In Our Time - The Science of Glass

Episode Date: May 28, 2015

While glass items have been made for at least 5,000 years, scientists are yet to explain, conclusively, what happens when the substance it's made from moves from a molten state to its hard, transparent phase. It is said to be one of the great unsolved problems in physics. While apparently solid, the glass retains certain properties of a liquid. At times, ways of making glass have been highly confidential; in Venice in the Middle Ages, disclosure of manufacturing techniques was a capital offence. Despite the complexity and mystery of the science of glass, glass technology has continued to advance from sheet glass to crystal glass, optical glass and prisms, to float glasses, chemical glassware, fibre optics and metal glasses.With:Dame Athene Donald Professor of Experimental Physics at the University of Cambridge and Master of Churchill College, CambridgeJim Bennett Former Director of the Museum of the History of Science at the University of Oxford and Keeper Emeritus at the Science MuseumPaul McMillan Professor of Chemistry at University College LondonProducer: Simon Tillotson.

Thank you for downloading this episode of In Our Time. For more details about In Our Time and for our terms of use, please go to BBC.co.com.uk slash Radio 4. I hope you enjoy the programme. Hello, around 5,000 years ago, the Egyptians were using glass to make beads, melting sand at very high temperatures and cooling it rapidly in water. Ever since glass has been deeply puzzling. Scientific advances reinforced the urge to understand glass

from spectacles, prisms and telescopes to optical fibres and the windows on space rockets to mobile phone screens today. How can it be made stronger or clearer, and what does it do to light? Many glassmakers have kept their methods secret. In Venice in the Middle Ages, disclosure was punishable by death. Scientists still don't precisely understand what happens when sand moves from a molten state to a hard, transparent phase, when it appears to have solid and liquid properties.

With me to discuss glass, one of the great scientific puzzles, are Dame Atheney Donald, Professor of Experimental Physics at the University of Cambridge and Master of Churchill College, Cambridge. Jim Bennett, former director of the Museum of the History of Science at the University of Oxford and Keeper Emeritus of the Science Museum. And Paul Macmillan, Professor of Chemistry at University College London. Athenny Donald, glass starts as something solid, mostly sand,

then it's molten and once cooled it's a very different solid. What do we need to know about the different states of matter in that process? The three familiar states of matter are gases, liquids and solids. And if we think of something like water with whichever one's familiar in each of the three states, it's quite easy to describe what's going on. So in a gas, the atoms or molecules are far apart. They're moving very fast. You cool down into the liquid phase.

The molecules of water, in this case, would get much closer together. Their movement is much less. And you can think in the liquid state of the molecules, mainly just sort of vibrating in quite large distances, but just moving around a set position and occasionally swapping positions, which is why the liquid can flow. And if you take water, when you cool it to ice,

the molecules sit on a lattice in a regular way, and that's what we're all familiar with. Now, in the case of forming a glass, you go from the liquid state when these molecules or atoms are moving around a fair bit, and you cool it down and at some point they stop moving but they are not in a regular lattice, they're not in a well-defined position.

They've essentially just got frozen in from the positions they were in the liquid and they are not moving very fast. So the transition from the liquid to the glass is actually not a very well-defined transition. It depends on how you prepare the glass exactly where you would measure that transition. whereas if you freeze water, you know exactly when that's going to happen. It's a well-defined phase transition.

So we owe glass to a not very well-defined transition. That's right. And how you prepare that glass, you will end up with glass with different properties. They will vary. So it's one of those things that seems extremely simple and gets better the more you're digging into it. Can we dig into it a bit more with glass? I mean, everybody knows about water to ice. That's fine.

So that's established. That's the platform. Now what happens with glass? You've got sand and other bits with it, and then what happens? Okay, so when you've got your molten sand and you cool it down. You just heat it and heat it. You just heat it up. There's nothing very magical about it.

I mean, one of the interesting things about many of the glasses that we're so familiar with is that they are very familiar materials. So you heat up the sand and you make it into a liquid, and then you just cool it down fast. And the trick is to cool down sufficiently fast that with a material like that, you prevent it crystallising. If you cooled molten sand down very, very slowly, it does have the potential to crystallise. Some materials, you have to cool down incredibly fast if you were going to suppress that crystallization, because the crystallization is the atoms or molecules

slotting into regular positions, and they've got to rearrange themselves in a way to get to that state. So you don't want it to be, you don't want the molecules to be in a regular position, You're trying to avoid that. That's right. When did they discover that they had to try to avoid that? Well, I mean, I think historically it was just an empirical finding that you got this transparent stuff. Probably people didn't really understand the structure until they could start doing things like x-ray scattering and actually analyze the packing in detail. And you mentioned the phrase phase phase transition.

Can you just explain once more precisely what you mean by that? A phase transition is a point at which you move from, say, the liquid transition. phase to the solid phase and it is a thermodynamic transition temperature which will be defined by thermodynamic laws. But the glass transition, it only looks sort of vaguely thermodynamic. It isn't a true thermodynamic phase transition because it depends on how you do the measurement and how you prepare the glass. So it's not a well-defined transition.

So Jim Bennett, what is glass? Despite its familiarity, its everyday familiarity, it is in many ways a paradoxical, of material. It's extraordinary and paradoxical. So you'll get a number of answers to that question. One I would like to emphasize is it's manufactured nature. It seems obvious that this is a manufactured material, but that's very important to the way people thought about glass at the beginning.

As to say, it doesn't occur in nature, or at least only very rarely. Naturally occurring glasses are quite rare. So we make glass. And that affected the way people thought about the whole process Sand, of course, affected the way it was developed. So how do we make it? Well, as we've heard, sand is a basic component. We add sand to, we add lime and soda.

We heat it up. We heat it up in a particular way. It's not quite, even in ancient glassmaking, it's not just a matter of heating it vaguely and entirely empirically. There are furnaces for doing this. There may be a couple of stages and so on. And then we cool it down and it becomes solid. One of the things I would, a simple distinction that I would like to make, which is perhaps helpful,

if we think of the other common material which is used for everyday objects, pottery, ceramics, with ceramics, you mould the material when it's cold at ordinary temperatures, and you fire it to make it rigid and permanent. With glass, it's the opposite. You mould it, it's malleable and shapable when it's very hot, and you cool it down to make it rigid and to retain that shape. And that's a simple difference, but it is a fundamental one, and it's a helpful one to think about it.

But I would want to emphasize that glass is made by people, by an art, the art of glass making. It's an artificial product. Is there any sense in which you're trying to retain some of the liquid nature in the solid finish? Not at the start, of course. As we have heard, there's a lot of them empirical, unknowing, if you like,

aspects to the original manufacture of glass, except that I want to push back a bit against that mere empirical story. You'll read that in most of the books, that's to say that glass until the 19th century is just an empirical process, and then we understand it scientifically. Glass doesn't come out haphazardly from any process.

It's a complicated business, despite the fact that it's a complicated, practice, even if we can describe it simply. There are large facilities, complicated facilities for making it. There are teams of very specialist, trained, experienced glassmakers, even in ancient times. So there is knowledge there. It's encoded differently from the way we encode scientific knowledge. It's encoded in a plant, in the skill and experience of the makers.

And in the recognition of all of that through guild structures and so on. but it isn't, it's a form of knowledge, maybe not scientific, but it is a form of knowing. Let's go back two steps. First with the Egyptians, who were the first we know about, is it 3.5,000 BC who were making glass. How are they doing it? Yes, it's exactly that process, heating sand, soda and lime and cooling it in furnaces. They are indeed... It's a fantastic temperature. Yes, well, we would, as we would say, perhaps in the first melt it, it.

200 degrees centigrade, a thousand degrees centigrade eventually. You're right, it's a third millennium BC that we're finding Egyptian jewelry, beads and so on. And then from the second millennium, about 1,500, there are a number, a large number of vessels and objects that are formed in glass. The notion that how do they understand it is an interesting one. And that's where I want again to emphasize the manufacturedness of it.

Historians now think of these early practices in an alchemical way, if you like, or even in a pre-alchemical way. That's to say we as artificers can make objects that look like precious stones, that look like minerals, and have some of the properties of precious stones. So there's a transmutational process going on here. and it encourages the notion that maybe we can do that with metals as well. The Romans did a great deal with glass from, let's say, for 1,500 years they were in charge of the operation, right across the Roman Empire.

And you said they developed methods, I think it was your, all of you said in Europe, that they developed methods which did go through those 15 centuries. Absolutely. The Romans can do pretty well all of the processes that become commonplace in glass making. the Romans are blowing glass, free blowing glass, for instance, it is understood in the first century BC. And indeed, it's through the Romans that glass making becomes very widespread through the empire.

And we find glass works in all parts of Europe, really. Refined glass windows and... Well, well, the manufacturing plant. Windows would... It's true that Romans did have glass windows, but they would have been very special. That would have been a very special. that would have been a luxury item, a glass window.

But it was it a big manufacturing business than making of glass? It is indeed. In the Roman environment. Yes, indeed. And the factories, if you like, are large. That's what's so special about glass that this is made in large facilities with teams of specialist workers. Paul McMillan, can you tell us about the atomic structure of glass? Okay.

That's a really interesting question because it goes right to the. heart of why scientists are interested in glass because they're trying to work it out. I'm going to go back to some of the things that Atheney said and then also that Jim was said. So first of all, the reason for the challenge and also for the interest is because the atoms in a glass are disordered compared with the atoms in a crystal where they're all in perfectly regular array of positions. I'd like to imagine if you've got a box of of marbles. You can put them in a biscuit tin, for example. It makes lots of noise. And if you shake that around really gently, or else if you put the marbles in just one by one, then you can place them

in perfectly regular rows where all of the marbles or atoms are touching each other equally. And then once you've completed that first layer, you can make rows in the second layer. And that's a crystal. That's an atomic crystal. Everyone perfectly ordered in perfect position. Now, you take your box of marbles and you shake it around and then you drop it down really fast. The marbles or the atoms don't have time to get back to their proper positions. Some of the holes get blocked up with ones that are in the wrong positions. That's a glass.

That's a disordered solid. It's the glass. Now, that works really well for a glass that contains only a single kind of atom. That's not the kind of glasses that we, that we, used to. Those glasses are based on two or more different kinds of atoms. They've got metals in there like silicon and sodium and so on. And those are held together with non-metals or anions like oxygen. The oxygen provides the glue that holds together the metal atoms. Now, in a crystalline mineral like quartz, that's the main mineral in sand, then you've got these

little tetrahedra, a little pyramid with silicon in the centre and oxygen atoms on the four corners. That's the basic building block. And then those are all linked together to form polymers, inorganic polymers. In the crystalline mineral, all of those tetrahedra are the same as you go through the structure and all the linkages between them are the same. In the glass, the tetrahedra are all in red, random orientations and the linkages are all random and so that's where the disorder comes from. Now there's even more in a glass that if you change the glass composition slightly away from sand, away from pure sand, put a little bit of soda in or a little bit of lime in, then you start to break up this glassy network and so you can get a whole set of different linkages present at the same time.

And it's this mixture of all these different structural units that make glass different to crystalline structures. Is sand essential to the making up glass? That's the main component for most of the glasses that we are familiar with. The trouble with trying to melt pure sand, if you go out and find yourself a really nice white sand beach, then most of the tiny little crystals that make up the sand are pure quartz, pure silicon dioxide.

The trouble is that that doesn't even begin to melt until you get to about 1,700 degrees centigrade. It's just far too high for most furnaces. You just can't work it. So that's why even early people, the Egyptians and Romans, started to put a bit of soda ash in. Because as you put in the soda, that drops the temperature. You make this mixture and it drops the melting point. So you can make a liquid at much lower temperatures. at the temperatures that you could get underneath a beach fire, for example,

and then you quench that rapidly. Then you can start to add other things into it. You can add other metals, other metal oxides. You can add potash, a bit of alumina. But then you can move away, and instead of silicon, you can replace that with phosphorus.

Athenia, thank you very much. Do we properly understand, that's a very clear explanation, as clear as you, and I could possibly. Do we still properly understand the structure of glass? You can determine it. I don't think you can predict it. So I make... Because of what Paul's been saying. Exactly.

And as I said earlier, it does depend, the structure you end up with does depend on how you prepare it. So to go back to the analogy of the marbles and the biscuit tin, depending on exactly how you shake it. You have exactly the same marbles and biscuit tin, but you will end up with different structures. And so it's very hard to predict what you're going to end up with, given any starting information, and that's not true for crystals.

So it's very hard to understand that, and it's very hard to predict when you will pass through this glass transition, that the theories are really not well developed. I mean, there are theories and they work up to a point, but I don't think anyone would say that they had an absolutely fundamental, we understand it all approach to the glass structure. Is it true that no two glasses are identical?

In a sense, yes, because the precise positions of all the atom-stroke molecules will be very specific to exactly how you've got to that point. And, I mean, you can take a piece of glass, and of course this is hugely important in preparing good window glass, and you can take a piece of glass and prepare it and then you can anneal it. So you hold it at some temperature and let the molecules or atoms just rearrange slightly and you will get an improvement in perfection. You will remove some of the internal strains.

You will actually be able to measure an increase in the density. And so take exactly the same material and hold it at temperature for a bit and you change its fairly fundamental properties. Jim, Jim Bennett, after the Romans, we still interle with the Venetians took over. There's lots of people doing things in between. We're taking the peaks here, and they went into a glass development. Can you tell us something about that and what they did? Yes, well, we know of glassmaking from archaeological evidence around the Venetian area from about the 8th century,

but it's really in the later medieval period that it comes into its own, famously on the island of Marano where the Venetians were pushed their glassmaking and their plant, particularly their furnaces so as to preserve the city and to take all the noxious fumes and so on off elsewhere. And that meant in effect

that there was a kind of a research institute, if you like, or a sort of cluster of skill on an island. As you said in your introduction, It was a capital offence to take your knowledge and the secrets of glassmaking. Some people did, of course. But nonetheless, it was highly prized. And in Murano, there was the possibility of specialist work. It isn't that there was a particular secret that you could identify very precisely.

But there's a concentration of effort. There's a focus on the purity of materials. For instance, we were talking about what sound you might use. in Morano, they found that if you used quartz pebbles, powdered, heated and cooled and powdered, that would make a purer basic material than everyday sand. And other materials were brought in. The process was repeated and refined.

There was a lot of skill involved and so on. And you ended up with a very... For example, they discovered that if you add manganese oxide to the melt, you could get a glass with a greater clarity. So Venetian glass, as a result of all this... combined effort became the envy of Europe. And everyone wanted Phoenician glass and many people wanted to know how to make it.

And how long was it a luxury good before it became trickled down to the... And we're difficult to talk about classes. It's a lot difficult. How much of a luxury good was it for how long? The nice thing about glass is that it spreads throughout that whole spectrum. It's an everyday good and it's a luxury good. And that's still the case.

That's very interesting that it, that it has that spread. So even the Romans were making glass objects of all different sorts, and so were the Venetians. One thing I would want to say that we mustn't lose sight of is that by this time, we've already got lenses. So we have spectacles from the end of the 13th century. We're coming to spectacles.

Paul McMillan, can you tell us it's been developed a bit? here, but how the properties of glass change, glass has changed when they're made from different substances? Well, that's one of the great beauties of glass, is that you can change its chemical composition a little or quite a lot and still have glass, but you can use these changes in the composition to change any property that you want. So, for example, the first property that I talked about was actually the ease of making glass. So that just by varying the amount of soda or potash that you put into the mixture, you vary the temperature that you melt at.

And so you make it easier or more difficult to make glass. But then you discover that when you've put some soda into the glass, it's no longer chemically resistant. It starts to dissolve in water. And that's even worse if you use plant ash, if you use potash to make the glass. So then what you can do is you can add a little bit of lime into it, and that hardens the glass up. And then you can add a little bit of alumina,

and that will improve the mechanical properties as well. So you've got this very, very wide range of possible chemical changes, and a lot of that was empirical right at the beginning. Some of it's still empirical now. There's a really good cooking analogy that making a good glass, is a little bit like making really good pizza. You've got a whole set of possible ingredients and you just put a little bit of this one in,

a little bit of that one in, until you get the flavour that you want, it's still a pizza, but you can change it almost constantly. People who want to know, what I want to do, about colour. When did the variety of colours, the depths of colours, come on board?

Again, that's where we need to go back to Jim and back to the Romans, and also to a question that you asked about how readily available glass was. one of the great reasons that the Romans liked glass was to make glass bottles because the Romans like wine still do and so this let them have a container it was chemically unreactive they could blow it into these complex shapes to hold liquid in

and they could appreciate the colour of the wine which is a major part of its appreciation and it would store for a long time the problem is that the glasses that were that were made at that time were impure. And so the colours, these were sort of brownish, brownish green. Gradually, as people started to experiment with using different sands of different purities,

they began to realise that they could control the colour of glass and then making perfectly transparent glass was a major advance. By that time, we'd started to develop knowledge that different impurities like iron or manganese, or chromium or copper would cause the colour of glass to change in specific ways and that then led to these specific additives

that we used to create all of these beautifully coloured stained glass windows. I think Adonnell, what effect does glass have on light? Well, obviously the key thing, as we've just heard, is that glass by and large is transparent exactly how transparent and what colour it comes across out. will vary. But that's a key factor in the use of glass. Our windows would not be nice if it was

not transparent. It wouldn't serve the purpose at all. And that arises because of, again, because of the disorder. So that the way light interacts with glass, unlike, say, a metal is very, very different and all the visible light will essentially get through. So that is one key finding. Was it a bit of a puzzle that glass being a solid, light went through it, it didn't go I don't know when that became a puzzle, as it were. I think it was possibly only, as people understood better what the nature of the interaction of light was, that it became a puzzle. I think it was probably something that people just accepted that that's how it was and it was wonderfully useful. I think where glass started being used as a material for scientific experiments, if you like,

and another key point about the interaction of light with glass was when people started using it, like Newton, for instance, as a prism. When they were shining the light through, so the light would go through, but you can separate the wavelengths of light, and this is key in understanding what white light is, because of the phenomenon known as refraction.

So that when light enters a prism, different wavelengths in that white light spectrum will be refracted to different extents. Refraction is the phenomenon that the light is bent when it enters, in this case, the denser material that is glass. So you separate the white light out into a spectrum and that was hugely important

in understanding what light was. What I really like about Newton's experiments and it's beautiful piece of doing the right experiment was having separated the light into its component parts, and people could say, well, it's something in the glass. You know, that's not a property of the light itself. It's something about the prism. So he then put the light back through a second prism

and recombined it to make white light, showing that it wasn't something in the glass. And I just use that in my lectures because it's such a nice example of designing an experiment well. But it all comes back to this phenomenon of refraction, which people will be very familiar with if you look, well, if you look into it, put a straw in a glass of water, I'm looking at the glass of water on the table,

you would see that it looked bent, and that is all about refraction. And Jim Bennett, we're back to your beloved spectacles. Now then, the demand for spectacles and telescopes brought glass into a new and powerful world, and it's never stopped really since then. Can you give us a resume of how that happened and Dave, I thought to do that. Yes, I think it's very important. important that spectacles appear on the scene because that gives an imperative to the refining

and development of optical glass as it turned out to be. And we have spectacles for long-sightedness, I say convex lenses from the end of the... What period are we talking about? The end of the 13th century. And then we have 14th century portraits with people proudly wearing their spectacles. It shows you are scholarly, because of course if you have a pair of spectacles, then you can continue your scholarly life into older age. And then in the mid-15th century you have concave lenses for short sight. So you have the two components of a telescope, of the early telescopes of a convex and a concave lens. The telescope the Galileo used, for example. And Galileo got his glass for the lenses that he made or helped

to make from Murano. So it was very important that he had access to Venetian glass for his telescope making. And the coming of lenses and spectacles then created a new sort of cadre of glass making and glass working. Spectacle makers formed a specialist group within the glass working fraternity and then even a more specialist group with optical instrument makers. So this dynamic to make improved optical instruments

and of course the telescope and the microscope are at the foundation of so much of what science has achieved. But again you seem to be talking about, which is very attractive really. There's a closed group of what we would now call artisans craftsmen, craftsmen really, master craftsmen, aren't there? Keeping inside the group and there's almost a necessary secrecy about it, which sometimes works against the development. Well, that's true. You can identify moments when it works against the development.

There is secrecy in glassmaking, that's true, and in glass working. I don't think that it's much more secret than many trades. There are obviously gill structures for many artisanal practices, and the glassmaking makers are better organized than most. On the other hand, they are a very specialist trade and they require specialist processes and apparatus. It's not like a carpenter or a joiner,

for instance. It's much more refined and special than that. So I think there is indeed secrecy, and Morano is a good example of the secrecy, but it's probably much similar to other artisanal practices in the time. So, Paul McMillow, when we're getting to

spectacles, we're getting to people, demanding things, aren't the individuals saying, well, they aren't strong enough, I can't read Prince too small, or is it your glasses? You've made the wrong spectacle. So we have a more demanding public and where does that take us? And also

a more demanding scientific public because they want telescope lenses that can focus better. People developing microscopes because they want to be able to see smaller and smaller things. And

All these fields are linked. So before I carry on to answer the question, because it's related back to what I was saying about the continuously variable composition of glass, we've got to go back and think a little bit more about what Athene was saying, about how light gets through a solid. Because light in free space travels at this constant speed,

really high speed, three times 10 to the power 10 centimeters per second, right? Once it enters a solid, it's got to get through, and it does that by moving the electrons around inside the solid. That slows the light down. So basically, the more electrons you've got, and the more closely packed they are together, than the more you slow the light down.

Now, in a glass, if you decided that you wanted to make a glass that would really slow the right light right down, maybe to about half of its speed in air, then you would want to put in lots of heavy elements. And so that was done by putting lots of barium in. It's a really heavy metal with lots and lots of electrons. And that made the set of flint glasses that were developed in the UK.

Brown Henley, there's a lot of flint. Yeah, exactly. And so with these flint glasses, people could then make lenses that would focus the light much better and those were used for some of the best telescopes and microscopes. Problem is that all these heavy elements started to make glass a wee bit difficult to wear on your nose. Nowadays your glasses and ours, Athene is not wearing any. I'm wearing lenses.

There we go. Those are even worse. but those are not made out of the silica glass anymore. Instead, they're made out of a polymer glass. They're made out of polycarbonate glass. Can we talk about that for a second then? The polymeric glasses, how do they relate to what we've discussed? Okay, so what we've been talking about in the case of the silica glasses are materials that, if you cool fast enough, you can stop them crystallising.

Now, many of the polymeric glasses are inherently disordered. there is no way they can crystallise. So you can cool at any rate you like and you cannot form a regular structure. So if you think a polymer, it's a long chain molecule, carbon, carbon, backbone, various other things hanging off the side

and those side groups are often fairly irregularly distributed. So you are prevented, well, it prevents it from forming a regular crystalline structure. So polymeric glasses are exactly the same as we've been talking about,

we're talking about huge molecules which simply cannot form a crystalline structure. And that means they're really easy to work with because you don't have to worry about the cooling rate. Some of the more obscure glasses, we haven't talked about the metallic glasses, for instance, you have to cool at phenomenal rate, sort of a million degrees a second or something. Polymeric glasses, you can do anything you like and you still end up with an amorphous material, which is then useful for contact lenses, four optical fibres, for crash helmets, for all kinds of purposes, and they are very valuable. We're moving now into developments, Jim,

and most strange things are now happening with glass because of glass. Let's start with a leap that a lot of people, including myself, when I read it, I think is rather odd. Suddenly glass becomes very important in the discovery of electricity. Yes, absolutely vital. We've talked about moving electrons around. Well, the other way you can move electrons right is just by rubbing them around. I know that sounds a bit crude.

but attrition can strong attrition can move the electrons around on the surface of the glass or close to the surface and electricity as a phenomenon is discovered then by the rubbing wool on glass on some of the materials as well but

glass becomes the material of choice because it's fairly common and it works very well and it's a it's an insulator so if you move the electrons around and you take some of them off with the woolen cloth, the static charge that you create on the glass

will not disappear because it's preserved on the glass. And then you can draw it off. You can draw it off with glass spikes and you can take it somewhere else and you can store it.

You can store it, oddly enough, in a bottle, the luyden jar. You can store it in a bottle or you can store it on a big globe or something. And then you can do things with it. Now, the whole of electrical research which was substantial

in the 18th century is carried on through that process. through static electricity. And electricians, as they were called, were making better and better machines for doing this rubbing, for mechanising the rubbing, turning glass discs round, rubbing them against leather and against wool,

and drawing the glass off. And then at the end of the 18th century, well, suddenly you have current electricity, and that whole process loses its dynamism. But that's where the study of electricity begins. So Paul McMilland, glass is now moving into industry, and engineering, and you think of it very highly in those areas.

I mean, optical fibres differ, but they differ from the glass in windows, having many different purposes. But you say it's one of the greatest elements in engineering. Well, I'm not quite sure that I said that, but it certainly is... You wrote something like that. I'm sorry. I couldn't make that up, so... No, no, but it's certainly true that glasses of all different kinds

enter into all sorts of different technologies and some are just emerging. First of all, we spent most of our discussion talking about transparent oxide glasses that are electrical insulators like Jim was talking about. However, there's a whole different family of glasses that conduct electricity.

These are semiconductor glasses and instead of having any oxygen in there, they're based on sulfur or selenium or tellurium. And they're semi-conducting in the dark, but often they start conducting electricity when they're exposed to light. And that's the basis, for example, for the Xerox process, that you build up charge on the surface of the glass. The glass is coated round the drum of the copying machine. You build up charge in the dark. And then what you do, when you flash light on it, you've exposed your sheet to be photocopied.

the light reflects from the white bits. It causes electricity to flow through those bits on the glass surface that have become conducting. And so you've created an image. Anthony, Donald, why is some glass so brittle and other so strong? Here, an interpolation is that one reading about this, yet again in Pyrex, the hard glass was discovered for laboratory purposes. And within a couple of decades, it's used all over the world, it's manufacturing in its millions,

it's a great commercial product. That's yet another example of experiments in a lab saying, oh, we'll do this because it helps us to move to the next stage, turning into a huge... That's as maybe, right. No, that is as maybe. That's as worse. Well, I mean, the familiar thing about glass is it breaks. And if you've got...

I mean, this is true as ceramic tiles too. If you've got a piece of glass which you're trying to cut shape, you will usually introduce a notch and then it breaks much more easily. And the point about the amorphous nature is it's quite hard for any kind of local motion that the atoms and molecules are frozen in their position. So there's not much local motion.

And that local motion is what you would associate with toughness, ductility and that kind of stuff. So if you've got an atomic material like copper or something, it's very ductile because the atom, planes can slide past one another. In a glass, they can't do that. So if you introduce a notch, if you start a crack, it will just zip through the material without there being much defamation and that's what you detect as brittle. So if you want to toughen the glass, you've got to do something different to it. And one of the ways of doing that is to, again, it's back to

this annealing treatment so that you change the surface of the glass. If I get this right, and I may get this the wrong way around, you cool the outside very fast and keep the inside warmer and then you get a sort of external tension, internal compression. I hope I've got that the right way around. They'll tell you back in Cambridge if you have. They will indeed. They're sending me tweets as I speak. But that will give you something that is, it changes the way it responds to the external

mechanical stress and becomes tougher. Jim Bennett, how was glass developed for use in chemical experiments and how important Well, glass was used. We know of alchemists using glass vessels from the 12th century in the Islamic culture. And it's used in the 18th century, but only rarely. I think it only becomes commonly used in chemistry in the 19th century, a later 19th century, because it has to be resistant to chemical action, of course, and it has to be reasonably resistant to heat. But some of the glasses that are particularly good at that come out of an optical. Institute in Niena with Carl Zeiss and Otto Schott and Ernst Abbey who are looking for optical glasses and they come up with this glass

based on oxides of boron if I understand it and that's highly resistant or much more resistant to heat and that's where your parox comes from because that gets manufactured by Corning in New York under the name of parrex and so that it's sort of there's an awful lot of spinning around in

in this discipline. Because glass is so flexible in its uses, there's lots of spin-offs between the developments of different sorts of glasses. Paul McMillan, can you give us a summary, sorry when you're at the end of the programme, that's the way it goes, of the application of the newer metallic glasses? Yeah. This goes back

to something that Athenie said about how glasses break. The metallic glasses don't have any crystalline domains. They don't have little crystals and so they're just a single continuous matrix and so a crack can't propagate through them

so they're incredibly strong. And that's why they're so useful in engineering. And what's driving the development of new glasses today? Is it just people working in laboratories, Athenia? What's driving it? Well I think places like Pilkington's they have huge research efforts devoted to it. And as we've heard, I mean,

has identified lots of interesting properties. And as you said, about PowerX, people can develop this. And then the use, to some extent, comes afterwards. We haven't talked about optical fibres at all, for instance, which are hugely important in telecommunications. I know, another day. I mean, there's just so many ways in which glasses have use now and potential use in the future.

Right. Well, thank you all. Very much indeed. I'm not going to say we should raise a glass of this. I'm not going to say, and now we are speaking face-to-face. Sorry, right. We might as well get them in. Thank you very much for seeing it, Donald. Jim Bennett, Paul McMillan. Next week we'll be talking about the legend of Presta John in the Middle Ages. He was supposed to be the ruler of a lost Christian nation.

Thank you very much for listening. And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin and his guests. It's good to know something like that. I mean, I think with a lot of people, I don't know. You tell me, I think a lot of people have been amazed by what you said. It seems very obvious to use. It's amazing to most people.

I really wanted to try and slip in something about glass in cooking because all that interesting sugar work that people do, that's sugar glass. That's a glass made out of sugar with a little bit of water in it. And the glazing on a donut, that's sugar glass. So most people actually eat glass and don't even think about it. Yes, I mean, if you'd unwrapped your sweets, we'd have been very important.

A glass brunch. Yes. Glacier mints. Yes. Absolutely. Just think about what. Oh, you've got glassier mints. Yes.

No, absolutely. They're transparent, but you can cover them and you don't have to boil it to a very high temperature. And then the other thing, when you get into the polymer glasses, you get into this really interesting state of matter that's not a solid or a liquid. It's sort of stuck halfway in this glass transformation range. And we call that rubber.

One thing I wanted to try to get in I wasn't doing it deliberately by it and I forgot all about it but it's very interesting that we're sitting here 200 yards roughly speaking from Glass House Street which is at Piccadilly Circus and if the readers were thinking about

where they live there are lots of street old street names in listeners we have on radio I'm like I know Richard I know sorry about that I'm trying to dragging in the 21st century but they will

themselves know about street names which reflect the widespread manufacturer of glass So that was a local thing down that Yes and that that name was first recorded in the 17th century Just at Piccadilly Circus And it's nothing to do with

Lettuces and market gardening and so on They were making glass there So when did Mirano lose its domination? In the quite quite early in the later 16th century people were taking that technology throughout Europe and we, I mean, it's interesting, one of the things

I didn't quite agree with in the discussion is the idea of the scientific development driving everything and for example we talked about the heavy metal oxides. In fact, of course you have lead crystal before. Yes. Again, it gets adapted to optical use and lead crystals being made as you know from you

You knew about Clintass obviously in the Lits and empirical. I know you took exception to what I said. But I mean, there is a difference between having what I would call scientist knowledge and empirical incredibly smart people. All I was saying, of course, it's completely different. I mean, it's a different knowledge culture. All I was trying to say was that empirical doesn't mean unknowing. No, absolutely.

It means there's a different structure of knowledge and it's encoded in different ways. It's not. I mean, the big example that I know, and I don't know, The Industrial Revolution was way ahead of the subject, it seemed to me, from the science. Well, if you take Wedgwood and people. Yes. Yes. The other thing that a lot of people don't really understand is this incredible way in which science is embedded in culture.

Because science is done by people, for people, usually. and for example you look at the interactions the appearance of glass windows suddenly you had a way to open up the inside of houses to light you keep out the weather

you keep out dirt people become cleaner it improves public health yes you're one of the surprises going to the globe is that there's no windows when you go to the precursor this is this is Simon Gillett's on the ball

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