The science of stained glass

24 September 2019

Interview with 

Jeremy Baumberg, University of Cambridge

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A stained glass rose window

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Stained glass can be absolutely stunning, but what is it that makes stained glass stained? Adam Murphy took a trip to Jesus College Chapel in Cambridge to meet with Jeremy Baumberg from the Cavendish Laboratory in Cambridge, who explained the physics in the light of something truly stunning...

Adam - I have always loved stained glass. How vivid the colours can get when the light streams through it. But what makes stained glass stained, how does it get that way? To find out, I visited the absolutely stunning stained glass at Cambridge’s Jesus College Chapel, along with Jeremy Baumberg from the Cavendish Laboratory at the University of Cambridge who explained the physics to me in the light of something truly beautiful.

Jeremy - Staining is actually... they added something to the glass when they were making it, and it was discovered in fact by the Romans accidentally several thousand years ago. So there's something in the glass which gives it the colour and that's actually tiny chunks of metal.

Adam - What kind of metal? and how tiny?

Jeremy - So the scales we're talking about are a billionth of a meter, about a thousandth of a human hair across. And so they're made of, you know, chunks of atoms, maybe 100 atoms across, a bit bigger than that. These chunks of metal behave very differently than when you have a big chunk of metal. So metals like gold... if I have a gold bar, imagine chopping my gold bar into drill bits. So gold looks golden, but at a certain stage, it stops looking golden. But I have to chop it down very very small, and that size is about the scale of the wavelength of light, so a millionth of a meter. Once I go smaller than that, the way that the electrons in the gold interact with light changes and I start to see colours.

Adam - Stained glass can hit every colour in the rainbow though. What's the difference between a regal red and a blinding blue?

Jeremy - The interesting thing is if I change the shape of my chunk of gold, I start to get a different colour. So if I just put in a spherical chunk of gold into my glass, then it looks red typically when I look at it in a stained glass. If I'm looking at scattered light it looks green. So I get some strange sort of iridescent properties to it. If I change that from a sphere into a rod it would actually go deep blue. And if I actually change into plates I get a greeny colour as well. If I use silver, again I get a different colour, and copper… So gold, silver and copper were some of the main metals used because when they go into the glass, they give you these tiny little chunks of metal. So you don't actually add, you don't pour liquid metal into the glass. What you do is you put in some oxides of the metals, so ores that you dig out of the ground which are purified, and during the firing of the glass, the ions of the metal start to stick together and start to form these tiny chunks, and it is the way that they do that - that is the skill of the glass maker, to actually create these different really luminous colours that we see in all the churches and chapels around.

Adam - Jeremy though doesn't spend his days looking at stained glass, as lovely as that would be. How do we apply this kind of thinking today?

Jeremy - The same properties that the light interacts really strongly with the metal is why it's interesting for us now. So some of the projects we’re, for instance, trying to do are to use two tiny chunks of metal which come very very close together, and then light gets trapped into the gap in between them. The light is trapped into a very small region, and it allows us to look at very small numbers of molecules, so we can use it for sensing. In fact, we're trying to pour these same metal nanoparticles into toilets to actually make an intelligent toilet that can tell if people's brain state is working well or what dosage of antipsychotics or antidepressants they should take. So that's if you like on the biomedical side. Another thing we're trying to do is to make colour-changing wallpapers. Just as these colours can actually colour stained glass, we actually can make wallpapers out of them now, but we want to make them change colour: you flip a switch and your building turns from red to green. So nobody's been able to do that. The Romans would be aghast if you could take their stained glass and flip a switch and we could change all the colours, but that's what we're trying to do, and it's the same principle. We trap the light very close to the metal, and if we change the properties of the materials just around them, then we can change the colours that scatter.

Adam - Now, the Romans, they didn't know they were at the forefront of nanotechnology, they were just mixing stuff and it worked. So today, how do we go about making these things?

Jeremy - Even a professor can do this in a chemical lab. So I actually brought some with me. So here's a little bottle. This is actually... I made it a couple of years ago and it's still the same. Again, it looks a bit like wheat stained glass, doesn't it? The way you do that is you take a metal salt and then you reduce it; so you add a chemical which will actually add electrons to the metal, or take them away. In this case, it takes them away, so they become neutral. And what that starts is an aggregation, and these little tiny seeds of metal start to grow. The trick however is to get them to grow in the right way, and to all stop growing at the same time. So if you could imagine it's like a whole load of little tiny chunks of metal and they all start slowly growing together, and then you stop it when it reaches the right size or you've used up all the metal ions in the solution. Even though it's using gold, there's only a tiny amount of gold in this bottle here and already gives a very strong colour. So the interactions with the light are very strong so you don't need much gold to do this.

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