The Lycurgus Cup is a remarkable Roman artefact, a decorative glass cup from about AD 400 that is encased in a metal cage depicting the mythical King Lycurgus. The glass has a very peculiar property – it is dichroic, meaning that the glass shines in different colours according to the angle of the light. It is red when lit from behind, but green when lit from the front. While there are a few other fragments of Roman glass with this effect (using a variety of colours), this is the only complete surviving example.
The Roman glass in this decorative cup from 400 AD is dichroic, meaning that the glass shines different colours according to the angle of the light.
In modern manufacture this effect can be achieved by layering glass with incredibly thin layers of metals or oxides. However, for a long time the process used to create the Lycurgus Cup remained a mystery. More recently, a few fragments of the glass were subjected to high-resolution microscopy. The strange effect turns out to be created by tiny traces of nanoparticles of silver and gold suspended within the glass. However, we have no way of knowing how the Roman artisans created this effect, particularly how they ensured that the silver and gold particles ended up in an even distribution throughout the molten glass before it set. While we can’t reproduce the effect, it is at least possible to go and admire the beauty of the cup in the British Museum (although it is frequently on loan to other collections).