Academic Reading — All-Types Test 25

Reading passage

How glass is made A Glass is one of the oldest manufactured materials known to humanity, yet the way it is made can still seem faintly miraculous. Take a handful of ordinary sand, heat it until it flows like syrup, allow it to cool, and you are left with a hard, transparent solid that lets light pass straight through it. Few everyday substances undergo so complete a transformation. The windows of a house, the screen of a telephone and the bottle in a kitchen cupboard all began as loose, gritty mineral grains, and understanding how that raw material becomes glass reveals a good deal about the chemistry hidden inside familiar things. B The chief ingredient of most glass is silica, the mineral that makes up common sand. Silica on its own can indeed be turned into glass, but only at extremely high temperatures, well above what early furnaces could reach, so manufacturers add other substances to make the process easier. The most important of these is a flux, usually soda ash, which lowers the temperature at which the silica melts and so saves a great deal of fuel. Unfortunately, glass made from silica and soda alone will slowly dissolve in water, which would make it useless for holding liquids. To prevent this, a third ingredient, lime, is added to stabilise the mixture. This combination of sand, soda and lime accounts for the great majority of the glass produced in the world today, from drinking glasses to the panes in office towers. C Once the ingredients have been carefully weighed out and blended, they are fed into a furnace and heated to around 1,500 degrees Celsius. At this temperature the mixture does not boil away but melts into a thick, glowing liquid. Crucially, glass does not behave like water, which turns abruptly from solid to liquid at a single fixed point. Instead it softens gradually over a wide range of temperatures, passing through a stage where it is soft and workable, rather like warm toffee. It is this unusual property that allows glass to be shaped in so many ways, for a worker has time to blow, press or draw the material into a chosen form before it hardens again. Molten glass is also remarkably sticky, and skilled makers learn to judge its readiness by eye, watching how it stretches and folds. D The methods used to shape the hot glass depend on what is being made. To produce a bottle or a jar, a blob of molten glass is dropped into a mould and air is blown into it, forcing the glass outwards against the walls of the mould until it takes on the required shape. Flat glass for windows was once made by blowing a large bubble, cutting it open and flattening it, a slow procedure that left the surface uneven and distorted. The modern answer, developed in the mid-twentieth century, is to float a continuous ribbon of molten glass on top of a bath of liquid tin. Because the tin is perfectly smooth and the glass spreads evenly across it, the sheet that emerges has two flawlessly flat surfaces and needs no polishing. Almost all window glass is now made in this way. E Whatever the method of shaping, one further step is essential if the finished object is to survive everyday use. Glass that has been allowed to cool too quickly hardens unevenly, leaving hidden stresses locked inside it, and such glass may shatter at the lightest knock or even, disconcertingly, for no obvious reason at all. To avoid this, freshly made glass is passed slowly through a long, temperature-controlled oven that lets it cool down in a gradual, carefully managed way. This process relieves the internal tensions and leaves the glass far tougher than it would otherwise be. Only after it has been treated in this manner is the glass ready to be cut, inspected and sent out into the world as the sturdy, dependable material we rarely stop to think about.