The Lycurgus cup
The Lycurgus Cup, an ancient relic from the late Roman Empire that changes colour depending on how it is lit.
The Lycurgus Cup is a mysterious ancient relic from the late Roman era (4th Century AD). It's a "cage cup" - a central cup surrounded by a decorative "cage" design around it. In this case the cage's design depicts the rage and death of King Lycurgus of Thrace.
According to legend, Lycurgus banned the Cult of Dionysus from his lands. There are various different tales of what Dionysus did in revenge. Many of them involve tormenting Lycurgus with vines (the vine was symbol of the wine-drinking cult). In the depiction on the cup, Lycurgus is attacking Ambrosia, a follower of the cult, who entraps him in vines, killing him Laocoön-style.
The incredible detail of the depiction on the cup is impressive enough - the whole thing is made out of a single work-piece and the figures are raised from the main body of the cup so presumably someone very skilled would have had to spend a lot of time painstakingly cutting out the figures individually. However, the most impressive thing about the cup is that it changes colour depending on where you are lighting it from.
As you can see in the above video (hopefully it loads correctly), when I shine the torch from behind the cup instead of onto it directly, the cup appears as a transparent red - almost like a medieval stained window. However, when I shine the torch directly at the cup the whole thing appears as an opaque green. The complete opacity of the green can be a bit surprising - a French couple watching me mess about with it were surprised to learn that the cup is in fact made out of glass.
There is one other notable trick. The torso of Lycurgus even appears as purple under the light - not just red. Presumably this is in reference to his legendary rage. I've seen other sources on the web claim that this is achieved via his figure being inset1 but in fact many of the figures are inset in the same way and they don't turn purple so I don't understand this explanation.
Manufacture - ancient and modern
The technique to make the Lycurgus cup is one of a number of ancient skills not to have come down to us. The cup is the only intact example we have of the technique at all2. The ability to manufacture dichroic glass (which is what the cup is) only seems to have been regained by western civilisation in the mid-20th Century when required for the American space programme3.
However, when NASA creates similar glass for its purposes it uses nanotechnology (nanometre-scale particles of gold and silver) - clearly not a branch of material science available to ancient Romans. So how did the Romans make a glass cup with nanometre-scale particles of gold and silver?
The best guesses I've read are theories that small amounts of gold and silver dust are added to molten glass which is then repeatedly diluted with more molten glass to get the ratios down to the nano-scale. This sounds plausible to me but I'm not sure how the gold and silver dust is then being broken down from dust mote size to the nanoparticle level - surely this dilution method would just put the dust into suspense in the glass and give you a gold leaf effect?
Accident or intention
Due to the fact that it's not clear how the Romans managed to fabricate the glass, there is debate about whether they made it "by accident" or not. There's a certain school of thought that the Romans might have had a glass blowing factory next to a goldsmiths somewhere and that there was some cross-contamination where kilns or tools somehow got small gold and silver particles on them which were then somehow transferred into the glass-making process.
The other point of view is that the Romans could have known something about glass-making that we don't. Glass-making was a very common profession in the Roman Empire but a lot of glass was recycled - which explains why there aren't huge numbers of Roman glass fragements in museums. If you have a society where there are a huge number of people working directly with glass on a daily basis as their primary profession it seems plausible that they might discover something about glass-making "empirically" that our present society might not know. We only have a small number of professional glassmakers.
Going to see it yourself
The cup is utterly transfixing in person in a way that my video doesn't really capture. There is an incredible sense that you are playing with an exceptionally pretty, one-of-a-kind 1600 year old toy whose mechanism still isn't really understood.
Unfortunately the position of the cup in the museum isn't ideal - it's off to the side in a room whose main subject is a series of objects of a different nature and era (the Sutton Hoo). Most people passing through fail to notice to cup at all and even the cut-out figures catch their eye the case is very dimly lit so the colour changing is not demonstrable without a torch. I actually struggled quite hard to light up Lycurgus' torso at all given the configuration of the case.
However you can just walk in off the street and see it anytime - the British Museum is free and open late on Friday.
Jonathan Blow's talk Preventing the collapse of civilisation, which is where I learned of the cup. When I looked it up I was surprised to discover I'd already walked past it several times in my life while looking around the Sutton Hoo.
The Name of the Rose by Umberto Eco, which has lack of access to (and loss of) ancient written material as one of it's main themes in the ultimate post-apocalyptic setting of all time: the medieval era.
The British Museum's object page for the cup. They have a lot of good images and generally release them under the CC BY-NC-SA.
The Wikipedia page, which has a lot of background information
It's hard to explain (I wish I'd taken a picture) but the inside of the cup isn't completely smooth - the major figures have been "pushed out" somehow - possibly having been placed over a mould when the glass was still soft. ↩
There are fragments of the same material available in other museums - enough to suggest that the Lycurgus Cup was not a one off at the time but not enough to clarify how widespread this technique was. My hunch is: not very widespread. ↩
"Dichroic glass" is used by NASA to protect human eyes and cameras from harmful radiation and glare. ↩