Characterisation of Roman and Byzantine glasses from the surroundings of Thugga (Tunisia): Raw materials and colours

Abstract

Roman and Byzantine glass shards collected during excavation campaigns around the ancient city of Thugga in northern Tunisia have been characterised by means of various analytical techniques. Portable X-ray Fluorescence (pXRF) was used to determine the glass matrix elemental composition and to identify the elements responsible for the colour. Fibre optic reflectance spectroscopy (FORS) was employed to investigate the chemical nature, the oxidation state and the coordination of the chromophores. X-ray photoelectron spectroscopy (XPS) was here exploited for characterising the outermost layers of the glasses. Elemental analyses by pXRF pointed out that Si, Fe, Al, Ca, K, Rb, Sr, Na and Mg are the main elements present in the glasses. The chromophores responsible for the different shades are Fe for the green shades, Cu and Co for the blue ones, Mn, also used as discolouring agent, for the colourless samples. In relation to the raw materials used for glass production, the comparison with control groups of samples from Northern Tunisia suggests a common provenance of sands

Degradation Products on Byzantine Glasses from Northern Tunisia

“…accordance with the presence of sand deposits of important rivers of that area. Concerning the flux, in most of the cases a mixture of natron and plant ash was used [28].…”

“…In a previous paper we have thoroughly investigated by means of numerous analytical techniques [28] a group of Roman and Byzantine glass shards collected during an excavation campaign at Aïn Wassel in the surroundings of the ancient city of Thugga in northern Tunisia [29,30], a territory belonged to the Roman province of Africa Proconsularis and particularly flourishing during the Roman and Byzantine periods. The study has been carried out with the aim to chemically characterize the objects and to study the production technology including the chromophores responsible for the different color shades.…”

“…As far as Thugga's glasses, for the raw materials a local origin of the sand has been hypothesized due to the homogeneity of the samples from the compositional point of view and in accordance with the presence of sand deposits of important rivers of that area. Concerning the flux, in most of the cases a mixture of natron and plant ash was used [28].…”

“…With regard to the glass chemical composition, it was found [28] that the chemical elements responsible for the different glass shades were generally Fe for the green shades, copper and cobalt for the blue ones and manganese (used as discoloring agent) for the colorless samples. Roman and Byzantine glasses generally show a good standard of technology, particularly in terms of color control.…”

Flash Thermography Mapping of Degradation Patterns in Archaeological Glass

“…The main degradation process of archaeological glass is the leaching out of alkali ions from the glass network [10]. This phenomenon is strictly connected to the burial conditions and to the amount of Sodium and/or Potassium in the glass itself [11]. Alkali migrates towards the surface moving through the 3D silicon-oxygen network and they are replaced by hydrogen ions coming from the water molecule.…”