Academic Reading — All-Types Test 10

Reading passage

How Glass Is Made A. Glass is one of the oldest manufactured materials known to humankind, and archaeologists have recovered glass beads made more than four thousand years ago. Remarkably, the way it is made has remained essentially unchanged in principle ever since. At its heart, glass is produced by melting sand until it fuses into a clear, hard substance that will hold whatever shape it is given as it cools. The sand used is not ordinary beach sand but a very pure form of silica, the mineral that gives glass its transparency and strength. Ordinary sand is often coloured brown or yellow by traces of iron, and even a tiny amount of such impurity is enough to tint the finished glass a faint green, which is why manufacturers who want a colourless product take great trouble to obtain sand that is as pure as possible. When silica is heated to around 1,700 degrees Celsius, it softens and eventually flows like a thick, syrupy liquid, and it is this molten state that allows workers to pour, roll or blow it into countless forms. B. Melting pure silica on its own, however, demands enormous amounts of energy, so manufacturers add other ingredients to lower the temperature at which the mixture liquefies. The most important of these is soda ash, a compound that reduces the melting point considerably and therefore saves a great deal of fuel. Unfortunately, soda ash has an awkward side effect, for the glass it helps to create will slowly dissolve in water, a property that would make it useless for windows or drinking vessels. To counter this weakness, a third material called limestone is introduced, which makes the finished product stable and durable. The combination of sand, soda ash and limestone forms the basis of what is known as soda-lime glass, the ordinary variety used for windows and bottles across the world. Other substances may be added for special purposes: a little lead produces the heavy, sparkling glass prized for fine tableware, while certain metal oxides are used to colour the glass deliberately, cobalt giving a deep blue and chromium a rich green. C. The raw materials are weighed out in careful proportions and mixed together with a quantity of recycled broken glass, known in the trade as cullet. Adding cullet is sensible for two reasons: it reuses waste that would otherwise be thrown away, and, because it is already glass, it melts more readily than fresh sand and so cuts the energy needed. The blended batch is fed into a furnace, a vast tank lined with heat-resistant brick that can hold hundreds of tonnes of molten material at once. Modern furnaces run continuously for years without ever cooling down, since starting and stopping them repeatedly would waste fuel and crack the lining. The heat is now usually supplied by burning natural gas, though the demands are so great that some manufacturers are experimenting with electric furnaces in an effort to reduce the industry's carbon emissions. D. Once the batch has melted and any bubbles have risen out of it, the glass is ready to be formed, and the method chosen depends entirely on the object being made. Flat glass for windows is produced by the float process, in which the molten glass is poured onto a shallow bath of liquid tin. Because glass is lighter than tin, it spreads out and floats on the surface, forming a sheet of remarkably even thickness with two perfectly smooth faces. Before this technique was invented in the middle of the twentieth century, sheet glass had to be ground and polished laboriously to achieve a comparable finish. As the ribbon of glass moves slowly along the bath it gradually cools, and by the time it leaves the tin it is solid enough to be lifted away on rollers without marking its surface. E. Containers such as bottles and jars are shaped by an entirely different technique. A blob of molten glass, called a gob, is dropped into a mould, and compressed air is blown into it so that the glass expands to fill the hollow shape, much as a craftsman once used his lungs and a hollow pipe to achieve the same effect by hand. Whatever the forming method, the newly made article must then be cooled with great care. If glass cools too quickly the outside hardens while the inside is still shrinking, leaving hidden stresses that can make the object crack or even shatter without any warning. To prevent this, the glass is passed through a long oven called a lehr, where the temperature is lowered gradually over a carefully controlled period. This treatment, known as annealing, relieves the internal strain and gives the finished glass its lasting strength.