R7 – Sheet glass manufacture: the float process

 

Sheet glass manufacture:

the float process

1

¹Glass, which has been made since the time of the Mesopotamians and Egyptians, is little more than a mixture of sand, soda ash and lime. ²When heated to about 1500 degrees Celsius (°C) this becomes a molten mass that hardens when slowly cooled. ³The first successful method for making clear, flat glass involved spinning. ⁴This 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’. ⁵However, the process took a long time and was labour intensive.

2

¹Nevertheless, demand for flat glass was very high and glassmakers across the world were looking for a method of making it continuously. ²The first continuous ribbon process involved squeezing molten glass through two hot rollers, similar to an old mangle. ³This 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. ⁴This part of the process rubbed away around 20 per cent of the glass, and the machines were very expensive.

3

¹The float process for making flat glass was invented by Alistair Pilkington. ²This process allows the manufacture of clear, tinted and coated glass for buildings, and clear and tinted glass for vehicles. ³Pilkington 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. ⁴The 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). ⁵The best metal for the job was tin.

4

¹The rest of the concept relied on gravity, which guaranteed that the surface of the molten metal was perfectly flat and horizontal. ²Consequently, when pouring molten glass onto the molten tin, the underside of the glass would also be perfectly flat. ³If 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. ⁴Once 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. ⁵The glass settled to a thickness of six millimetres because of surface tension interactions between the glass and the tin. ⁶By fortunate coincidence, 60 per cent of the flat glass market at that time was for six- millimetre glass. ⁷Pilkington built a pilot plant in 1953 and by 1955 he had convinced his company to build a full-scale plant. ⁸However, it took 14 months of non-stop production, costing the company £100,000 a month, before the plant produced any usable glass. ⁹Furthermore, once they succeeded in making marketable flat glass, the machine was turned off for a service to prepare it for years of continuous production. ¹⁰When it started up again it took another four months to get the process right again. ¹¹They 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.

5
¹Float plants today make glass of near optical quality. ²Several processes – melting, refining, homogenising – take place simultaneously in the 2000 tonnes of molten glass in the furnace. ³They occur in separate zones in a complex glass flow driven by high temperatures. ⁴It 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.

6
¹The principle of float glass is unchanged since the 1950s. ²However, 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. ³To ensure the highest quality, inspection takes place at every stage. ⁴Occasionally, a bubble is not removed during refining, a sand grain refuses to melt, a tremor in the tin puts ripples into the glass ribbon. ⁵Automated on-line inspection does two things. ⁶Firstly, it reveals process faults upstream that can be corrected.⁷Inspection 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. ⁸Secondly, it enables computers downstream to steer cutters around flaws.⁹Float 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

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.

Early methods of producing flat glass

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

Pilkington’s float process

 

Questions 9-13

Do the following statements agree with the information given in Reading Passage?

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.