Sunday 3 October 2021

Tracing the origins of glass tesserae from the Roman city of Gerasa in northwest Jordan.

The city of Gerasa in northwestern Jordan was an important economic and cultural centre in the Eastern Roman Empire and later the Byzantine Empire, and one of the cities that made up the Decapolis (self-governing cities with populations largely made up of Greeks and Romans in an area otherwise dominated by local cultures). The city covered about 800 000 m² in Early Roman times, and was home to a number of important public buildings and temples, including the Artemision, a temple to the goddess Artemis, and one of the largest religious buildings in the Roman world. From the fifth century these were largely succeeded by Christian churches, with a new phase of building showing the importance of the city in the Byzantine period.

In a paper published in the journal Archaeometry on 7 January 2021, Cristina Boschetti of the Centre National de la Recherche Scientifique and the Université d’Orléans, Achim Lichtenberger of the Institut für Klassische Archäologie und Christliche Archäologie and Archäologisches Museum at the Westfälische Wilhelms-Universität Münster, Rubina Raja at the Danish National Research Foundation’s Centre of Excellence for Urban Network Evolutions and the Department of Classical Studies at Aarhus University, Will Wooton of the Classics Department at King’s College London, and Nadine Schibille, also of the  Centre National de la Recherche Scientifique and the Université d’Orléans, report on the work of the Danish-German Jerash Northwest Quarter Project, which has been investigating a section of the Northwest Quarter of the city since 2011, with an emphasis on understanding long-term change and development of this quarter. 

One aspect of this investigation has been the study of the numerous mosaics found in the area, and the materials used to make them. These mosaics were largely made up of glass tesserae (tiles), of which many have survived even if the mosaics have been lost (much of the city was destroyed by an Earthquake in the eighth century AD. The chemical composition of these tiles can be used to identify their age and origin (by comparison to other tiles of known origin) helping to understand the trade in glass in northern Jordan in Roman and Byzantine times.

Scaled plan of Gerasa, with the North Western Quarter outlined in green at top left. Boschetti et al. (2021).

Previous studies of glass tesserae from northern Jordan have found that the overwhelming majority of tiles dating from the fourth to eighth centuries AD are of Levantine origin, with very few examples of Egyptian origin - the exception to this being a gold-glass tesserae from the Petra Church. Other glass artefacts in the region are also predominantly of Levantine origin, although some Egyptian artefacts are known from the Deir ‘Ain ‘Abata sanctury and the Petra Great Temple. The closeness of Gerasa to the glass furnaces of the Levant makes it likely that Levantine tesserae glass would have been easily available in the city. However, examination of fifth to sixth century glass objects from Gerasa, and fifth to eighth century artefacts from Umm el-Jimal (about 50 km to the east of Gerasa), have found them to be made almost exclusively of recycled glass. Recycled glass also dominates in third to fourth century artefacts from Petra and Khirbet et-Tannur, and early fifth century artefacts from Petra. However, with the exception of the material from Petra, no tesserae from the Byzantine period of northern Jordan have previously been examined for their chemical composition.

Both the Romans and Byzantines made extensive use of tesserae in mosaics used to cover floors and walls. However, relatively few Roman mosaics have been uncovered at Gerasa, largely because the residential district of the Roman city is largely covered by the modern town. At the moment only a single Early Roman period mosaic is known, along with a number fragmentary examples. However, a number of Late Roman and Early Byzantine mosaics are known, along with a larger collection of later Byzantine examples, predominantly from the late fifth to early seventh centuries. The majority of these mosaics are from the floors of religious buildings, some of which also had mosaics on their walls, although some are also known from civic buildings and private homes. Recent excavations by the Danish–German Jerash Northwest Quarter Project have added a sixth century 'Mosaic Hall' (attached to the Synagogue Church) and an early Islamic home to this collection.

Late second to early third century Roman mosaic from Gerasa, Jordan. Yale University Art Gallery.

Boschetti et al. selected 76 glass tesserae from early fourth and eighth century backfills, excavated at Gerasa between 2013 and 2016. These tesserae included examples of all colours known from the site, and were excavated from fifteen different trenches, across the area of the city excavated by the Danish–German Jerash Northwest Quarter Project. These tesserae were examined by laser ablation inductively coupled plasma mass spectrometry at the Centre Ernest-Babelon at the Université d’Orléans, with a single spot analysis being carried out on each sample.

Specimens from the North Western Quarter divided by base glass composition and colour, with an indication of sample number, trench letter and date of the backfill. Boschetti et al. (2021).

All of the tesserae were found to be comprised of low magnesium and potassium silica-soda-lime glass, which is indicative of the use of natron (sodium carbonate) as a fluxing agent. This way of glassmaking was the dominant method from the Hellenistic period till the ninth century AD, and therefore fits with expectations for the Gerasa tesserae. 

The proportions of aluminium oxide to silicon dioxide, and of titanium oxide to aluminium oxide can be used to determine the proportions of feldspars and heavy minerals mixed with the original silica from which the glass was made, and therefore can be used to identify the origin and date of manufacture of tesserae, by comparison to other glass artefacts of known origin. This methodology was used by Boschetti et al. to split the sample into a number of groups. 

The first group was distinguished by the presence of a low titanium oxide to aluminium oxide ratio and a moderate aluminium oxide to silicon dioxide ration. This group of tesserae date from the first to fourth centuries AD, and is identified as being of Roman origin, and specifically from the Roman Levantine coast. As this glass does not have a mixed manganese-antimony composition, it is likely to have been made from manganese-decoloured raw glass close to the original workshops of the Levantine coast. During this time antimony compounds were commonly used to colour glass; calcium antimonate was used to produce a blue colour and lead antimonate to produce yellows and greens. This method of colouring glass predominated from the Hellenistic period till the third century AD, when it was largely replaced by a new method with replaced the antimony compounds with tin compounds.

Specimens from the North Western Quarter divided by base glass composition and colour, with an indication of sample number, trench letter and date of the backfill. Boschetti et al. (2021).

The majority of the examined specimens (57 of the 76 tesserae) had a composition with a low titanium oxide to aluminium oxide ratio and a slightly higher aluminium oxide to silicon dioxide ratio, combined with higher calcium and potassium levels and lower sodium levels than the Roman tesserae. These examples are considered to belong to a type known as Levantine I, which was primarily made at Apollonia (in modern Israel) in the sixth and seventh centuries. However, while this glass is assigned to the Apollonian Levantine I type, the majority of the tiles are in the overlap between Leventine I, Roman manganese glass, and Levantine glasses produced in Jalame in the fourth century. Thus, it is quite likely that these tesserae were made from recycled glass a mixture of sources, dominated by Apollonian glass, but with some earlier Levantine and Roman glass. A previous study of glass vessels from the same area found many appeared to be a mixture of Apollonian and Roman glass, so finding this composition in tesserae is not unanticipated. These samples also typically have higher potassium and phosphorus levels and lower chlorine levels than is typical for Apollonian glass, which is known to be a symptom of repeated remelting of glass.

Previous studied of glass from Jordan have shown a strong correlation between their potassium and phosphorus contents, something which has been taken as evidence for the use of Olive stones as a fuel. Curiously, the Gerasa glass tesserae showed no such pattern, possibly due to the methods used to opacify the glass. The white, blue, and turquoise tesserae show raised levels of calcium phosphate, which is probably indicative of the use of bone ash as an opacifier, a practice known in the Mediterranean and Near East from the beginning of the fifth century onwards. However, it is difficult to be confident about this, as repeated recycling of glass also raises phosphorus levels. Tesserae from the Petra Church have been shown to contain crystals of calcium phosphate by electron microscopy, and both the Gerassa and Petra Church tesserae have phosphate levels unrelated to their potash (potassium salt) levels, something which differs from other glass objects from Jordan. These other glasses all have a phosphorus pentoxide content of less than 0.2% by weight, while in the tesserae have levels between 0.12 and 2.23% by weight, with textures ranging from opaque to bubbly translucent, which may be another indication of bone ash in the opacification process.

This use of calcium phosphate as an opacifyer, combined with the use of lead stannate (or lead-tin-yellow) to produce green and yellow-green glass provides an extra distinction between the Roman and Levantine tesserae. Comparison between cobalt and nickel compositions of the blue tesserae. The Roman tesserae typically have low nickel levels, and therefore a high cobalt/nickel ratio, whereas tesserae made from the fourth century onwards have a higher nickel content, and therefore a lower cobalt/nickel ratio. 

Eight gilded tesserae were present in the sample, and these show a different base glass composition to all the other tiles. The base colour of these tesserae varies from green to amber-yellow, something done deliberately to modify the appearance of the gold foil covering the tiles. One of these tesserae shows raised levels of titanium, vanadium, chromium, zirconium, and hafnium, something typical of HIMT (High Iron, Manganese, Tin) glass, which is thought to have originated from northern Egypt in the fourth and fifth centuries. Another six tesserae have a low titanium oxide/aluminium oxide ratio which is associated with Foy 2.1 group glasses, which again came from Egypt, and which are found across Europe and the Mediterranean in the fifth and sixth centuries. One of these final gilded tesserae showed raised levels of transition metals, notably cobalt, copper, and iron, which is probably indicative of recycling, as well as a high cobalt/nickel ratio, which may indicate the inclusion of some older Roman glass into its base mix. Another gilded tesserae has a unique composition, with low levels of calcium and aluminium oxides and a high titanium oxide/aluminium oxide ratio, combined with raised levels of antimony, lead, zirconium, europium, and hafnium. Boschetti et al. suggest that this composition might indicate a recycled origin involving Roman and HIMT glasses, which might also imply that it is of Egyptian origin.

Chemical analysis of the Gerasa tesserae indicates a definable archaeological sequence. This begins with Roman glasses from the first to fourth centuries AD, which were found in Roman occupied areas from the second century onwards, a period when Gerasa was entering a long, prosperous interval, and mosaics were installed in public buildings and the homes of prosperous citizens. Few of these tiles appear to be recycled, indicating new tesserae were easily available. These are succeeded by Levantine tiles in the sixth century, which are found across the whole site, apparently linked to the widespread replacement of Roman sanctuaries with Christian churches, and a desire to decorate these new structures with mosaics, provoking a sudden demand for locally made coloured glass. This seems to be a general pattern in Byzantine Jordan, with recycled glass becoming common, something which suggests the availability of imported glass went down, provoking the development of an efficient glass recycling system. Analysis of Byzantine glass artefacts from other sites in Jordan has suggested that this process often involved colouring clear recycled glass with reused tesserae, but the pattern at Gerasa appears to have been different, with fresh colouring agents and opacifiers being added to the glass mix. Apart from the single HIMT tesserae, all the gilded tesserae from Gerasa appear to date from this Byzantine period, and therefore are also probably associated with the church building episode. However, at other Byzantine and early Islamic sites in Jordan and nearby, gilded tesserae appear to have been made locally, whereas those at Gerasa appear to have been imported from Egypt, which is unusual in that few other objects of Egyptian origin are found in Gerasa or northern Jordan at this time.

Base glass compositions thus far identified in fourth- to eight-centuries Jordan. Boschetti et al. (2021).

Mosaics made up of glass tesserae appear to have been common at Gerasa from at least the second century AD. The earlier 'Roman' tesserae were probably made on the Levantine coast to a Roman recipe, with the later 'Levantine' tesserae probably being made more locally, possibly within the city itself. Examination of material from late Byzantine and early Umayyad backfills revealed an abundance of older tesserae but none that could be comfortably assigned to the eighth century, suggesting that by this time mosaics were being made entirely with recycled tesserae, although these backfills could represent recycling centres, where tiles were collected for reuse, and new tesserae would not be found even if they were being produced at this time. The gilded tesserae appear to have been imported from Egypt, a rare example of Egyptian goods in Jordan at this time, highlighting their status as a specialist product, although why they were imported from Egypt, and not the Levantine coast, where such tesserae are known to have been manufactured, is unclear.

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