Glass production

Glass production

As the analysis of the use and production of glass requires large amounts of material, this topic (WP 9) is restricted to large-scale urban excavations at Sagalassos.

The great majority of ancient glass was based upon silica, fluxed with soda or potash. Chemically, ancient soda-lime-silica glass falls into two broad categories: (1) plant ash glass, combining a soda-based flux and lime from plant ash with quartz pebbles as a silica source and (2) natron glass, combining a soda-based flux of mineral matter with quartz sand. Natron glass is the predominant type of ancient glass in the Mediterranean and Europe from the middle of the first millennium BC through to the ninth century AD. Plant ash glass is mainly produced before that time in Egypt and Mesopotamia and the use of plant ash as a flux became the dominant practice from the ninth century onwards throughout the Mediterranean and Europe.

Whereas it was previously assumed that glass was made from silica and alkali in the same workshops where glass was formed, it is now clear that raw glass was already traded as ingots in the Late Bronze Age and as lumps of glass (chunks) in the Roman and early medieval periods. Primary workshops made raw glass and were distinct from secondary workshops that shaped glass. It is clear from excavation that large quantities of 4th to 8th century AD natron glass were made in a limited number of primary glass production centres mainly in Egypt and Syro-Palestine. Suggestions towards the existence of similar units in the Levant in Roman times have been made but were never proven. Some authors have suggested that primary production may have taken place elsewhere.

Local secondary production (working) of natron glass from imperial to early Byzantine times was proven at Sagalassos. Research of post 9th century structures in the town have also revealed the presence of chunks of plant ash glass in the town. Distinct archaeological indications for the craft organisation of glass working in Sagalassos or Turkey in general are scarce. Some of the evidence at Sagalassos is therefore unique in glass studies. Due to the detailed analysis and documentation of the glass finds, amongst others, several chunks, moils and knock-off’ s (direct indicators of glass working) were found. In this regard, also a ceramic pipe -made out of Canakli clay (also used for the local table ware) and attached to a chunk of glass- needs to be mentioned. Only further study of this artefact can answer the question whether we are dealing here with a terracotta variant of a regular metal blowpipe. Although well know in metallurgy, this ceramic object is unique in glass studies.

It is suggested that the raw blue raw glass at Sagalassos had several production sites, likely in the Levant. H(igh)I(ron)M(anganese)T(itanium) raw glass at Sagalassos corresponds very well to earlier described material, of which the primary production site is placed in Egypt. Strontium and lead isotopes have successfully been applied to determine the origin of these different types of glass found at Sagalassos. This was only possible after the methodology for the analyses of strontium in glass was developed. This isotopic study also nicely demonstrates local recycling of glass (secondary production) from primary (imported) HIMT and blue glass. In the application, the analyses of minute mineral inclusions in the glass, that could be remnants of the raw materials, by Raman spectroscopy was planned. Raman spectroscopy of the glass, however, did not confirm the presence of minute inclusions that could be used to identify the origin of the raw material.

http://mill.arts.kuleuven.be/IPA-V-09/glass.html