Few examples of dichroic (from Greek dikhroos , two-coloured) glass vessels from Hellenistic and Roman antiquities are known, the most famous being the Lycurgus cup exposed at British Museum. This cup, prepared by ancient roman glassmakers (4th cent. AD), is ornamented by a carved freeze with the scene of Lycurgus death and its dichroic nature is still exciting the researchers. In fact, the optical properties of the used glass are very unusual even for a modern observer: it has a red wine color in transmitted light and olive-greenish in reflected light, the dichroic properties being observed for the whole cup, not only for some specific coating.

Previous studies reported that the Lycurgus cup contains some quantity of bimetallic Ag-Au nanoparticles (NPs) and suggested that these NPs are responsible for both red plasmonic color and green reflections. It is known that red glasses are colored by nanoparticles: ruby gold glass colored in various shades of red, pink or crimson with gold nanoparticles is well known since at least the end of the 17th century. At the same time, the small size of the nanoparticles prevents them from scattering light. Gold ruby glasses are often transparent and do not demonstrate all the optical properties that the Lycurgus cup glass exhibits.

Actually, the preparation of nanoparticles exhibiting dichroic properties is not simple: several efforts have been made in the past and it has been found that the differences in color are related to the size and shape of the particles. Very recently it has been reported that it is possible to synthesize photonic crystals with specific thickness exhibiting two-way structural colors in the reflection and transmission directions similar to those of Lycurgus cup.

Therefore, to reveal the nature of unusual optical properties of the Lycurgus cup glass and to understand the role of different additives in dichroism, the authors of the paper Lycurgus cup: the nature of dichroism in a replica glass having similar composition, by Andrey Drozdov et al., published in the Journal of Cultural Heritage, 51, 2021, 71-78, doi.org/10.1016/j.culher.2021.07.002 , decided to investigate the role of NPs and phase separation on the dichroic nature of antique glasses. In the article, they reported the production of glass samples with compositions close to Lycurgus cup glass, their electronic transmission, diffuse reflectance and polarization spectra, and finally the measurements of sizes of silver and gold NPs by computer modeling.
A series of replica glasses with optical properties and composition similar to Lycurgus cup glass were prepared by secondary heat treatment. The glass microstructure was investigated by SEM showing the occurrence of a phase separation in the form of droplets enriched with silica which crystallizes over time into the α-quartz. The phase separation is due to the Fe2 +, Fe3 + and P2O 5 species that could enter the glass batch occasionally as also made by Au and Ag nanoparticles (NPs). The NPs size has been estimated according to Mie theory.
The authors propose that the wine-red color in transmitted light is due to the absorption of light by the metal NPs, whereas the olive-green color of glass, in reflected light, is mainly caused by Fe2 + and Fe3 + absorption and Rayleigh scattering on droplets. The significant role of light scattering at the droplets of silica-rich phase was evidenced; together with the light absorption of iron ions near the surface, it explains the green color of glass in reflected light. This is in accordance with the role of scattering in optical properties earlier demonstrated in the case of Venetian chalcedony and girasol glasses. The role of microcomponents (such as phosphate, iron) that came with admixtures to local sand or were specially added to the batch (gold, silver) in the phase separation was shown. Only their joint introduction leads to the production of dichroic glass.

This paper gives a new important contribution to the current hypothesis related to the dichroism of Lycurgus cup.