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 1 - 8
Questions 1-8
Complete the table and diagram below.
Choose NO MORE THAN TWO WORDS from the passage for each answer.
Write your answers in boxes 1-8 on your answer sheet.
7 8 |
Question (9)
Questions 9-13
Do the following statements agree with the information given in Reading Passage 1?
In boxes 9-13 on your answer sheet, write
TRUE if the statement agrees with the information
FALSE if the statement contradicts the information
NOT GIVEN if there is no information on this
9
The metal used in the float process had to have specific properties.
10
Pilkington invested some of his own money in his float plant.
11 Pilkington’s first full-scale plant was an instant commercial success.
12
The process invented by Pilkington has now been improved.
13
Computers are better than humans at detecting faults in glass.
Question (14)
Questions 14-17
Reading Passage 2 has six paragraphs, A-F.
Choose the correct heading for paragraphs B and D-F from the list of headings below.
Write the correct number, i-ix, in boxes 14-17 on your answer sheet.
List of Headings
i Predicting climatic changes
ii The relevance of the Little Ice Age today
iii How cities contribute to climate change
iv Human impact on the climate
v How past climatic conditions can be determined
vi A growing need for weather records
vii A study covering a thousand years
viii People have always responded to climate change
ix Enough food at last
14
Paragraph B
15
Paragraph D
16
Paragraph E
17
Paragraph F
Questions 18 - 19
Questions 18-22
Complete the summary using the list of words, A-I, below.
Write the correct letter, A-I, in boxes 18-22 on your answer sheet.
Weather during the Little Ice Age
A | climatic shifts | B | ice cores | C | tree rings |
D | glaciers | E | interactions | F | weather observations |
G | heat waves | H | storms | I | written accounts |
Questions 18-19
Documentation of past weather conditions is limited: our main sources of knowledge of conditions in the distant past are ___and____
- A
- B
- C
- D
- E
- F
- G
- H
- I
20 We can deduce that the Little Ice Age was a time of _____rather than of consistent freezing.
21 Within it there were some periods of very cold winters, others of _____ and heavy rain.
22 yet others that saw ______ with no rain at all.
Question (23)
Questions 23-26
Classify the following events as occurring during the
A Medieval Warm Period
B Little Ice Age
C Modern Warm Period
23
Many Europeans started farming abroad.
24 The cutting down of trees began to affect the climate.
25
Europeans discovered other lands.
26
Changes took place in fishing patterns.
Question (27)
Questions 27-32
Reading Passage 3 has six paragraphs, A-F.
Choose the correct heading for each paragraph from the list of headings below.
Write the correct number, i-viii, in boxes 27-32 on your answer sheet.
List of Headings
i The difficulties of talking about smells
ii The role of smell in personal relationships
iii Future studies into smell
iv The relationship between the brain and the nose
v The interpretation of smells as a factor in defining groups
vi Why our sense of smell is not appreciated
vii Smell is our superior sense
viii The relationship between smell and feelings
27
Paragraph A
28
Paragraph B
29
Paragraph C
30
Paragraph D
31
Paragraph E
32
Paragraph F
Question (33)
Questions 33-36
Choose the correct letter, A, B, C or D.
Write the correct letter in boxes 33-36 on your answer sheet.
According to the introduction, we become aware of the importance of smell when
- A
- B
- C
- D
The experiment described in paragraph B
- A
- B
- C
- D
What is the writer doing in paragraph C?
- A
- B
- C
- D
What does the writer suggest about the study of smell in the atmosphere in paragraph E?
- A
- B
- C
- D
Questions 37-40
Complete the sentences below.
Choose ONE WORD ONLY from the passage for each answer.
Write your answers in boxes 37-40 on your answer sheet.
Tests have shown that odours can help people recognize the 37 belonging to their husbands and wives.
Certain linguistic groups may have difficulty describing smell because they lack the appropriate 38
The sense of smell may involve response to 39 which do not smell, in addition to obvious odours.
Odours regarded as unpleasant in certain 40 are not regarded as unpleasant in others.