Friday, June 21, 2024

Extraordinary origin of Viking glass beads in glittering Roman mosaics uncovered

White beads prized by 8th-century Scandinavians were made from gilded clear glass cubes stripped from Roman and Byzantine mosaics hundreds of miles to the south, new analysis reveals.

It was previously known that early medieval Scandinavians recycled Roman glass, including glass from mosaics and drinking vessels. However, the finding that they used clear glass rather than opaque white glass to manufacture opaque white beads illustrates their technical expertise.

The study also uncovers a remarkable supply chain from the production of glass in Roman Egypt and the Levant to its use in mosaics in cities such as Rome and Ravenna and eventual recycling for fashion accessories in the North.

The researchers used innovative chemical analysis methods to study glass found in excavations at Ribe in southwest Denmark. Ribe was an important trading centre from the early 8th century and attracted craftsmen and merchants from far and wide to manufacture and deal in goods such as brooches, buckles, combs and coloured glass beads.

In 8th-century Ribe, with no local production of raw glass, beads were made from small glass tiles removed from mosaics in grand Roman and Byzantine buildings and transported over long distances. Over 8,200 of these tiles, or tesserae, have been recovered at Ribe, alongside about 9,500 beads. The beads are widely known as Viking beads, although they slightly predate the Viking Age proper from around 790-1050.

“This knowledge is almost like an alchemist’s . . . It is quite extraordinary”

Professor Søren Sindbæk

Until now, archaeologists had assumed that the bead makers used opaque white glass tesserae as the raw material for the production of opaque white glass beads. However, analysis of the beads’ chemical composition reveals that, in fact, the artisans crushed clear glass tesserae and melted them at a relatively low temperature, stirring to form tiny air bubbles resulting in the opaque white finish.

These clear glass tesserae contained a layer of gold leaf close to one surface and had been used to create dazzling gold sections of mosaics. Although the Ribe artisans attempted to salvage the gold before crushing the glass, this was difficult to achieve and the researchers found traces of the metal distributed unevenly through the beads. These gold flecks, along with the air gaps and lack of chemical colourants, revealed the beads’ gilded origin.

Roman and Byzantine gilded clear glass tesserae were crushed and melted at Ribe in Scandinavia to create opaque white beads. Graphic: Museum of Southwest Jutland

The researchers observed that opaque white glass tesserae were available to the bead makers, but their use of translucent glass to produce white beads, through the bubble-making method, saved fuel and time by requiring lower temperatures and less labour. Given the extensive gold sections of Roman and Byzantine mosaics, gilded tesserae would also have been more numerous than white glass examples.

Traces of gold leaf were also found in translucent blue beads from the same workshop. For these beads, the analysis indicates that the bead makers used a mixture of blue and clear glass tesserae. The combination was necessary because the blue tesserae contained high concentrations of chemicals that made them opaque. By diluting these substances they could obtain a deep blue translucent glass popular in Scandinavia. This method was more complex than the production of white beads and involved heating the glass to high temperatures.

Glass beads were often worn between brooches in medieval Scandinavia. Photo: Shutterstock

Both the white and blue beads also contained small quantities of glass melted down from Roman drinking vessels, or from Frankish drinking vessels made from old Roman glass. Barford said this vessel glass was used sparingly because it had already been repeatedly melted down and reused and was heavily contaminated.

“The glassmakers in Ribe were clearly connoisseurs who preferred the clearest glass they could get their hands on,” said Gry Hoffmann Barfod of the department of Geoscience at Aarhus University, lead author of the paper in Archaeological and Anthropological Sciences.

Among other methods, she said the team had used a new technique which analyses hafnium isotopes. This had only previously been used in archaeology in the study of sandstone artefacts. “It was able to pinpoint the difference between glass coming from Egypt and glass coming from the eastern Mediterranean, which are the two places where the Romans and Byzantines were making this glass and that was the first time you could pinpoint it.”

The study found that the glass of the tesserae repurposed at Ribe was originally produced in the 4th to 6th centuries in both regions, from where it would have been exported across the empire. Barfod pointed out that a slight yellow or green tinge in these clear glass types caused the gold leaf to shine with an especially beautiful colour.

The researchers discovered that many of the tesserae were produced after the fall of the Western Roman Empire in 476 and could not therefore have been obtained from Roman ruins in northwestern provinces such as Germania and Gaul. They speculate that the gold-leaf tesserae were obtained from Italy where they were used both in late Roman buildings and Byzantine ones, such as the 6th-century Basilica of San Vitale in Ravenna.

Graphic from the study illustrating the production processes for white and blue beads at Ribe. Image: Museum of Southwest Jutland

Co-author Søren Sindbæk, professor of archeology at the Danish National Research Foundation’s Centre for Urban Network Development at Aarhus University, said: “It demonstrate that there were Mediterranean contacts even at this very early stage. As we come into the Viking Age, there were contacts with the Middle East and all sorts of places. But the earliest workshops we are analysing date from 100 years before that. So we’re very curious about how that story starts. The tesserae could have arrived in several steps.

“In fact, we have suspicions that it may have been people within the orbit of the Church who brought this material from Italy to Northern Europe because they knew it was valuable up here.”

The authors noted that glass bead production methods at Ribe changed and evolved over time. The first workshops, operating from about 700–720, mostly produced monochrome blue or white beads. In the following decades white, red and yellow bands were applied to translucent blue cores. In the second half of the 8th century production shifted towards production of monochrome red, green, dark brown and black beads, some of which, so-called “wasp beads”, were decorated with yellow bands.

Commenting on the artisans’ technical abilities and knowledge of colouring methods, Sindbæk said: “This knowledge is almost like an alchemist’s. They really had chemical knowledge. It is quite extraordinary that we find expertise in a faraway trading place that we wouldn’t be surprised if we had seen in Rome or Alexandria.”

Glass-melting crucibles found at Ribe. Photo: Museum of Southwest Jutland

Among the artefacts recovered from the former workshops at Ribe are fragments of two large ceramic crucibles. In one of the crucibles, the team found traces of glass that was in the process of being coloured yellow using a lead stannate, an oxide of lead and tin. Barford said: “All the archaeologists I’ve shown it to say you’ve just found the process preserved in situ, as it happened. That’s pretty amazing.”

By the 8th century, the use of beads had decreased in Western Europe following Christianisation. However, beads remained in widespread use in early medieval Northern Europe and were commonly hung across the wearer’s chest between two brooches.

Around 800, as long-distance Viking trading networks became more established, local glass bead production at Ribe was largely replaced by the import of new glass beads from the eastern Mediterranean and further afield.

The interdisciplinary study was a collaboration between Barfod, Sindbæk and Claus Feveile, curator at the Museum of Southwest Jutland specialising in the Viking Age and Ribe’s earliest history. The artefacts described in the paper are currently exhibited in a reconstructed bead-making workshop at the museum.

The top image shows gold leaf tesserae used in a 6th-century Byzantine mosaic depicting Empress Theodora in the Basilica di San Vitale, Ravenna, Italy. Photo: Shutterstock.

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