Sheet Glass Manufacture: The Float Process - IELTS Reading Answers & Explanations
From Cambridge IELTS 08 Academic Reading Test 2 · Part 1 · Questions 1–13
Reading Passage
You should spend about 20 minutes on Questions 1-13, which are based on Reading Passage 1 below.
Sheet glass manufacture: the float process
Glass, which has been made since the time of the Mesopotamians and Egyptians, is little more than a mixture of sand, soda ash and lime. When heated to about 1500 degrees Celsius (°C) this becomes a molten mass that hardens when slowly cooled. The first successful method for making clear, flat glass involved spinning. This method was very effective as the glass had not touched any surfaces between being soft and becoming hard, so it stayed perfectly unblemished, with a ‘fire finish’. However, the process took a long time and was labour-intensive.
Nevertheless, demand for flat glass was very high and glassmakers across the world were looking for a method of making it continuously. The first continuous ribbon process involved squeezing molten glass through two hot rollers, similar to an old mangle. This allowed glass of virtually any thickness to be made non-stop, but the rollers would leave both sides of the glass marked, and these would then need to be ground and polished. This part of the process rubbed away around 20 per cent of the glass, and the machines were very expensive.
The float process for making flat glass was invented by Alistair Pilkington. This process allows the manufacture of clear, tinted and coated glass for buildings, and clear and tinted glass for vehicles. Pilkington had been experimenting with improving the melting process, and in 1952 he had the idea of using a bed of molten metal to form the flat glass, eliminating altogether the need for rollers within the float bath. The metal had to melt at a temperature less than the hardening point of glass (about 600°C), but could not boil at a temperature below the temperature of the molten glass (about 1500°C). The best metal for the job was tin.
The rest of the concept relied on gravity, which guaranteed that the surface of the molten metal was perfectly flat and horizontal. Consequently, when pouring molten glass onto the molten tin, the underside of the glass would also be perfectly flat. If the glass were kept hot enough, it would flow over the molten tin until the top surface was also flat, horizontal and perfectly parallel to the bottom surface. Once the glass cooled to 604°C or less it was too hard to mark and could be transported out of the cooling zone by rollers. The glass settled to a thickness of six millimetres because of surface tension interactions between the glass and the tin. By fortunate coincidence, 60 per cent of the flat glass market at that time was for six-millimetre glass.
Pilkington built a pilot plant in 1953 and by 1955 he had convinced his company to build a full-scale plant. However, it took 14 months of non-stop production, costing the company £100,000 a month, before the plant produced any usable glass. Furthermore, once they succeeded in making marketable flat glass, the machine was turned off for a service to prepare it for years of continuous production. When it started up again it took another four months to get the process right again. They finally succeeded in 1959 and there are now float plants all over the world, with each able to produce around 1000 tons of glass every day, non-stop for around 15 years.
Float plants today make glass of near optical quality. Several processes – melting, refining, homogenising – take place simultaneously in the 2000 tonnes of molten glass in the furnace. They occur in separate zones in a complex glass flow driven by high temperatures. It adds up to a continuous melting process, lasting as long as 50 hours, that delivers glass smoothly and continuously to the float bath, and from there to a coating zone and finally a heat treatment zone, where stresses formed during cooling are relieved.
The principle of float glass is unchanged since the 1950s. However, the product has changed dramatically, from a single thickness of 6.8 mm to a range from sub-millimetre to 25 mm, from a ribbon frequently marred by inclusions and bubbles to almost optical perfection. To ensure the highest quality, inspection takes place at every stage. Occasionally, a bubble is not removed during refining, a sand grain refuses to melt, a tremor in the tin puts ripples into the glass ribbon. Automated on-line inspection does two things. Firstly, it reveals process faults upstream that can be corrected. Inspection technology allows more than 100 million measurements a second to be made across the ribbon, locating flaws the unaided eye would be unable to see. Secondly, it enables computers downstream to steer cutters around flaws.
Float glass is sold by the square metre, and at the final stage computers translate customer requirements into patterns of cuts designed to minimise waste.
Questions
Questions 1–5 Table Completion
Complete the table below.
Choose NO MORE THAN TWO WORDS from the passage for each answer.
| Method | Advantages | Disadvantages |
|---|---|---|
| 1 | • Glass remained
2 |
• Slow
• 3 |
| Ribbon | • Could produce glass sheets
of varying 4 • Non-stop process |
• Glass was 5
• 20% of glass rubbed away • Machines were expensive |
Questions 6–8 Diagram Labeling
Complete the diagram below.
Choose NO MORE THAN TWO WORDS from the passage for each answer.
Questions 9–13 True / False / Not Given
Do the following statements agree with the information given in Reading Passage 1?
TRUE if the statement agrees with the information
FALSE if the statement contradicts the information
NOT GIVEN if there is no information on this
Answers & Explanations Summary
| # | Answer | Evidence | Explanation |
|---|---|---|---|
| Q1 | spinning | The first successful method for making clear, flat glass involved spinning | Excerpt/Passage Explanation: The passage says that the very first way people used to create clear and flat glass was called spinning. Answer Explanation: The answer is the name of the first way people were able to successfully make flat and clear sheets of glass. Reason For Correctness: The correct answer is "spinning" because the text identifies it as the first successful method for making flat glass. The table asks for the name of a method that is slow and produces unblemished glass. The first paragraph of the passage mentions that "spinning" was very effective at keeping the glass unblemished, but it was also "slow" and "labour intensive," which matches all the points in the first row of the table. |
| Q2 | perfectly unblemished / unblemished | This method was very effective as the glass had not touched any surfaces between being soft and becoming hard, so it stayed perfectly unblemished, with a ‘fire finish’ | Excerpt/Passage Explanation: The passage explains that this way of making glass worked well because the glass did not touch anything while it was changing from soft to hard, which kept its surface perfect and free of marks. Answer Explanation: The answer means that the glass was completely free of any marks, scratches, or spots. Reason For Correctness: The correct answer is found in the first paragraph. The table asks for an advantage of the spinning method using the word "remained." The text says that because the glass did not touch any surface while cooling, it "stayed perfectly unblemished." In this context, "stayed" is a synonym for "remained," and "unblemished" describes the perfect condition of the glass. |
| Q3 | labour-intensive / labor-intensive | However, the process took a long time and was labour-intensive | Excerpt/Passage Explanation: The passage explains that this specific way of making glass was not fast and required many workers to complete the task. Answer Explanation: The answer means that the work needed many people to do it by hand. Reason For Correctness: The correct answer is found in the first paragraph, which talks about the first successful way to make flat glass using a spinning method. While this method made glass that was clear and perfect, it had two negative sides: it took a very long time (which the table describes as 'slow') and it was 'labour intensive', meaning it required a lot of hard work from many people. |
| Q4 | thickness | This allowed glass of virtually any thickness to be made non-stop | Excerpt/Passage Explanation: The passage explains that this method made it possible to create glass in almost any size (from thin to thick) while the machine was running continuously. Answer Explanation: The answer "thickness" refers to the measurement of how deep or wide an object is from one surface to the other. Reason For Correctness: The correct answer is found in the description of the "first continuous ribbon process." The passage explains that this specific method used hot rollers, which enabled the factory to produce glass of "virtually any thickness" without having to stop the machines. This matches the advantage mentioned in the table regarding the production of glass sheets of different sizes. |
| Q5 | marked | This allowed glass of virtually any thickness to be made non-stop, but the rollers would leave both sides of the glass marked, and these would then need to be ground and polished | Excerpt/Passage Explanation: The passage explains that while the ribbon method could produce glass of different thicknesses without stopping, the metal rollers used in the machine left marks on the glass that had to be cleaned off later. Answer Explanation: The answer 'marked' refers to the physical imperfections, such as scratches or spots, left on the surface of the glass during the manufacturing process. Reason For Correctness: The correct answer is 'marked' because the passage describes the continuous ribbon process as using two hot rollers. While this method was faster, these rollers physically touched the glass, which resulted in the glass having marks on both sides. These marks were a disadvantage because they meant the glass had to be ground and polished afterwards to make it smooth again. |
| Q6 | molten glass / glass | Consequently, when pouring molten glass onto the molten tin, the underside of the glass would also be perfectly flat | Excerpt/Passage Explanation: The passage explains that when the liquid glass is poured onto a layer of liquid tin, the bottom part of the glass becomes very flat. Answer Explanation: The answer is the liquid material that is used to create the flat sheets of glass. Reason For Correctness: The correct answer is "(molten) glass" because the passage explains that the process starts by heating a mixture until it becomes a liquid, referred to as "molten glass." This liquid substance is what flows through the machine to be shaped into flat sheets. The word "molten" is used to describe something solid (like glass) that has melted into a liquid because of high heat. |
| Q7 | molten tin / tin / molten metal / metal | Pilkington had been experimenting with improving the melting process, and in 1952 he had the idea of using a bed of molten metal to form the flat glass, eliminating altogether the need for rollers within the float bath The best metal for the job was tin |
Excerpt/Passage Explanation: The passage describes how the creator of the float process decided to use a layer of hot, liquid metal to shape the glass. It also states that tin was the most effective metal for that specific purpose. Answer Explanation: The answer refers to the hot, liquid material—specifically metal or tin—that provides a smooth, flat surface for the glass to float on as it hardens. Reason For Correctness: The correct answer is (molten) tin or metal because the passage explains Alistair Pilkington's invention where glass is made flat by pouring it onto a 'bed of molten metal'. The text later identifies that the most suitable 'metal' to use in this process is 'tin' because of its specific melting and boiling points. |
| Q8 | rollers | Once the glass cooled to 604°C or less it was too hard to mark and could be transported out of the cooling zone by rollers | Excerpt/Passage Explanation: The passage explains that when the glass becomes cold and hard enough, it can be moved away from the cooling area using rollers without damaging the surface. Answer Explanation: The answer "rollers" refers to the rotating cylinders or wheels that carry the glass out of the cooling area once it is hard. Reason For Correctness: The correct answer is "rollers" because the passage explains how the glass moves after it is formed. When the glass cools down and becomes hard, it is no longer at risk of being scratched or marked. At this point, the machine uses rollers to move or transport the glass out of the cooling zone toward the next steps of production. |
| Q9 | TRUE | The metal had to melt at a temperature less than the hardening point of glass (about 600°C), but could not boil at a temperature below the temperature of the molten glass (about 1500°C) | Excerpt/Passage Explanation: The passage explains that the metal had to stay liquid within a specific range of temperatures to be used in the process. Answer Explanation: The answer means that the text says the metal needed special features or characteristics to work correctly. Reason For Correctness: The correct answer is TRUE because the passage describes exact melting and boiling points that the metal had to have. These requirements are 'specific properties'. For example, the metal must become liquid (melt) at a lower temperature than when glass gets hard, and it must not turn into gas (boil) at the very high heat where glass is still liquid. The passage identifies these unique needs and names tin as the metal that fits them. |
| Q10 | NOT GIVEN | However, it took 14 months of non-stop production, costing the company £100,000 a month, before the plant produced any usable glass | Excerpt/Passage Explanation: The passage says that the factory was very expensive for the company, but it does not mention anything about Pilkington’s own money. Answer Explanation: The answer is NOT GIVEN because the story does not tell us if Alistair Pilkington spent his own private money to build the plant. Reason For Correctness: The correct answer is NOT GIVEN because the text only talks about the money spent by the company. It says the plant was "costing the company £100,000 a month." There is no information in the whole passage about whether Pilkington used his personal money or not, so we cannot say if the statement is true or false. |
| Q11 | FALSE | However, it took 14 months of non-stop production, costing the company £100,000 a month, before the plant produced any usable glass | Excerpt/Passage Explanation: The passage shows that the factory had to run for more than a year and spend a huge amount of money each month before it finally made good glass. This means it did not succeed quickly. Answer Explanation: The answer means that the statement is not true because the text says something different. Reason For Correctness: The correct answer is FALSE because the statement says Pilkington's first big factory was a success right away ("instant"). However, the text explains that the factory had many problems at the start. It took 14 months and cost the company a lot of money before they could make any glass that people could actually use. Since it took over a year to work correctly, it was not an "instant" success. Important keywords include "14 months" and "usable glass." |
| Q12 | TRUE | The principle of float glass is unchanged since the 1950s. However, the product has changed dramatically, from a single thickness of 6.8 mm to a range from sub-millimetre to 25 mm, from a ribbon frequently marred by inclusions and bubbles to almost optical perfection | Excerpt/Passage Explanation: The passage explains that even though the main idea for making the glass is still the same as when it was invented, the final glass has changed a lot. It used to have flaws like bubbles and only came in one size, but now it is made much better, looks perfect, and is available in many different thicknesses. Answer Explanation: The answer is TRUE because the way they make glass using Pilkington's method has gotten better over the years. Reason For Correctness: The correct answer is TRUE because the passage describes how much better the glassmaking results are now compared to the past. While the basic idea (the principle) has not changed since the 1950s, the process has been upgraded to make glass that is nearly perfect and comes in many different sizes. The text mentions that old glass had bubbles and marks, but new technology allows for better quality control and more variety. Important words to notice are 'changed dramatically' and 'optical perfection,' which show that the process and its results have been improved. |
| Q13 | TRUE | Inspection technology allows more than 100 million measurements a second to be made across the ribbon, locating flaws the unaided eye would be unable to see | Excerpt/Passage Explanation: The passage says that special machines can do millions of checks every second to find tiny mistakes in the glass that a person's eyes are not strong enough to see by themselves. Answer Explanation: The answer means that the statement is correct: computers are indeed more capable than people when it comes to finding small mistakes or marks in the glass. Reason For Correctness: The correct answer is TRUE because the text explains that 'inspection technology' (the computers and machines used for checking) can find 'flaws' (marks or mistakes) that the 'unaided eye' (a human eye without help) would not be 'able to see'. This confirms that the technology is superior to a person's natural vision for this job. |