Chemical characterisation of glass mosaic tesserae from sixth-century Sagalassos (south-west Turkey): chronology and production techniques Nadine Schibille a, * , Patrick Degryse b, 1 , Markku Corremans c, 1 , Christian G. Specht d, 2 a Research Laboratory for Archaeology & the History of Art, University of Oxford, Dyson Perrins Building, South Parks Road, Oxford OX13QY, UK b Celestijnenlaan 200E, Division of Geology, Centre for Archaeological Science, K.U. Leuven, 3001 Heverlee, Belgium c Sagalassos Archaeological Research Project, Department of Archaeology, K.U. Leuven, Blijde-Inkomststraat 21 e Bus 3313, 3000 Leuven, Belgium d Biologie Cellulaire de la Synapse, Inserm U1024, IBENS, École Normale Supérieure, 46 rue d’Ulm, 75005 Paris, France article info Article history: Received 29 August 2011 Received in revised form 4 January 2012 Accepted 15 January 2012 Keywords: Byzantine Roman Glass tesserae Colourants Opacifiers Antimony abstract Little is known about the origin, supply pattern and production technology of Byzantine glass mosaic tesserae. In this study, we have analysed forty-eight glass tesserae from Sagalassos (Asia Minor) of different colours and from two archaeological contexts that were stratigraphically dated to the sixth century CE. The main aim was to identify the raw materials, colourants and opacifiers as well as secondary working practices that are reflected in the composition (EPMA, LA-ICP-MS analyses) and the microstructure (XRD, SEM-BSE) of the tesserae. The set of samples retrieved from the Roman Baths complex at Sagalassos is compositionally very homogeneous, representing possibly a single commission, and can be tentatively dated to the late Roman period. In contrast, the assemblage associated with the construction of a Byzantine church around the turn of the sixth century CE is more diverse, suggesting that these tesserae were produced from more than one silica source. This highlights a diversification in the supply and manufacture of glass tesserae during the Byzantine period. Although all analysed samples are natron-type soda-lime-silica glasses, a strong correlation between MgO, K 2 O, P 2 O 5 and CaO was observed, pointing to the contamination of the glass batch through fuel ash either intentionally in case of the red tesserae or accidentally as regards the green, peacock and white samples. The degree of contamination reflects differences in the duration of the secondary production stage according to the colour, which is also evidenced by the shapes and sizes of the opacifying crystals present in the opaque tesserae. X-ray diffraction measurements identified calcium antimonate (in the blue, peacock and white tesserae) and lead antimonate (green and yellow) as the main opacifying agents. The morphological analysis suggests that the calcium antimonate particles crystallised in situ, whereas a lead antimonate pigment was synthesised prior to its addition to the glass melt. Taken together, the compositional and microstructural characteristics clearly demonstrate different production processes that are related to the colours of the tesserae, thus indicating colour-specific secondary workshop practices. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Recent work has shown that throughout the Byzantine period glass mosaics were much more widespread and more commonly produced than has previously been thought (James, 2006, 2010). Yet, there is an almost complete lack of archaeological evidence for the making of tesserae from the Byzantine period. It is still unknown how the mosaicists obtained their material, whether, for instance, glass tesserae were produced locally or whether the manufacture of tesserae was a specialised trade and mosaicists procured their material from a single supplier or from numerous glass workshops with an expertise in the production of specific colours. No written records on the making of mosaics survive from Byzantium. Some of the earliest extant documentary information stems from fourteenth-century CE Orvieto (Italy), according to which local magistri vitri were responsible for the production of all but the blue tesserae for the decoration of the cathedral. A century * Corresponding author. Tel.: þ44 1865 285204. E-mail addresses: nadine.schibille@uclmail.net (N. Schibille), Patrick.Degryse@ ees.kuleuven.be (P. Degryse), markku.corremans@arts.kuleuven.be (M. Corremans), specht@biologie.ens.fr (C.G. Specht). 1 Tel.: þ32 16 326460. 2 Tel.: þ33 1 44322300. Contents lists available at SciVerse ScienceDirect Journal of Archaeological Science journal homepage: http://www.elsevier.com/locate/jas 0305-4403/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jas.2012.01.020 Journal of Archaeological Science 39 (2012) 1480e1492