A new study reveals new manufacturing techniques for how artisans in Denmark recycled glass from 8th-century AD Roman glass mosaics.

Glass became a rare commodity in the early medieval period, with mosaics from abandoned Roman and Byzantine structures often being looted for the colored glass cubes called tesserae.

A trade network carried them north to imperial cities such as Ribe in Denmark, where they were melted down in large vessels and fashioned into decorative beads.

Until now, archaeologists have assumed that bead makers used white and opaque tesserae as raw material for producing white and opaque beads.

Glass, a rare commodity in the early medieval period

However, a new study by Aarhus University on an early bead-making cult in Ribe has revealed that the chemical composition of Viking white beads was actually produced by crushing gilded transparent tesserae, which were then melted from new at low temperatures.

The molten glass would then have been agitated to trap the air in the form of bubbles and then rolled to form the beads. The thin gold leaves on the tesserae were removed before the melting process.

However, the study shows that some of the gold inevitably ended up in the melting pot. This process is indicated by the tiny gold droplets in the white beads, the numerous air holes (which is why the beads are opaque), as well as the lack of chemical color tracers, he writes HeritageDaily.

Roman mosaics, often looted for the colored glass

Traces of gold were also found in blue beads from the same workshop. Here, the chemistry shows that the glazier’s recipe consisted of a mixture of blue and gold mosaic stones.

The mixture was necessary because Roman blue mosaic stones contained high concentrations of chemicals that made them opaque – and therefore ideal for mosaics, but not for blue beads.

Diluting the chemicals resulted in the deep blue transparent glass we know from Viking Age beads.

The study was published in the journal Archaeological and Anthropological Sciences.

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