Raw Glass From Syria Used in European Works of Art Medieval Period
PLoS Ane. 2017; 12(7): e0182129.
Glass import and production in Hispania during the early on medieval period: The glass from Ciudad de Vascos (Toledo)
Jorge de Juan Ares
IRAMAT-CEB, UMR 5060, CNRS, Orléans, France
Nadine Schibille
IRAMAT-CEB, UMR 5060, CNRS, Orléans, France
John P. Hart, Editor
Received 2017 Apr 21; Accepted 2017 Jul 12.
- Supplementary Materials
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S1 Tabular array: LA-ICP-MS information of the glasses from Ciudad de Vascos. (PDF)
GUID: CC0F7E0C-0C35-4A4A-818E-B77E35C8B054
S2 Table: LA-ICP-MS data of glass standards in comparison with published values. (PDF)
GUID: 0B5F8CDC-53A5-4D3C-BBD0-0312E5CFE664
- Data Availability Statement
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All relevant data are within the paper and its Supporting Data files.
Abstract
1 hundred and xl-one drinking glass fragments from medieval Ciudad de Vascos (Toledo, Spain) were analysed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The glasses fall into three types according to the fluxing agents used: mineral natron, soda-rich plant ash, and a combination of soda ash and atomic number 82. The natron glasses tin can be assigned to various established primary production groups of eastern Mediterranean provenance. Different types of establish ash spectacles indicate differences in the silica source besides as the plant ash component, reflecting changing supply mechanisms. While the before plant ash groups can exist related to Islamic spectacles from the Near E, both in terms of typology and composition, the chemical signature of the later on samples appear to be specific to drinking glass from the Iberian Peninsula. This has of import implications for our agreement of the emerging glass industry in Spain and the distribution patterns of drinking glass groups and raw materials. The plant ash that was used for the Vascos spectacles is rich in soda with low levels of potash, similar to ash produced in the eastern Mediterranean. Information technology could therefore be possible that Levantine plant ash was imported and used in Islamic catamenia drinking glass workshops in Spain. Unlike central and northern Europe where an contained glass manufacture based on potassium-rich forest ash developed during the Carolingian menses, the prevalence of soda ash and soda ash lead glass on the Iberian Peninsula indicates its commercial and technological interconnection with the Islamic east. Our study thus traces several stages leading to the evolution of a specifically Spanish primary glassmaking industry.
Introduction
Recent research on the chemistry of ancient and medieval glass has established compositional groups that are linked to the materials' origin and date of production. For virtually of the outset millennium CE, the overwhelming majority of drinking glass from the Mediterranean and primal Europe was manufactured on the Levantine declension and in Egypt, using Egyptian natron as the main fluxing agent [1, 2], although the geochemical characteristics of some early Roman glass raise the possibility of a western Mediterranean provenance [3]. In the eighth century, mineral natron was gradually replaced by vegetable fluxes: soda-rich found ash in the Islamic world, and potassium-rich forest ash in primal and northern Europe. Around the same time, loftier lead glasses besides appear, both in Europe equally well as in the Islamic east [iv–vi]. Little is known well-nigh glass from the Iberian Peninsula, mainly due to the pocket-sized number of analyses carried out to date. Natron-type spectacles of known compositions have been identified, simply their regional and chronological distributions remain unclear. Similar to the eastern Mediterranean, vegetable soda glass had replaced natron glasses by the tenth century, but at what point exactly this transformation took place and where these soda constitute ash spectacles came from is uncertain [7]. Systematic large-scale studies of well-dated assemblages are needed to better sympathize the distribution patterns of glass in Iberia.
The present commodity reports the chemic information of 141 glass samples from Ciudad de Vascos (Navalmoralejo, Toledo) next to the river Tajo in Fundamental Kingdom of spain [8]. In the tenth and eleventh century CE, Ciudad de Vascos was a small fortified urban settlement with mosques, baths and cemeteries on the border of Islamic al-Andalus and the Christian kingdoms to the north. The archaeological excavations additionally yielded Roman and early medieval remains, such as ceramics, structure materials and epigraphic stones [nine], as well as a substantial number of late Roman coins dated to the reigns of the Roman emperors from Constantine to Honorius (303–423 CE). Hardly any material tin be unambiguously attributed to the Visigothic or the earlier Islamic flow. Later on the Christian conquest of Toledo in 1085 CE, a modest Christian settlement, express to the kashbah, existed until the starting time decades of the twelfth century, after which the site was abandoned [10, xi]. The stratigraphic sequence is more or less preserved from the centre of the ninth and tenth centuries and most of the vestiges of occupation date, in fact, to the eleventh century. The diverse archaeological materials (metals, ceramics, lithic, glass, etc.) have been widely published, making the finds from Ciudad de Vascos the best-known repertoire of 10th- and eleventh-century material from central Iberia.
The archaeological finds from Ciudad de Vascos let us to trace some of the chronological developments and commercial networks of medieval Spain, about which our knowledge is even so limited. The numismatic finds from the early Islamic menses of the Umayyad Emirate of Cordoba, for instance, are very deficient. No more than a couple of coins recovered from Vascos date to the eighth and ninth centuries. Caliphal tenth-century coins from the mints of Cordoba are likewise very express. The vast majority of coinage date to the 2d half of the eleventh century, minted mostly at Toledo in the name of al-Maʻmun (1043–1075 CE), al-Qadir biʻllah (1074–1090) and, since 478-479H (1085–1086), in the name of Alfonso VI. The coins from other locations are less represented, but some have been identified from Cuenca (1081) and Valencia (1061–1064), both by al-Maʻmun, from Badajoz by al-Mutawakkil (1072–1094), Seville by al-Muʻtamid (1068–1091), and from León by Alfonso Half-dozen (1072–1109), probably a Fatimid fragment, too as some coins of uncertain provenance from Alpuente and Ceuta dating to the late eleventh century [11–13].
The scientific analysis of the glass from Ciudad de Vascos can provide further data on the site's cultural and economical connectivity during the tenth and eleventh centuries CE. Here we present new analytical results of a substantial set up of samples, thus producing quantifiable data about the transmission of goods, both on a local as well as long distance scale between medieval Spain and the wider Mediterranean world. The belittling results reveal distinctive patterns of production and consumption, documenting the technological and historical developments in central Spain. At that place is vitreous evidence for the late Roman occupation of the site in the grade of common belatedly antique glass groups originating in the eastern Mediterranean. A clear interruption during the Visigothic and early on Islamic period is evident from the absenteeism of early Islamic glass groups, while a certain degree of prosperity during the tenth and eleventh centuries is reflected in the presence of several singled-out plant-ash spectacles. As will be discussed, the earlier plant ash spectacles show similarities with examples from the eastern Mediterranean and may be imports, whereas the afterward ones were possibly made somewhere on the Iberian Peninsula.
Materials and methods
Archaeological context and samples
Over the entire forty-year period that excavations have been carried out at Ciudad de Vascos, just about 165 different glass objects have been recovered, 141 of which were sampled and analysed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for the present study. The drinking glass fragments are stored at the Santa Cruz Museum in Toledo (Spain) and were sampled and analysed with the official authorization from the Consejería de Educación, Cultura y Deportes de la Junta de Comunidades de Castilla-La Mancha, Spain. The material comes from dissimilar stratigraphic contexts and from different find-spots throughout the archaeological site (Fig 1). Unfortunately, the intense occupation of Ciudad de Vascos during the Islamic menses altered the previous stratigraphy and prevents well-defined chronological attributions of the pre-Islamic and earlier Islamic material except for a few fragments. The stratigraphy of the tenth and eleventh century, in dissimilarity, allows for the distinction of chronological trends (S1 Table). Typological considerations can furthermore assistance in establishing a relative chronology of the finds. For case, practically all glasses of the natron family unit can exist attributed to pre-Islamic types such as goblets and glasses of greater thickness. Some of the vessel types circulating during the Islamic flow include beakers, bottles and unguentaria besides as several pieces of decorative canvas glass (S1 Table). Near of the drinking glass is gratis blow with a limited presence of mould-blown samples. Decorations are restricted to very few fragments like unguentaria with practical trail decorations, a few examples of scratch-engraved ornaments and mould decorations that have parallels in drinking glass objects from the Near Due east. The assemblage exhibits a relatively wide chromatic variety with a preponderance of colourless and weak colours (greenish, bluish, yellowish) and a few strong colours such as blue, light-green and purple [fourteen].
Spatial distribution of chemical glass groups in Ciudad de Vascos.
Analytical method
Minor samples of iii mm size were mounted in epoxy resin and polished to remove any surface contaminations and corrosion layers. The mounted and polished sections were then analysed by LA-ICP-MS at IRAMAT-CEB, following the aforementioned protocol as described in [15–17]. The mass spectrometer used is a Thermofisher Element XR and the laser ablation arrangement is a Resonetic UV light amplification by stimulated emission of radiation microprobe (193 nm Excimer laser). The operating conditions were set at five mJ with a frequency of 10 Hz and a spot size diameter of 100 μm that was reduced where necessary to avoid saturation. Pre-ablation fourth dimension of 20 seconds was followed by 40 seconds analytical time, measuring l-viii elements, including all major constituents of archaeological glass, and various small-scale and trace elements (S1 Table). Quantitative results were calculated based on the principle of an internal standard and a fix of international glass standards (Nist SRM610, Corning B, C and D) as well as an archaeological glass of known composition (ALP1) for the decision of chlorine [16]. Glass standards NIST SRM612 and Corning A were analysed at regular intervals to institute the accuracy and precision of the analyses (S2 Table). The belittling precision is meliorate than v% relative for most elements with very few exceptions amidst some of the trace and rare earth elements. The accurateness of nearly of the major and modest elements is within v% relative, and better than x% for most of the trace elements (S2 Table). The calcium values deviate more than substantially in the Corning A standard (> 10%), but show a much better consistency with the published values for NIST SRM612 (< 4%). The detection limits vary between 0.ane and 0.01% for major elements and betwixt xx and 500 ppb for trace elements, depending on the analytical parameters [16].
Results
All 141 spectacles analysed from Ciudad de Vascos are soda lime silica spectacles that fall into three main compositional groups, co-ordinate to the use of dissimilar fluxing agents (S1 Table, Fig ii). By far the largest group (93) consists of soda-rich plant ash glasses, typical of Islamic glass from the eighth/ninth century onwards [eighteen–20]. This type of glass seems to be present on the Iberian Peninsula since at least the ninth century [7, 21]. The second largest group represented at Vascos (37) are natron-type glasses of the Roman tradition [22]. Finally, there is a small set of six fragments with high lead contents (40–48%), similar to glasses previously described in medieval Islamic contexts [4–7, 23]. V samples are classified as outliers, because they show unusual compositional features and cannot be attributed to any of the groups with certainty. Two samples (VS021 & VS027) have exceptionally high tungsten levels that cannot be easily explained. VS027 has together with samples VS012 and VS092 the highest potash concentrations. Sample VS091 seems to be a relatively typical natron glass except for its elevated magnesium concentration (S1 Table). The five outliers are not considered further.
Soda concentrations versus the sum of potash and magnesia of all glasses from Ciudad de Vascos.
The three main glass types are easily distinguished based on the sum of the alkali and alkaline world components MgO and K2O.
Natron glasses
The samples of this family are characterised by their low potassium and magnesium oxide levels (<one.5%) and high soda concentrations [24]. The comparison of the samples from Ciudad de Vascos with known natron glass groups reveals a great multifariousness with the presence of at least five different types: HIMT / Foy-one, Foy-two, Levantine, Roman Sb and Egypt Ii.
HIMT / Foy-1
Almost half of the natron glass is made upwards of so-called HIMT [25] or 'série 1' [1] (S1 Tabular array). Either green or amber-yellow, it is characterised by high iron, manganese and titanium contents as well as elevated trace and rare earth elements (REE), while lime levels are relatively depression (CaO < vii%) (Fig 3). The production location of these glasses is unknown, merely the relative proportions of major and trace elements besides as the isotopic signatures for neodymium, lead, oxygen and strontium advise an Egyptian provenance [i, 26, 27]. HIMT / Foy-1 was widespread during the fourth and 5th centuries (Republic of albania, Republic of bulgaria, Cyprus, Egypt, France, Italy, Tunisia, and the Great britain), but information technology seems to have been conspicuously absent from the Syrio-Palestinian expanse [i, 28, 29] (and references therein). On the Iberian Peninsula, it has been identified in Portugal, Extremadura, Madrid, Catalonia, Cuenca, Valencia, Murcia, Leon and in Seville [seven].
Compositional differences related to the silica sources of the natron glasses from Vascos.
(a) Titanium versus aluminium contents highlight differences in heavy chemical element contents of the dissimilar drinking glass groups; (b) unlike strontium to lime ratios imply unlike sources of calcium for Levantine I, HIMT and Arab republic of egypt 2; (c) means of selected trace element and REE data of the unlike natron groups normalised to the continental chaff (MUQ [30]).
Levantine-type glass
13 samples take compositional characteristics of drinking glass produced from a Levantine silica source [18, 19, 31–34], with relatively loftier lime and alumina and low heavy elements such as titanium, iron and zirconium (Fig 3). Compared to the HIMT / Foy-1 grouping, the Levantine spectacles are more depleted in REE (Fig 3C). The Levantine-type glass from Vascos perhaps includes both the so-chosen Levantine I grouping, related to the glass produced in the 6th- to seventh-century workshops of Apollonia [eighteen, 19, 32], and earlier glasses from Jalame [31] equally well every bit Roman manganese decolourised glasses [1, 5, 33, 34]. Given a sure degree of recycling manifest in the contamination levels of these spectacles (S1 Table) and the poor chronological resolution of the samples in this group, it is not possible to separate them strictly into Roman and Levantine I. In Hispania, Levantine glass has been institute in Tarragona [35] every bit well as Guadalajara (Recópolis) (in preparation).
Miscellaneous
The remaining seven fragments encompass Roman antimony, Foy-2 and Egypt II types. Two samples (VS057, VS097) appear to be related to the Roman antimony group even though 1 of the two (VS097) has no antimony (S1 Tabular array) [five, 33, 34, 36, 37]. They both have low concentrations of lime, alumina and heavy elements likewise as the everyman REE levels (Fig 3). This glass type has been linked with an Egyptian origin because of its loftier soda levels and TiO2 to Al2O3 ratios as well equally its prevalence in Egypt [1, 34, 37, 38]. It has been proposed that Roman Sb glass was referred to as the glass from Alexandria in the Toll Edict of Diocletian [34, 39], but no master production eye has up to now been archaeologically ascertained. It is found throughout the Roman Empire and generally dates to the showtime to fourth century CE [33, 34]. In Iberia, information technology has been identified among glass assemblages from key and northern Portugal, in Leon and in Asturias [7].
Iii samples correspond to the so-chosen Foy-2 production grouping (série ii.1), also known as HLIMT, weak HIMT or HIMT2 [1, 17] (and references therein). It has somewhat higher aluminium, titanium, atomic number 26, sodium and magnesium oxide concentrations than Levantine glass, but lower values than HIMT across all these elements (Fig iii). This compositional grouping was supplied to secondary workshops throughout the Mediterranean and raw glass chunks were found in France (Montpellier, Marseille, Vendres and Bordeaux) [one]; Republic of bulgaria (Serdica) [40] and Tunisia (Carthage) [34]. Iberian workshops used raw glass of the Foy-2 composition in Lugo, Vigo [41] and Recópolis (in preparation). Foy-ii drinking glass is commonly dated to the sixth to seventh century CE [17, 34].
Interestingly, two samples (VS018, VS031) stand for to Arab republic of egypt 2 [19, 42]. This is the start fourth dimension that Arab republic of egypt II has been identified in Spain. One of the samples is dated on grounds of the stratigraphic sequence earlier the second half of the tenth century CE. It is a bottle fragment with a funnel-mouthed rim recovered from a small neighbourhood mosque [43]. Bottles of this type are well known in the Near East from the Abbasid-Fatimid periods, for case, from Ramla, Caesarea, Tiberias and Beth Shean, and they can probably be attributed to an Early on Islamic date [44–46]. No main workshop of Egypt 2 glass is known, but the high proportions of iron, titanium, zirconium and niobium advise an Egyptian origin (Fig 3). Commensurate with the characteristics of Arab republic of egypt 2 glasses, the ii samples from Vascos have high amounts of calcium, low aluminium and low strontium feature of calcareous rocks instead of sand enriched with shells, thus indicating the use of a continental silica source [47]. The precise chronology of this glass has been established through the analysis of Abbasid glass weights from Fustat to 750 to 868 CE [42]. Spectacles of this composition have since been found in Raya in South Sinai [48], in the seventh- to eighth-century secondary workshops of Khirbet al-Ḥadra (Israel) [49], in the ninth-century contexts of El Ashmunein (Egypt) [50] and San Vincenzo al Volturno (Italian republic) [51].
Soda plant ash drinking glass
With 93 soda plant ash glasses the study of the assemblage from Ciudad de Vascos represents the largest gear up of plant ash glasses ever published from the Iberian Peninsula. Almost fragments are dated to the tenth and eleventh century CE, with some dates confined to within half a century (S1 Table). The glass is by and large colourless or weakly coloured, with some exceptional night bluish, purple or green samples. Their high levels of soda and low potash (S1 Table, Fig ii) are congruent with ash obtained from soda-rich halophytic plants from saline semi-arid or coastal environments [52, 53]. The lime concentrations are highly variable (S2 Tabular array), due to a combination of factors. The calcium content of plant ash varies substantially [54, 55] and can exist modified through purification processes [56]. Additionally, calcium can also partially derive from the silica source. As is quite mutual in Islamic glasses, the plant ash glasses from Vascos take elevated manganese contents (MnO > 0.five%), indicating its deliberate add-on as decolouring agent [57].
The silica groups
The compositional variability of plant ash makes the estimation of this type of glass more complex than that of natron spectacles. Nevertheless, when considering elements that are (almost) exclusively introduced equally office of the silica source such every bit titanium, aluminium, zirconium and thorium, five different groups can be differentiated among the Vascos plant ash samples (Fig 4A and 4B). These groupings are confirmed by distinctive trace element and REE patterns (Fig 4C). Intriguingly, the archaeological assay shows furthermore the existence of chronological, typological and functional differences betwixt the compositional groups.
Elements associated predominantly with the silica source of the found ash spectacles from Vascos.
(a) Titanium and aluminium concentrations identify different silica sources; (b) different correlations of zirconium and thorium carve up groups 1, 2 and iii more clearly; (c) ways of trace and REE data of the five dissimilar plant ash silica groups normalised to the continental crust (MUQ [30]) confirm variations in the silica source.
Group 1 is the smallest grouping with only seven fragments, mostly colourless and belonging to flat or sheet drinking glass, some of which are cut into regular pieces, possibly indicating a decorative part [14]. All these fragments derive from domestic contexts southward of the citadel, perchance the richest area in the city (Fig 1). The composition of these samples is characterised past relatively depression levels of alumina, titanium and niobium combined with elevated heavy elements zirconium and hafnium (Fig four). The dissimilar ratios of zirconium to niobium and titanium point to the use of a silica source different from the other constitute ash groups. All spectacles of grouping 1 have pocket-sized amounts of contaminants such equally lead and copper (< 0.5%), suggesting that these spectacles have undergone some caste of recycling.
Groups 2 and iii seem closely related, except for varying zirconium to thorium ratios as well as vanadium and yttrium levels (Fig 4). They both have aluminium and titanium values like to grouping ane, while grouping 2 has the lowest thorium, tantalum and uranium. Establish ash group 3, in dissimilarity, has the lowest overall trace elements and REEs (Fig 4C). The spectacles of group ii and 3 were found mostly in areas that mostly comprise the oldest deposits from the early on Islamic occupation, namely in the citadel and surrounding houses and baths. The samples are predominantly colourless or have a slightly green and blue tinge, and some samples accept been intentionally coloured through the addition of copper or lead (S1 Table). Group 2 entails pocket-size tubular flasks, bottles and almost all mould-blown and engraved vessels. Group 3 shows a wide typological multifariousness, it was used in jewellery, bottles, unguentaria, dishes and goblets, ane of which is decorated with circular boss at the eye (VS093) (Fig ii.19 [14]) characteristic of some Near Eastern spectacles (eastward.g. No. 123 [58]).
The samples of group 4 make upwardly about a third (31 samples) of all the soda plant ash glasses with a very distinctive chemical make-upwardly (Fig 4). The sand source utilised for these glasses was rich in accompaniment minerals such equally feldspar, magnetite, monazite, sphene and zircon every bit judged past the elevated levels of aluminium, iron, niobium, titanium, yttrium and zirconium, too every bit higher REE contents relative to the other constitute ash groups. In fact, all samples of group four have alumina contents in excess of 3.5%, which is unusual for contemporary plant ash spectacles from the eastern Mediterranean and European sites with the exception of Italian republic, Spain and Portugal [59–64]. To this group belong fragments of tubular unguentaria, some with applied decoration, too equally some small bottles, fragments of lamps and a single piece of decorative flat glass. Based on the archaeological context the samples of group four date to the eleventh century CE, although for some of the fragments an before, tenth-century date, cannot be ruled out.
An intermediate limerick characterises the spectacles of plant ash grouping 5, with aluminium, titanium, thorium, zirconium, trace and rare earth chemical element contents all situated between groups iii and four (Fig four). Information technology is a relatively heterogeneous grouping that might result from the recycling and mixing of other plant ash spectacles. That the mixing of groups three and four could event in the group v composition, can exist mathematically simulated (Fig 5). First, the theoretical mixing ratio was estimated past calculating the relative deviations of groups 3 and 4 from the group v composition for each element and the median was determined (median ratio of group three to group 4 is ii:1). Then, a theoretical mixed composition was calculated and compared to the analytical information of group 5 (normalised data). The hypothetical drinking glass composition closely matches that of grouping 5 as regards the base of operations glass components (orange trace in Fig 5). It differs in terms of the colouring agents such every bit copper, tin, antimony and atomic number 82 and associated minerals, probably because of recycling. Four samples have lead contents of up to 8% and all only one are deep bluish, turquoise or dark-green (S1 Table). This atypical limerick may be indicative of colouring techniques similar to those described in Islamic sources [4] that specify the add-on of moderate amounts of atomic number 82 (from four% to 8%) and copper (> 1.5%). The remaining samples of group 5 are mostly colourless or accept a blueish, dark-green or purple tinge. This type of glass was used for small bottles and unguentaria, a few bowls and jewellery. Numerous samples of group 5 were retrieved from archaeologically well-stratified contexts dating to the last quarter of the eleventh or the first decades of the twelfth century CE (S1 Table), thus generally postdating the glasses of groups 3 and iv.
Computation of a mixed glass limerick corresponding to group 5.
Averages of the group three (due north = 12) and iv (n = 31) compositions were normalised to the hateful of group 5 (northward = 28). A theoretical limerick (groups iii and 4 at a mixing ratio of 2:1) was calculated and normalised to group 5. Greyness areas stand for the standard departure of the group five composition.
Fluxing agents
Judging from the high soda and low potassium concentrations, the alkali source in the Islamic spectacles from Vascos was ash from halophytic plants. Institute ash is a complex composite fabric that contains numerous minor and trace elements that are determined by various factors. Its limerick depends on the constitute species, the location and soil where it grows, the parts of the plant that are used and the flavor when the plants are harvested every bit well as the manner the ash is prepared [54–56]. Elements commonly associated with the constitute ash component (Na, Mg, P, K, B) and to a lesser degree with the silica source can potentially distinguish unlike types of found ash and may reveal differences in the preparation techniques [60, 63–65].
It turns out that in that location is substantial overlap of the silica groups in terms of the plant ash components, except for a cluster consisting of groups 1 and ii. This cluster can exist clearly discriminated because of its lower sodium, magnesium and boron contents coupled with the highest lime concentrations (Fig vi). Even though some of the differences tin can be accounted for past the differences in the quantity of ash used, the ratios of boron to sodium vary sufficiently to deduce that different plant ashes were used in the production of these glasses. It furthermore needs to be borne in mind that a meaning fraction of the calcium derives from the silica source, significant calcium is not a suitable discriminant for the plant ash component alone, unless quartz pebbles were used instead of sand. The glasses of the separate cluster include near all fragments considered decorative or architectonic pieces and most of the mould-blown and scratch-engraved glasses. All other institute ash spectacles bear witness a broad compositional variability in their institute ash elements with college sodium, magnesium and boron and overall lower calcium levels.
Elements related to the plant ash component in the five different plant ash silica groups.
(a) Bivariate plot of CaO against the sum of NatwoO, MgO and Thousand2O; (b) B versus NaiiO differentiates between types of plant ashes.
Soda-ash lead glass
Six glass fragments from the Vascos assemblage proved to be of soda-ash lead glass with pb concentrations between xl% and 50% and approximately v% to 7% of soda (S1 Table, Fig 2). These lead glasses take uncommonly high chlorine values (ane.7%–2.5%), negatively correlated with calcium. It has been shown that the chlorine content decreases with increasing temperature and calcium oxide content [66]. This could explain the high chlorine level, because the lime concentrations are very depression and the melting temperature of glass with loftier lead content (PbO > twoscore%) is drastically reduced. Other distinguishing features of these lead glasses are the pronounced levels of arsenic, silver and bismuth that are unlikely associated with the silica or alkali sources (S1 Table), but rather with the lead-begetting component such equally galena, litharge or slag [67–68]. Copper, tin and antimony are likewise elevated and it is not clear at this point whether they are remnants of colouring agents or part of the atomic number 82 source or both. The colours range from colourless and light yellow, green and bluish to dark green. All the soda-ash lead spectacles date to the tenth and eleventh century and include unguentaria, a canteen, probably the base of a glass lamp and an unusual fragment of an alembic pipe. Islamic lead glasses were first defined by Sayre and Smith [5] every bit soda silica glasses with contents of lead between 33% and 40%. Many other examples with varying amounts of lead have since been identified in Fustat (Arab republic of egypt), Timna (Saudi Arabia), the Serçi Limani shipwreck [23], Nishapur [69], Libya [seventy], Morocco [71] and in numerous sites on the Iberian Peninsula dated to before the thirteenth century CE [21, 59, 72, 73].
Discussion
The Vascos glass in its historical context
Ciudad de Vascos was a small settlement during the belatedly Roman flow, located virtually the Tajo river meadow and a secondary Roman road [xi, 74], which may have facilitated the sporadic arrival of glass objects. Our analytical data ostend that the fragments with typological characteristics of Roman objects were all natron glasses, testifying to their survival equally residual cloth in the later Islamic contexts. The natron spectacles reflect the import of well-established compositional groups of Levantine and Egyptian origin. In accordance with the numismatic evidence dating to the reigns of the emperors from Constantine to Honorius, the largest group of natron glasses correspond to the HIMT type that is attributed to the fourth and fifth centuries. Consistent with the archaeological show is the almost complete lack of early Islamic natron glasses. Only 2 Islamic Egypt II specimens are attested. This type of drinking glass appears from the commencement of the eighth century and then became, for some fourth dimension, a dominant drinking glass group in the Near Eastward [19]. The two Egypt II samples thus reflect the latest imports of natron drinking glass into the Iberian Peninsula in the eighth or ninth century CE. Natron glass was subsequently replaced past plant ash drinking glass in Iberia, analogous to the developments in the eastern Mediterranean [xix, 22, 42, 75].
Unlike the medieval glass making tradition in key and northern Europe, no potash glass was constitute in Vascos. Instead, the samples of the tenth and eleventh century correspond to Islamic plant ash glass, testifying to the continuous commercial and technological human relationship of Islamic Espana with the eastern Mediterranean. Overall, glass was a rather scarce commodity in Ciudad de Vascos, which is reflected in the quality and quantity of the vitreous materials. Even though the occupation during the late antique flow was of much lower density, natron drinking glass makes up a quarter of the entire glass assemblage. This phenomenon might exist related to a full general reject in the volume of drinking glass that was circulated within the domestic market of Hispania during the early on Islamic menstruum, when drinking glass all simply disappears from the archaeological tape, specially in more than rural areas. A break in the supply and working of glass in Iberia during the early Islamic menstruation is further suggested by an apparent lack of mixed alkali spectacles. Whereas both the natron and the plant ash glasses evidence some signs of recycling in the grade of elevated elements usually associated with colouring agents (Co, Cu, Pb, Sb, Sn), no mixing between the 2 appears to accept occurred. The absenteeism of a mixed alkali glass at Vascos is probably due to a decline in the population and near abandonment of the site during the early medieval / early Islamic period and its revival but from the ninth or early tenth century onwards. The renewed occupation was accompanied by the arrival of new glasses made from vegetable soda. The glass was no doubt imported to Vascos as at that place is no testify of glass working on site [14]. This has of import implications for our understanding of the supply networks and their changes during the Middle Ages.
The constitute ash drinking glass evolution
Our data indicate that the plant ash glasses from Ciudad de Vascos derived from different sources: groups 1, two and iii brandish lower feldspars and heavy mineral concentrations with distinct zirconium (group 1) and thorium (group two) signatures, while group 4 was made from a relatively impure silica source with loftier pocket-size and trace element contaminations. Equally suggested by our simulation, group 5 may take been the upshot of mixing between groups 3 and 4 and does therefore not constitute a split up silica source every bit such. When compared to published data of plant ash drinking glass assemblages from other Mediterranean sites, 2 major developments go evident: groups 1 and 2 have shut similarities with glasses from the eastern Mediterranean both in terms of their typology and composition, whereas no compositional friction match tin exist established for grouping 4. Silica groups 1 and ii correspond closely to glasses from the early eleventh-century Serçe Limani shipwreck, twelfth-century Caesarea [23], and eleventh-century Tyre [76] every bit regards the aluminium and titanium levels (Fig 7A). Previously, a correlation has been observed betwixt the geographical origin of glass and the ratios of yttrium to zirconium versus cerium to zirconium [77, 78]. According to these studies, glasses from Egypt typically show depression values for both ratios (Y/Zr < 0.08 and Ce/Zr < 0.18), while Syrio-Palestinian glasses accept notably college ratios (Y/Zr > 0.10 and Ce/Zr > 0.24). Even though this comparison works best for natron type glass and caution needs to exist exercised when applying it to plant ash spectacles, information technology is noteworthy that group ii from Vascos meets the criteria for a Levantine provenance as opposed to group ane that seems more congruent with an Egyptian origin, due to its elevated zirconium (Fig 7B). The data of grouping 4 (and to a bottom extent group 3), on the other hand, deviate from the Y/Zr versus Ce/Zr ratios typically encountered in eastern Mediterranean spectacles [77]. These compositional characteristics strongly suggest that the glasses of institute ash groups 1 and 2 may take been imported from the eastern Mediterranean, whereas group 4 may originate from somewhere else.
Possible affiliations and provenance of the Vascos plant-ash drinking glass.
(a) Aluminium and titanium concentrations in comparison with comparative material from the eastern Mediterranean in light grayness from Tyre, Caesarea, and the Serçe Limani shipwreck [23, 76]; (b) distinction of the found ash groups as a function of yttrium to zirconium and cerium to zirconium ratios.
In this context, it is important to point out that the fragments of groups i and ii were recovered from areas with the earliest Islamic remains. Group 1 was confined to the affluent dwellings in the environment of the kashbah, and the samples of group two were found either in the citadel or in the surrounding houses and baths (Fig 1). Even though only one fragment can be securely dated to the mid-ninth to early tenth century (VS105, S1 Table), information technology seems safe to assume that silica groups 1 and 2 represent the earlier of the plant ash glass compositions from Ciudad de Vascos. Like compositions were identified in Spain amid the eighth- to tenth-century spectacles from Gauzon in Asturias (in grooming), 10th-century samples from Cordoba [21] too as 12th-century Murcia [59] and Almeria (in training). Hence, the belittling and archaeological prove, together with comparative material advise that the primeval constitute ash groups were imported to Vascos from the eastern Mediterranean, just like Islamic natron glasses (Egypt Ii) had been imported some years before. This indicates that the long-established Mediterranean trade routes were still largely intact as belatedly as the ninth, possibly fifty-fifty the tenth century CE.
Regional glass production
No comparative material for grouping 4 could be identified among published information of contemporary plant ash glasses from the eastern Mediterranean. However, some compositional affinities were found with material from the western Mediterranean. A few isolated examples of a similar composition in terms of high aluminium and titanium contents exist among glasses from Qsar es-Seghir [23], ninth- to tenth-century glass chaplet from al-Basra [71], as well as Iberian assemblages from Murcia [59] and Silves (in preparation) [79], both dating to the eleventh to thirteenth century, and Beja [60] from the fourteenth and fifteenth centuries (Fig 8A). There is, furthermore, a grouping of like glasses from Tuscany and Liguria dating to the thirteenth to sixteenth century [64, lxxx]. Taking into consideration other trace elements such every bit zirconium, there is a shut correspondence between grouping four and a few samples from Silves and Tuscany, although most of the Tuscan glasses have a different titanium to zirconium ratio (Fig 8B), implying the use of an unrelated silica source.
Possible affiliations and provenance of grouping 4 from Vascos.
(a) Aluminium and titanium concentrations in comparison with comparative assemblages from Qasar es-Seghir [23], al-Basra [71], Murcia [59] and Beja [lx] in light grayness besides equally samples from Tuscany [64] and Silves (in preparation); (b) zirconium versus titanium contents highlight differences betwixt Iberian glasses from Vascos and Silves (Portugal) and glasses from various Tuscan sites [64].
Unfortunately, no trace chemical element data are available for the other assemblages and just preliminary conclusions can thus be drawn. Nonetheless, there is evidently a higher incidence of this blazon of soda establish ash drinking glass in Iberia during the eleventh and twelfth centuries than in any other Mediterranean region studied. It may therefore be hypothesised that the glasses of group 4 represent the output of an emerging Iberian glass industry, characterised by loftier aluminium and high heavy element concentrations equally reported for some Spanish sand deposits [81]. When because the chronology of these developments, about half the samples of grouping 4 from Vascos tin can exist attributed with some certainty to the second half of the eleventh or the showtime decades of the twelfth century, while the vitreous material from Silves (Portugal) and Murcia dates to the 12th and offset half of the thirteenth century [59, 79]. These results suggest that master plant ash drinking glass production on the Iberian Peninsula started sometime between the 10th and eleventh century CE.
Historical sources about the industry or working of drinking glass in medieval Iberia are scarce and notoriously unreliable for the early Middle Ages, simply they become more abundant in the second half of the eleventh century [7, 82]. The harvesting of soda-rich halophytic plants in medieval Spain is documented since at least the end of the eleventh century CE [83]. A study of medieval and mail service-medieval glass from northern Italian republic plant that western Barilla, possibly from Spain, was used alongside Levantine ash in the glass workshops of Altare in the Liguria region already from the thirteenth century onwards [fourscore]. However, Spanish ash (barilla) tends to be significantly richer in potassium than the ash that was used for the Vascos spectacles [63]. At the same time, the extensive trade in Syrian plant ash to the western shores of the Mediterranean during the Middle Ages is well documented [84, 85]. For example, Levantine ash was employed in thirteenth- to fourteenth-century Tuscany [63, 64]. The import of plant ash from the eastern Mediterranean to Spain is therefore a possibility, although this must remain tentative every bit non sufficient evidence is available at this indicate.
A regional glass product is also indicated by the relative frequency of Islamic soda-ash lead glasses inside the Iberian Peninsula. Glasses with compositional characteristics shut to the Vascos soda-ash lead glasses were found in Córdoba [21], Murcia [59] as well as Toledo [72], the Baleares [73], Pechina (Almería), Albalat (Cáceres) and Gauzon (in preparation). It has been proposed that these high lead glasses from al-Andalus were made by calculation a pure form of pb to either a pre-formed glass or its raw ingredients [21, 59]. A combination of quartz, wood ash and lead oxide was causeless also for wood-ash lead glasses from key European sites [4, 86, 87]. Even so, given the contamination levels and relative abundance of modest elements such as arsenic, silvery and bismuth in the soda-ash atomic number 82 glasses from Vascos, it appears unlikely that a pure form of pb oxide was used in their production. Instead, the source of atomic number 82 might have been a slag, the waste product from the lead mining industry flourishing on the Iberian Peninsula since Roman times and well documented in historical sources [88–ninety].
How the establishment of a Spanish glass manufacture relates to historical and political developments is not even so articulate. The collapse of the Caliphate of Cordoba at the first of the eleventh century and the ascent of the Taifa Kingdoms most certainly involved a reorganization of the commercial networks. The chemic assay of glass assemblages can conspicuously offering valuable new insights into the inter-regional connections and geographical distributions of a high-end product, just there is a demand for further investigations of well-defined Iberian drinking glass assemblages.
Conclusion
The assay of the glass assemblage from Ciudad de Vascos addresses a lacuna in compositional data of Iberian glasses from the late antique and early on Islamic period. The glass compositions reflect the historical development of the site and its integration into a wider network of exchange that was subject to political change. We accept shown that the pre-Islamic glass encompasses well-established natron-blazon glass groups that originated in the eastern Mediterranean. The eighth or ninth century saw the import of the last natron glasses in the form of Egypt II, for the starting time time identified in the archaeological record of the Iberian Peninsula. Subsequently, soda plant ash drinking glass is the main variant used and it is eventually also produced in Spain. The chronological and the chemic data of Ciudad de Vascos provide the clearest indication yet of an emerging primary found ash glass production on the Iberian Peninsula. This might appointment to the showtime half of the eleventh century, although earlier dates cannot be ruled out at this point. Interestingly, soda-ash lead glass appeared at around the same time as wood ash lead glass in central Europe, possibly in response to the declining availability of natron (or soda ash) from the Levant. The exploitation of waste product products from metallurgical activities could have made this technological choice economically viable. Further research is needed to analyze the origin and regional distribution of these technological innovations.
Supporting information
S1 Table
LA-ICP-MS data of the glasses from Ciudad de Vascos.
(PDF)
S2 Tabular array
LA-ICP-MS data of glass standards in comparing with published values.
(PDF)
Acknowledgments
Our thank you go to Professor Ricardo Izquierdo, Miguel Angel Bru, Yasmina Cáceres, Jaime Gallardo and Jacobo Fernandez for their efficient collaboration. We are grateful to the Consejería de Cultura de Castilla-La Mancha and the Santa Cruz Museum in Toledo for allowing us the sampling of the archaeological materials.
Funding Statement
This project has received funding from the European Inquiry Council (ERC) nether the Eu's Horizon 2020 research and innovation programme (grant agreement No. 647315 to NS). The funding organization had no influence in the report design, data drove and analysis, conclusion to publish, or preparation of the manuscript.
Information Availability
All relevant data are within the paper and its Supporting Data files.
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