IELTS Reading Practice Test 9 · Passage 1
The story of glass
How an ancient luxury became part of every modern building
Glass is so familiar that it is easy to forget how remarkable it is. Made largely from sand, one of the most common substances on Earth, it can be almost as transparent as air, yet strong enough to form the walls of skyscrapers. It is found in everything from bottles and telephone screens to the optical fibres that carry internet data beneath the oceans. For most of its long history, however, glass was a luxury, and the clear, flat sheets that we now take for granted became cheap only in the second half of the twentieth century.
The earliest known glass objects are beads made in the Middle East around 4,500 years ago, which may originally have been produced by accident, as a by-product of metalworking. For many centuries afterwards, glass was shaped by pressing it into moulds or winding it around a core, laborious methods that produced only small and costly items. The decisive change came in the first century BC, when craftsmen on the eastern coast of the Mediterranean discovered that molten glass could be inflated by blowing air into it through a hollow tube. Glassblowing allowed vessels to be made quickly and in large numbers, and under the Roman Empire, glass cups and bottles became everyday objects. The Romans also produced the first glass windows, although these were small, thick and far from clear, and only the wealthiest households could afford them.
In medieval Europe, the most celebrated glassmakers worked in Venice. In 1291, the city's government ordered all glass furnaces to be moved to the nearby island of Murano, officially because of the danger that fires might spread through Venice's crowded wooden buildings. The move also made it easier to protect the industry's secrets, since glassmakers were forbidden to leave the Republic. Window glass, meanwhile, was made by two main methods. In the crown process, a glassblower spun a bubble of molten glass until it flattened into a disc more than a metre across; in the cylinder process, a long cylinder was blown, cut along its length and flattened. Both methods produced glass that was uneven and slightly distorted, and the panes that could be cut from it were small.
Large, perfectly flat glass, which was needed for mirrors and shop windows, could be made only by casting molten glass on an iron table and then grinding and polishing both sides, an expensive process that wasted a great deal of material. Even so, demand grew rapidly in the nineteenth century. The Crystal Palace, built in London for the Great Exhibition of 1851, contained around 300,000 panes of cylinder glass, and it demonstrated that glass could be used not merely for windows but as a building material in its own right.
The method that transformed the industry was developed in England by Alastair Pilkington and Kenneth Bickerstaff of the glassmaking company Pilkington Brothers. Their idea was simple in principle but extremely difficult to put into practice: instead of polishing glass to make it flat, they would allow it to flatten itself by floating it on a bath of molten metal. Years of experiments between 1953 and 1957 produced large quantities of unsaleable glass, and the first profitable sales did not come until 1960. During the following decade, the process was licensed to manufacturers around the world, and float glass is now by far the most widely produced form of flat glass.
The process begins with a carefully measured mixture of raw materials, chiefly sand, soda ash and limestone, to which crushed recycled glass is added because it melts more easily and therefore saves energy. The mixture is heated in a furnace to around 1,500°C, and the molten glass then flows continuously onto a shallow bath of molten tin. Because tin is denser than glass, the glass floats on its surface and spreads out to form a perfectly flat ribbon. Tin is also chosen because it stays liquid across the whole range of temperatures involved. As the ribbon moves along the bath, it gradually cools from around 1,100°C to about 600°C, by which point it is firm enough to be lifted onto rollers without being marked. It then passes through a long oven known as a lehr, where it is cooled slowly to prevent stresses that might later cause it to shatter, before being cut into sheets by machines. A modern float line may run without stopping for years at a time.
The story is far from over. Researchers are now developing windows that generate electricity from sunlight. Others are working on glass that can change from clear to dark at the touch of a switch, suggesting that this ancient material still has new roles to play.