Creating plastics that are hard enough to withstand scratching is a major goal for materials science. Such a discovery could then be used to make lighter windows for aeroplanes, trains and cars and screens for mobile phones, but so far it seems completely out of reach. In the meantime, we have found another solution to the problem: instead of replacing glass, just make it safer.
Such glass is called toughened glass and was invented by the automobile industry in order to reduce the injuries caused by shards of glass in car crashes. The scientific origin of toughened glass is to be found in a famous curiosity of the 1640s known as Prince Rupert’s drops. These are teardrop-shaped pieces of glass that can withstand intense pressure at their rounded end, but if they incur the slightest damage at the tail end they will explode. Prince Rupert’s drops are very simple to make: all you need to do is drop a small piece of molten glass into water. The extreme and rapid cooling of the outside of the droplet puts the surface layers of the glass into a state of mechanical compression. All of the glass here is pushing in against itself, and as a result cracks find it very hard to form since the compression stress is always pushing the sides of the crack back together. This has the effect of toughening the outside of the glass to the point where the glass drop can, incredibly, withstand even a hammer blow. However, to maintain this compression stress in its surface, the laws of physics require an equal and opposite ‘tensile’ stress in its interior. As a result, the atoms in the middle of the drop are in a state of high tension: they are all being pulled away from one another. They are, in effect, like a small explosion waiting to go off. If the surface compression becomes ever so slightly unbalanced, which can be achieved by making a small indent in the tail of the drop, a chain reaction courses through the whole material as all the atoms in tension snap back into place – and the material explodes into countless tiny shards. These shards are still sharp enough to cut, but they are small enough not to do any major damage.
Getting windscreens to behave in a similar way was just a matter of finding a method for cooling the outside of the glass fast enough to create the state of compression found in Prince Rupert’s drops. The material that resulted has saved countless lives in car crashes, where it dissolves characteristically into millions of tiny shards. Over the years, glass has been made even safer. The latest generation of safety glass, called laminated glass, mostly used as automotive glass for windshields is a new product which although it can shatter in the manner of Prince Rupert’s drops, the fragments of the windscreen are hold together in a single piece.
This new generation of toughened glass has a layer of plastic in its middle, which acts as a glue keeping all the shards of glass together. This layer, known as a laminate, is also the secret behind bulletproof glass, which is essentially the same technology but with several layers of plastic embedded at intervals within the glass. When a bullet hits this material, the outermost layer of glass shatters, absorbing some of the bullet’s energy and blunting its tip. The bullet must then push the glass shards through the layer of plastic beneath it, which flows like tough treacle thus spreading the force over a wider area than the point of impact. No sooner has it got through this layer, then the blunted bullet encounters another layer of glass, and the process starts all over again . The more layers of glass and plastic there are, the more energy the bulletproof glass can absorb.
- 1 layer of laminate will stop A 9mm calibre pistol bullet,
- 3 layers will stop a 44 Magnum Pistol bullet,
- 8 layers will stop a person with an AK-47 from killing you.
Of course, there is little point having a glass bulletproof window if you can’t see through it, so the real challenge lies not so much in the layering of the materials but in matching the refractive index of the plastic with that of the glass, so that light is not bent too much as it travels from one to the other. This technologically sophisticated laminated safety glass is more expensive to produce, but it is increasingly a price we are prepared to pay to enjoy its benefits. The material is popping up all over the place, not just in cars, but across modern cities as they become more and more like glass palaces.
For example I remember when in the summer of 2011 there were riots in many city centres of the UK. Viewing the TV footage I couldn’t help but notice a difference between these and other such riots I had seen in the past: occasionally the rioters were unable to break the windows of shops by throwing a brick at them because many businesses had installed toughened safety glass. This trend is likely to increase, the shops using glass not just to present their wares but to protect them too.
This same laminated glass has been proposed as the material for new safety beer glasses, hopefully putting an end to the use of glass as a weapon in bars and pubs. It is now impossible to imagine a modern city without glass. On the one hand, we expect our buildings to protect us from the weather, this is what they are for, after all. And yet, faced with a prospective new home or place of work, one of the first question people ask is: how much natural light is there? The glass buildings that rise up every day in a modern city are the engineering answer to these conflicting desires : to be at once sheltered from the wind, the cold and the rain, to be secure from intrusion and thieves, but not to live in darkness. The life we lead inside buildings, which for many of us is the vast majority of our time, is made light and delightful by glass. Glass windows have come to signify that we are open for business, and that the business will be honest and open – a shop without a shop window is practically not a shop at all. This material is also instrumental in how we view ourselves. You may be able to see yourself reflected in a shiny metal surface or a pond, but for most of us it is the glass mirror that has become the final, intimate arbiter of our self-image. Even photographs and video representations are mediated through the glass lens.
It is often said that there are very few places left on Earth that have yet to be discovered. But those who say this are usually referring to the places that exist at the human scale. Take a magnifying glass to any part of your house and you will find a whole new world to explore. Use a powerful microscope and you will find another, complete with a zoo of living organisms of the most fantastic nature. Alternatively use a telescope and a whole universe of possibilities will open up before you. Ants build cities at their scale, and bacteria build cities at their scale. There is nothing special about our scale, about our cities, about our civilization, except that we have a material that allows us to transcend our scale – that material is glass.
Yet we have no great love for the material that has made this possible. People do not tend to wax lyrical about glass in the way that they do about, say, a wooden floor or a cast-iron railway station. We do not run our hands down the latest double-glazed panel and admire the sensuality of this material. Maybe this is because in its purest form it is a featureless material: smooth transparent and cold. These are not human qualities. People tend to relate more to coloured, intricate, delicate or simply misshapen glass, but this is rarely functional. The most effective glass, the stuff we build our modern cities from, is flat, thick, perfectly transparent, but it is the least likable, the least knowable: the most invisible. For all its considerable importance in our history and our lives, glass has somehow failed to win our affections. When we break a glass window it is shocking, annoying and painful in the case you have a car accident; but we do not feel that we have broken something that is intrinsically valuable. In these situations we are worried for ourselves, but as for the glass itself, it can be replaced. Perhaps it is because we look through it rather than at it that glass has not become part of the treasured fabric of our lives. The very thing that we value it for has also disqualified it from our affections: it is inert and invisible, not just optically, but culturally.