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Review Lecture. The Float Glass Process Author(s): L. A. B. Pilkington Source: Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences, Vol. 314, No. 1516 (Dec. 16, 1969), pp. 1-25 Published by: Royal Society Stable URL: http://www.jstor.org/stable/2416528 Accessed: 25-07-2016 22:34 UTC Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at http://about.jstor.org/terms JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Royal Society is collaborating with JSTOR to digitize, preserve and extend access to Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences This content downloaded from 128.111.121.42 on Mon, 25 Jul 2016 22:34:46 UTC All use subject to http://about.jstor.org/terms Proc. Roy. Soc. Lond. A. 314, 1-25 (1969) Printed in Great Britain REVIEW LECTURE The float glass process BY L. A. B. PILKINGTONt Pilkington Brothers Limited, St Helens, Lancashire (Delivered 13 February 1969-Received 14 April 1969) [Plate 1] My subject is the float process for making flat glass. I would like, first of all, to put the float process into perspective by describing briefly, and in simple terms, the methods used for making flat glass before and at the time of the invention of the float process and then to describe the development of the process itself and the position it occupies in the flat glass industry today. Finally, I would like to describe in as much depth as time allows, three of the main problems which had to be tackled in developing this process. The Egyptians seem to have been the first people to realize what could be done with glass when it is hot and plastic, and they made vessels for cosmetics and per- fumes by, it is assumed, trailing molten glass around a shaped core. By Roman times glass was being blown and moulded, cut and engraved, painted and gilded, and the Romans had mastered the plastic character of heat softened glass so funda- mental to today's processes. In the Middle Ages, the glass makers' main achievements were coloured glass windows, and while the decorative application of glass progressed fairly rapidly under the Venetians, and glass men like Ravenscroft in the seventeenth century, it is only in the last 50 years that there have been any appreciable advances in the development of flat glass for windows. Since it is with flat glass that my lecture is concerned, I hope you will therefore forgive me for skipping 3000 years of history! The importance of the float process can be placed in perspective by taking a look at the processes used for making flat glass before and at the time of its invention. There have, through the ages, been two basic methods of forming flat glass: the window glass processes and the plate glass processes. Window glass processes have all depended on forming a sheet by stretching a lump of molten glass. They all have the characteristic of brilliant fire finish. Three processes, crown, cylinder and finally drawn sheet, have been used to make windows. Up to the middle of the nineteenth century the crown process (figure 1, plate 1) was the most commonly used and Henry Chance writing in 1888 tells us how the t Elected F.R.S. 20 March 1969. I 1 ] Vol. 3I4. A. (i6 December I969) This content downloaded from 128.111.121.42 on Mon, 25 Jul 2016 22:34:46 UTC All use subject to http://about.jstor.org/terms 2 L. A. B. Pilkington crown or disk was spun after the initial blowing and shaping stages on the end of an iron rod. He says: 'A man ... with a veil before his face, stands in front of a huge circle of flame, into which he thrusts his piece rapidly, meanwhile revolving his ponty (the iron rod). The action of heat and centrifugal force combined is soon visible. The nose of the piece expands, the parts around cannot resist the tendency ... the next moment, before the eyes of the spectator, is whirling a thin transparent circular plate of glass which but a few minutes before was lying in the glass pot.' FIGuRE 2. Square panes were cut from a round crown, involving wastage. The numbers give the sizes in inches. Disks of 1.8 m diameter could be produced but the usual size was about 1.4 m (figure 2). This restriction in size and the fact that each crown had a bullion in the centre obviously dictated the scope for the product in windows. Another dis- advantage was that square panes were cut from a round disk, involving wastage. The next big advance in glass manufacture, which removed the limitations on window size imposed by the Crown process, came in the mid-nineteenth century with the introduction of the cylinder process which was capable of making much bigger panes. The process involved blowing a large cylinder with was allowed to cool before being split and flattened (figure 3). The process was mechanized some- what until cylinders up to 4 m long and 0.6 m in diameter could be blown. This content downloaded from 128.111.121.42 on Mon, 25 Jul 2016 22:34:46 UTC All use subject to http://about.jstor.org/terms Pilkington Proc. Roy. Soc. Lond. A, volume 314, plate 1 FIGURE 1. The crown process spun glass into a disk. FIGURE 4. In the sheet process the edge of the glass ribbon is gripped between rollers to prevent 'waisting in'. (Facing p. 2) This content downloaded from 128.111.121.42 on Mon, 25 Jul 2016 22:34:46 UTC All use subject to http://about.jstor.org/terms The float glass process 3 Sheet or window glass production was first mechanized on a large scale in the early part of this century. In 1903, the American Window Glass Company developed a method for the mechanical blowing of cylinders many times larger than hand blown cylinders. This drawn cylinder process drew cylinders up to 13.4 m long and 1 m in diameter from a molten pool of glass. The quality of the glass, however, was incon- sistent. FIGTTRE 3. Glass made by the cylinder process had to be split and flattened for windows. The problem with the cylinder process was that it was discontinuous and the cylinders, however large, had to be split and flattened which was costly and harmful to the surface. The logical evolution of this process was to draw a flat continuous sheet of glass from the molten pool. The modern sheet glass process was first developed by Fourcault in about 1914. The glass is drawn vertically in a ribbon from a bath of molten glass. The main problem in drawing such a sheet is to prevent the 'waisting in'. This is achieved by passing the edges of the ribbon between cooled rollers which stiffen them (see figure 4, plate 1). The surface of glass made in this way has what is called 'fire-finish' (figure 5). The fire finish surface is achieved by letting the glass cool down on its own without touching anything solid while it is still soft. However, the solidified ribbon of glass has a certain amount of distortion which cannot be avoided because of small differences in viscosity due to chemical and 1-2 This content downloaded from 128.111.121.42 on Mon, 25 Jul 2016 22:34:46 UTC All use subject to http://about.jstor.org/terms 4 L. A. B. Pilkington thermal inhomogeneities. The thickness of the ribbon of glass drawn from the bath is controlled by the viscosity, so that the effect of even small inhomogeneities is variations in thickness of the finished sheet. The machine needed to produce this sheet glass is relatively simple and the glass is therefore inexpensive. FIGURE 5. Sheet glass has a perfect fire finish. Only the edges gripped by rollers are marked. The major problem with all the glass made by these window processes was that it was bedevilled by distortion. All the manufacturing methods involved stretching the molten glass whether by spinning, blowing or pulling it and this stretching con- verted inhomogeneities into distortion. The processes also made only a com- paratively thin glass. Neither of these properties was acceptable for mirrors or coach windows, or, later, for car windows or for the large distortion free shop windows which came into vogue from 1850 onwards. The plate process was developed to meet these requirements (figure 6). To create distortion free glass the molten metal was cast on to a table, rolled into a plate and, after annealing, ground flat and then polished. Grinding involved several stages using finer and finer sand and polishing was done with rouge. The principles and, as we shall see later, the disadvantages of this process remained with the industry until the 1950s. Financial requirements were, from the beginning, This content downloaded from 128.111.121.42 on Mon, 25 Jul 2016 22:34:46 UTC All use subject to http://about.jstor.org/terms The float glass process 5 quite enormous. The ordinary small glasshouse for making crown or cylinder glass could be set up fairly cheaply. But much more was required for plate manufacture.