Multi-analytical study of the effects of secondary working and recycling in Roman glass

Abstract

The impact of secondary processing and recycling on the composition and redox equilibrium in archaeological glass remains difficult to assess. This paper reports the results of a multi-analytical study of glass finds from a Roman villa in southwestern France, revealing varying mixing and recycling strategies that affected the trace element profiles, redox conditions and structural properties of the glass. Laser ablation inductively coupled plasma mass spectrometry data allowed the assemblage of 146 glass finds to be classified into four primary compositional groups and three groups resulting from recycling and mixing. Optical absorption spectra show variations in the redox state of iron naturally present in the glass matrix. In primary Roman glass, antimony and manganese functioned as decolourants by promoting iron oxidation. During subsequent recycling, their oxidative capacity diminished, thus shifting the Fe2+/Fe3+ ratio in favour of Fe2+. Raman spectra corroborate this observation, as glass groups with higher proportions of Fe3+ display a more pronounced band at ∼980 cm-1. Only the 4th- to 5th-century high iron, manganese and titanium glass (HIMT) showing minimal evidence of recycling can be reliably attributed to local secondary production, consistent with the widespread use of this glass type and the chronology provided by the archaeological context. All earlier glass objects belonging to other compositional groups are therefore presumed to originate from secondary workshops elsewhere. The study highlights the value of an integrated, multi-analytical approach for revealing workshop-specific characteristics, technological practices, and distribution networks in the late Roman Empire.