In Our Time: The Science of Glass

Glass is odd stuff. We've been making it so long that one tends to forget that it's both artificial and really quite odd. The In Our Time episode about glass talked both the science of glass and glass-making, and the history of it. The experts discussing it were Dame Athene Donald (University of Cambridge, current Master of Churchill College, my old college, but here in her context as a physicist), Jim Bennett (University of Oxford) and Paul McMillan (University College London).

On the programme they intertwined the historical and the scientific discussion, but I thought the joins showed rather more than they usually do and so I'm going to split the threads up in my writeup. We first know of glass manufacturing about 5,000 years ago, by the ancient Egyptians who made beads of it initially. Over time they learnt to make larger and more complex objects like bottles & ornaments. The Romans developed the technology further. They invented most of the techniques that were used before the Industrial Revolution, like glass blowing for example. In ancient Egypt glass was primarily used for decorative or luxury goods, but the Romans used glass for both everyday and finer objects - including wine bottles (which struck me as an awfully modern way to store wine!).

In the Renaissance era the Venetians were famed for making particularly fine quality glass. The city attempted to keep a monopoly on glass-making by keeping their methods secret & forbidding glass-makers to leave the city. Which didn't entirely work, unsurprisingly. One of their secrets was a way of making very transparent glass which was useful for lenses. Something I learnt from this programme was that spectacles first appear in the 13th Century AD which is much earlier than I'd assumed. Once lenses were being made to correct people's sight it was only a relatively short step to making lenses for scientific instruments. Glass is part of the Enlightenment's scientific revolution - not just lenses but also for making scientific instruments or vessels. There is a feedback loop between the demands of the scientific experiments driving new glass making technology and better glass instruments expanding the possible experiments that can be done. Industrial production of glass as we know it today begins in the Industrial Revolution.

The whole of the history discussion was very Eurocentric so I had a little look on wikipedia after we'd listened to the programme to see whether this was a fair reflection of the world history of glassmaking. The answer (based on a tiny amount of effort on my part) is ... maybe? Glass making in China appears to've arrived late - during the Han Dynasty and probably influenced by trade goods from the Roman Empire. I didn't find anything about the Americas, so I don't know if that means they didn't invent glass making or if no-one cared enough to add it to wikipedia. It's odd to think that something so ubiquitous today might've been discovered once & once only.

Making glass (not good glass, just glass) is deceptively simple. In essence the process is to heat up sand till it melts, and then cool it very quickly and you end up with the transparent solid that we call glass. One of the experts pointed out that the necessary temperatures are those that would be reached by a bonfire on a beach - so it was probably discovered in Egypt by people (briskly) putting out campfires in the desert. Although a large body of empirical knowledge of how to make glass was built up over the next 5,000 years it was only relatively recently that we gained any understanding of what is actually going on, and the science of glass & glass-making is still not entirely understood. It's actually more difficult to make glass out of pure sand than when there are impurities present, pure sand needs a quicker cooling step. So when making glass other things are often added - like potash or lime.

One of the complicated things about glass formation is that the phase transition from liquid sand to glass is not well defined - which is an oddity in physics. An example of a well defined phase transition is that from liquid water to ice: it happens at 0°C no matter how you cool the water. But the point at which liquid sand becomes glass depends on the precise starting conditions and the precise heating & cooling regimen - and it isn't predictable using the current state of knowledge. Glass isn't even a usual solid - it's not crystalline, and that's why the speed of cooling is important. If it cools too slowly it will crystalise and you don't get glass. So instead of the atoms lining up in neat little rows they appear to just stop where they are. This non-crystalline nature of glass is what gives it some of its characteristic properties. It is brittle because there are no planes of atoms able to spread over each other when pressure is applied. I think they also said that the transparency is down to there being more routes for light to take through the structure, but I'm not sure that makes sense to me so I may've mis-remembered.

Glass in the technical sense is a broader term than just silicon glass (the stuff we generally call glass). You can make a glass using sugar - that's what sweets like glacier mints are made of. And something I knew but had never really thought about is that spectacles & things like motorbike crash helmet visors aren't made from silicon glass. Instead they are made using large polycarbon molecules - these can never crystallise so are much easier to work with. And the glass produced is not prone to fracturing, which is obviously important in those usages. I assume there are other downsides which mean we don't use these glasses for all applications.

From the title I hadn't expected this to be as interesting as it was - I didn't realise how much wasn't known about glass (nor how unique a discovery it was).

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