Copper-based opaque red glasses e Understanding the colouring mechanism of copper nanoparticles in archaeological glass samples F. Drünert a, * , M. Blanz a, 1 , K. Pollok b , Z. Pan a , L. Wondraczek a , D. M € oncke c, d, * a Otto Schott Institute of Materials Research, University of Jena, Fraunhoferstrabe 6, 07743 Jena, Germany b Institute of Geosciences, University of Jena, Carl-Zeiss-Promenade 10, 07745 Jena, Germany c Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue,11635 Athens, Greece d Department of Built Environment and Energy Technology, Linnæus University, Hus N 2086, 35195 V€ axj€ o, Sweden article info Article history: Received 23 October 2017 Received in revised form 15 December 2017 Accepted 29 December 2017 Keywords: Copper ruby Copper nanoparticles Mie-scattering Medieval glasses Opaque red glasses Historical glass making practices abstract Red opaque glasses of two different sites in central Germany, a medieval glassworks in Glashütten, Taunus Mountains, and an early modern glassworks in Wieda, Harz Mountains, were analysed with regard to their optical appearance. By scanning electron microscopy and X-ray diffraction, metallic copper nanoparticles were identified as a conspicuous constituent in these glasses. In addition, similar opaque red glasses were reproduced in the laboratory in order to better understand the manufacturing process. Detailed analysis of the optical scattering was conducted in order to evaluate the role of Cu 0 nanoparticles in the colouring mechanism relative to other possible reasons of colouration. We find clear differences between the possible contributions of Cu 2 O (cuprite) particles and metallic copper (Cu 0 ) nanoparticles. Through simulated backscattering spectra we were able to calculate an average copper particle radius in the archaeological glass samples resulting in a value of up to 95 nm, which matches well the results of SEM investigation (minimum 65 nm). Using the methods we applied in this study, it becomes possible to reconstruct various processing conditions as they were applied in medieval manufacture of these particular materials. © 2018 Elsevier B.V. All rights reserved. 1. Introduction One of the very first colouring agents which were applied in early glass manufacture is a red dye based on copper, dated to the 15th century BCE [1]. Starting in that time, the number of red- coloured glass artefacts produced until today has increased in countless variants. The underlying mechanism of colouring, how- ever, has become a subject of controversy. In his seminal book on glass colouration, in 1951, Weyl described four different types of copper-based reds, either opaque or transparent, all based on metallic particles [2]. In spite of this, as discussed within the same book, other authors assumed cuprite (Cu 2 O) to be the main col- ouring agent. According to their assumptions, the native red colour of crystalline cuprite gives rise to the appearance of many opaque red glasses (e.g. [3]). Since then, numerous articles have been published, usually following one of the two theories [4e7]. Today we know that either view can be correct in its own right, depending on the glass system and the fabrication process [8,9]: Whereas copper nanoparticles are found in glasses with low amounts of copper and lead oxide, cuprite can precipitate in high-copper-high- lead glasses and leads to a lighter red (often described as ‘sealing wax’ [9]) to orange colouration. Due to differences in crystal structure, chemistry and morphology the nanoparticles can be distinguished by combining X-ray diffraction and SEM techniques [9,10]. Chemically, the formation of metallic copper nanoparticles in glass matrices was investigated in several studies, usually focusing on particle size and the rules of Ostwald ripening at various tem- peratures between 500 + C and 800 + C[11e13]. In contrast, the for- mation of cuprite particles is less investigated; however, it is known from comparable glass systems that cuprite can form at tempera- tures above 700 + C[14]. In the present context, it should be noted that most of these studies focused on qualitative and quantitative analysis of the particles and did not attempt to correlate the number and type of the observed particles with the visual * Corresponding authors. E-mail addresses: ferdinanddruenert@yahoo.de (F. Drünert), dmoencke@eie.gr (D. M€ oncke). 1 Present address: Archaeology Institute, University of the Highlands and Islands, Orkney College UHI, Kirkwall KW15 1LX, UK. Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat https://doi.org/10.1016/j.optmat.2017.12.054 0925-3467/© 2018 Elsevier B.V. All rights reserved. Optical Materials 76 (2018) 375e381