and ordering of vessels in kilns. The interaction between vesseland fuel combustion-fumes seems to be influenced by the relativeposition of each object in the kiln and in turn to determine the occur-rence or absence of such amorphous carbon. The design of kiln observedin Motya and other Phoenician-Punic sites seems to support this hy-pothesis (Alaimo et al., 1997, 1998). Consequently, the presence of car-bon, exclusively in PBGW and L productions, allows us to distinguishthese ceramics from those of ABGW and SGC productions.
4.2.3. X-ray diffraction analysisXRD patterns of the black gloss indicate the same mineralogical
assemblage observed for internal body along with magnetite (Fig. 5) orhercynite (Fig. 9) and rare hematite in some samples. Themetallic lusterobserved in two samples is not related to mineralogical composition asthey contain the same phases recognized in other samples. To explainthe reasons of the visual differences of black gloss pottery, Gliozzo et al.(2004) suggested that starting from the same clayey materials, a fine re-finement of the clay, and/or a longer firing time and/or different oxygenfugacity fO2 conditions, could favor a metallic luster.
For samples of L production, because of low quantities of availablematerial, XRD analysis was performed directly on the sample's surface.These XRD patterns reveal hercynite (Fig. 9d). The occurrence of mag-netite and hercynite enables an estimate of the fugacity of oxygen andfiring temperatures (Rathossi and Pontikes, 2010). Magnetite crystal-lizes at the expense of oxidized forms of Fe (Nagy et al., 2000) andmost probably it forms in the following way: firstly, goethite isdehydrated during the initial oxidizing atmosphere according to the fol-lowing reaction:
2α� FeOOH→α� Fe2O3 þ H2O ð3Þ
then hematite is reduced to magnetite
3α� Fe2O3 þ CO→2Fe3O4 þ CO2 ð4Þ
Moreover, as noted earlier, magnetite can form from the breakdownof chlorite which releases Fe2+ in the system (Nagy et al., 2000;Rathossi and Pontikes, 2010). For PBGW and SGC productions, themechanism of Eq. (3) is supported by the occurrence of Terra Rossa
soils, composed mainly of goethite, hematite and maghemite, whichcover wide zones of Western Sicily (e.g., Bellanca et al., 1996). Magne-tite marks reducing conditions and remains stable because under oxi-dizing conditions it does not transform into hematite. The occurrenceof magnetite in the coating suggests that during oxidizing conditions,fluids or gasses did not re-oxidize this phase into hematite. This findingfurther supports the hypothesis that the black slip behaves as “an im-permeable structure”.
Hercynite originates at high temperature (over 950 °C and lower fO2
with respect to that of magnetite formation) from the breakdown of il-lite (Gliozzo et al., 2004; Jordán et al., 1999) and commonly after heatingof high alumina clays. It has also been demonstrated that from illitebreakdown an intermediate phase between spinel and hercynite occurs(Jordán et al., 1999), which transforms into mullite around 980 °C(Santos et al., 2006; Silva et al., 2011). In the case of fired ceramicfrom Motya, mullite has been detected, in a very low amount, in onesample of L production (Fig. 9d) suggesting a firing temperature aroundthe above mentioned threshold. The crystallization of magnetite orhercynite is driven by the Al content in the raw materials: hercynite-bearing black gloss was therefore produced using a more Al-enrichedpaste than that used for magnetite-bearing black gloss. The presenceof hercynite in L production deserves further discussion as some sam-ples of this group, on the basis of stylistic and morphological analysis,have been attributed by the archeologists to Etrurian productionsimported at Motya (Giura, 2011; Michetti, 2007). This hypothesisseems to be supported by our results as hercynite commonly occurs inblack ceramic from archeological sites of Etruria (Gliozzo et al., 2004).
A significant difference observed among the four productions is dueto the absence of hematite in PBGW and L. However, minor amounts ofhematite have been recognized in the XRD spectra of some samples ofthe other two groups. The occurrence of this phase may be the resultof a partial re-oxidation of the magnetite during oxidizing conditionsor possible contamination by the internal body. Also the occurrence ofabundant quartz and calcite in the gloss can be due to contaminationby the internal body during sample preparation. To solve these prob-lems, some samples were investigated with the preparation as a capil-lary to minimize the effects of possible contamination of the
Fig. 7 (continued).
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Fig. 8.Micro-Raman spectra of the black slip (sample n.24/MF.05.1287/22, PBGW) show-ing the presence of Mag. The featureless fluorescence backgroundwas subtracted for clar-ity (the acquisition time was 90 s with 3 accumulations).
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powdered gloss by the internal body. However, the presence in theblack gloss of the same main mineral phases recognized in the internalbody in this new set of acquired data, even if in different proportions,suggests that the nature of the starting raw materials for the internal
Fig. 9.XRDpatterns of the black gloss of representative samples of three productions: ABGW, sam(MF.06.1327/15) (c); L, sample n.49 (MC.04.715/1) (d) acquired on the sample's surface. Qtz =
body and the black gloss were similar, apart from the addition of Fe-rich materials or carbon during the preparation of the paste of theblack gloss.
4.2.4. Morphological and microchemical analysisThe BSE images reveal a fine grained structure of the external black
gloss composed of a vitrified phase inwhich very rare crystalline phasesoccur as detected by XRD. Indeed, as shown by Tang et al. (2001) thesephases have sizes b0.03 μmand therefore are not detectable using SEM.
The thickness of the black gloss ranges from 20 to 40 μm (Fig. 7 a,b).EDAX spectra of the external coating suggest a chemical compositionsimilar to that of the internal body, even if the coating is generallymore Fe-enriched with respect to the internal body (Fig. 7). On thebasis of EDAX results it is possible to infer that the black gloss shouldbe derived from a fine suspension of clay and probably Terra Rossasoils, to produce a homogeneous and polished layer. The suspension,as reported in previous studies (Tang et al., 2001) was extremely puri-fied, with almost complete absence of large size inclusions, and Fe-oxide enriched to involve a black slip with a predominant vitrifiedphase. The application of this fine grained paste on the ceramic artifactswas probably made by immersion of the main body before the firingprocess, attaining temperatures above 950 °C to obtain a completesintering of the coating (Tang et al., 2001). The reducing atmosphereand the high temperature are responsible for the formation and growthof magnetite and hercynite and of the almost complete sintering stageof the slip coating.
ple n.12 (MD.02.238/11) (a); PBGW, sample n.53 (MD.03.1036/72) (b); SGC, sample n.62quartz; Cal = calcite; Aug = augite; Hc = hercynite; Hem = hematite; An = Anortite.
212 C. De Vito et al. / Applied Clay Science 88–89 (2014) 202–213
5. Conclusions
Black-Gloss Ware artifacts were produced for several centuries inthe Mediterranean world, i.e., 6th–4th centuries BC, and for this reasona debate among archeologists about their provenance still exists. Astudy of Motye samples using optical microscopy showed that only onefabric is present and the main features in terms of microstructure,groundmass and inclusions are very similar in all specimens of the fourproductions. In addition, SEM investigations suggest that the high degreeof sintering and the low degree of porosity of the black ceramics fromMotya are the result of high technological manufacturing; moreover,the multi-analytical approach permits defining the detailed mineralogi-cal assemblage of both internal body and external black gloss. In partic-ular, both the black gloss and the internal body of the pottery fragmentsshow similar elemental compositions, but the coating is more Fe-enriched and different in color. The sampleswere exposed to similar fir-ing temperatures and fO2 as suggested bymineral assemblage, i.e., esti-mated T in the range 950–1100 °C under oxidizing–reducing–oxidizingconditions. The most significant discriminating elements are the pres-ence of carbon in the external black gloss of some samples that permitsdistinguishing the PBGW, locally produced potteries, from ABGW andSGC and the presence of apatite in the internal body of PBGW andSGC. Considering the nature of the rawmaterial, we infer that the potterused a mixture composed of clays and carbonates for the internal body,whereas the black gloss should be derived from a fine suspension ofclays, extremely purified and enriched in iron oxides. The potentialsources of clays used for ceramic production are difficult to infer asthe majority of ceramic artifacts were imported, even if a local originfor PBGW and SGC productions can be inferred.
Acknowledgments
This research was financed by the Sapienza University, Rome, Italyand was carried out on the basis of an agreement with SoprintendenzaRegionale BB.CC.AA. di Trapani. Authors are indebted to P. Ballirano forgenerously sharing his expertise in XRD. Authors thank Arch. P.Misuraca and Dr. Rossella Giglio, for their support to the excavations.The authors thank two anonymous reviewers and the editor J.B. Percivalfor useful comments and suggestions regarding the manuscript. S.Stellino and M. Albano for the assistance in the XRD and SEM laborato-ries are acknowledged.
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