Glass consumption and production at the turn of first millennium CE: chemical analyses of late antique and medieval glass from Koper (Capodistria, SI)

Abstract A series of glasses from the site Kapucinski vrt (garden of the Capuchin monastery) in Koper (Capodistria) in northern Adriatic was measured by a combined PIXE and PIGE method. Koper has been continuously populated since late Roman period till the present with a rich medieval history, so the measurements were expected to show the trends in glass production and consumption from Late Antiquity until the Middle Ages, notably the transition between the natron to plant ash glass and supply of the fresh glass. Among the set 22 glass finds, both natron and plant ash glass were identified. Natron glass was of the types Foy 2.1 (with a noticeable component of Roman Mn) and HIMTa, all with the signs of recycling, while four samples (about 20% of the whole set) were unrecycled: they involve 2 examples of Levantine glass, an example of probably Mesopotamian origin and a lamp of mixed glass between the natron and plant-ash glass; this may indicate a modest supply of fresh glass during the period. Plant ash glass may be attributed to the Early or High Middle Ages, exploiting purified alkalis of the Levantine coasts (known as alume catino in Venetian glassmaking), while the admixture of impurities in the siliceous sands suggest circulation and consumption of glass that was produced and traded in the eastern Mediterranean since the 10th centuries onwards.