One of the world’s oldest products faces the digital future

AFTER 4,000 years of development, you might assume that just about everything there is to be known about glassmaking has already been found out. Not so. Though the basic recipe of sand, soda and lime remains the industry’s core, first alchemists and then chemists have tinkered with the ingredients over the centuries to produce specialised products. For clarity and sparkle in tumblers and decanters, they added lead. For heat resistance in ovenware, they added boron. For a beautiful blue colour in drinking vessels and decorative bowls, they added cobalt. To increase the speed at which light traverses it, as may be useful in an optical fibre, they added germanium. To reduce that speed, they added fluorine. And so on.

So when, one day in 2006, Steve Jobs, the founder of Apple, came knocking on the door of Corning, one of the world’s biggest glassmakers and based in an upstate New York town from which it took its name, they were ready for him. The request was for a perfectly clear, tough and scratch-resistant glass to cover the screen of Apple’s newly designed “iPhone”. Jobs, being Jobs, wanted it in six months.

Scientists at Corning’s research centre produce thousands of new formulations of glass every year. Some are promising enough to go to a small glassworks within the centre, for trial production—but only a few make it to market. Everything that is learnt, however, is filed away for a rainy day. A search in the archives in light of Jobs’ request turned up a project from the 1960s to develop a toughened lightweight glass for industrial use. The new glass had been made in small volumes, but it never took off and was abandoned. Corning reworked the formula to produce a strong, thin glass suitable for touchscreens. They also reworked the name. And thus was born Gorilla Glass.

Touchy feely

Gorilla Glass’s unique selling point is not that it is tough, but that it stays tough when formed into sheets thin enough to protect the surfaces of the touchscreens of today’s increasingly skinny mobile devices without affecting those screens’ function. That means permitting the circuits within a screen to locate the position of a finger placed on the surface. In many portable devices that is done by detecting a tiny change in an electrical charge across the screen at the point where the finger touches. Too thick a screen can make this change harder to detect. Since its launch, Gorilla Glass has been getting thinner and tougher still. According to Corning, a sheet less than 1mm thick, made of the fifth generation of the stuff (the latest iteration, released last year), can survive four times out of five if dropped facedown from a height of 1.6 metres (63 inches) onto a rough surface. As a consequence, Jobs’ job was but the first of many. Gorilla Glass is now found in some 5bn smartphones, tablet computers, laptops and other devices produced by electronics companies around the world. It is beginning to appear in other things, too, including cars—an ironic development, as the motor industry, one putative destination of the original version from the 1960s, had rejected it back then.

Two tricks give Gorilla Glass its strength. One is its composition—or, rather, the way that composition is modified in the middle of the manufacturing process. The other is a detail of this process itself.

The material starts off as a mixture of silica, aluminium oxide (a standard strengthening agent) and sodium oxide. This mix, once molten, is turned into a sheet using the “fusion draw” process, a technique pioneered by Corning. Fusion drawing involves pouring molten glass into a V-shaped trough and letting it overflow down the sides of that trough, clinging to them and running down them as treacle might cling to and run down the outside of a bowl.

As the two streams of glass meet at the bottom of the V their inner surfaces fuse into a single, thin sheet. Because the outside surface of each stream has had no contact with a production surface, those surfaces do not pick up any contamination or other damage, and emerge flat and devoid of defects. Materials break at their weakest point. For a sheet of glass that is often an impurity, crack or scratch on its surface. Fusion drawing eliminates such weakness.

The next stage, modifying the glass’s composition to impart strength throughout its volume, involves immersing the fusion-drawn sheet in a hot bath of potassium salts. This results in a process called ion exchange, in which sodium ions within the glass are forced out and replaced by potassium ions from the bathing solution. Sodium and potassium are chemically similar, which is what permits this to happen. But potassium ions have about two-and-a-half times the volume of sodium ions. When the glass cools, this extra ionic volume compresses the material from the inside. That makes it more resilient to knocks and scrapes.

Gorilla Glass dominates the market for cover glass for electronic equipment, but it faces rivals, including Dragontrail, a chemically toughened glass manufactured by Asahi Glass in Japan. This is made using the float-glass system, in which molten glass is floated onto a bed of molten metal. (The technique was invented in the 1950s by Pilkington, a British glassmaker which is now owned by NSG, a rival Japanese glassmaker.) Another potential competitor is sapphire glass, which is not really a glass at all, but rather a crystalline material that is a synthetic version of the eponymous gemstone. Sapphire glass is extremely hard and is used in some high-end watches, but it can be heavy and is more expensive than Gorilla Glass—though researchers are trying to reduce both its weight and its cost.

Corning, meanwhile, is pushing Gorilla Glass, and other specialist glasses made by fusion drawing, into more areas. According to Jeffrey Evenson, the firm’s chief strategy officer, tough, lightweight glass opens up new possibilities for giant display screens and for use as part of the architecture of buildings. Entire walls and tabletops could become displays with touch-sensitive surfaces. Windows, too, will contain electronic layers, allowing their transparency to be tweaked as desired—and perhaps even permitting them to gather solar energy from non-visible frequencies, to generate electricity. Other types of glass will become more flexible, enabling portable devices to be bendable, or even foldable.

One of the biggest areas for growth, Mr Evenson reckons, will be in cars. Already, instruments and switches on the dashboards of new cars are being replaced with touchscreens. As glass can be formed into different shapes, these screens can be curved into the contours of a vehicle’s interior. But Gorilla Glass, or something like it, could also be employed to make car windows. These, being thinner than existing windows, would be lighter and thus save fuel (or, in a battery-powered car, electricity). A version of Gorilla Glass is already being used for the windscreen of Ford’s GT sports car. Ford reckons the new glass is about 30% lighter than what it is replacing. It is also stronger, and scratch-resistant. Further ahead, electronics could be incorporated into the glass, to project images onto the windscreen, to assist drivers.

Perhaps, one day, one of Gorilla Glass’s descendants will be strong and tough enough to abolish the windscreen altogether, and replace it, the other windows and the roof with a single, transparent canopy. For drivers, while drivers there continue to be, that will improve visibility. And, if cars of the future really do become driverless, it will let everyone on board relax and enjoy the scenery.