article posted 22 March 2016
Ferdinand Drünert graduated in chemistry in 2013. Since the start of his PhD project he is focussing on the spectroscopic analysis of historical glass samples, as well as comparison with modern, remade glass samples.
Why are opaque red glasses opaque and red? The link between optical spectroscopy and particle emergence
Ferdinand Drünert*, Magdalena Blanz, Zhiwen Pan, Doris Möncke & Lothar Wondraczek
Otto Schott Institute of Materials Research, Friedrich Schiller University, Fraunhoferstraße 6, 07743 Jena, Germany
Current discourse on how copper-containing particles influence the appearance of red glass artefacts has not yet reached a clear consensus, and in fact still bears many similarities to how the issue was discussed in a review by W. Weyl in 1950.
The controversy most likely continues because, depending on the techniques employed in the analysis of the origin of the opaque red colour, evidence either points toward the presence of copper nanoparticles (Cu0
) dispersed in the glass matrix (when using e.g. EXAFS  or XRD ), or cuprite particles (Cu2
O) were identified as colorant [4,5]. While most of these studies give a good impression about the system, explanations on the actual constituents and mechanisms causing the opaque red colour seem evasive. This is in clear contrast to transparent Cu-ruby glasses, which are much better understood.
In order to investigate the mechanisms of colouring and scattering in opaque red medieval hematinone glass, archaeological opaque red glass shards, slightly opaque medieval flash glass and remade laboratory glass samples (all ~1 mol% CuO) were studied together with a commercial transparent Cu-ruby flash glass (Lamberts Glass) using optical spectroscopy. The resulting spectra were further evaluated by scattering simulations using Mie’s theory (Mieplot 4.5) by assuming certain particle size distributions. Scanning electron microscopy was used to further characterise these samples as well as higher concentrated copper glasses (<10 mol% CuO).
By fitting of UV/Vis spectra to a computational model, this study showed that copper metal particles (<100 nm) are consistent with the colouration of transparent red glasses, whereas opacified glasses have an additional major contributor of a much larger size (in the micrometre domain).