(1–8 mm) and width (1.5–2.5 m) were possible for glass sheets formed with a relatively uniform thickness. In terms of disadvantages, continuous operation was impossible because of the need after about two weeks of operation to remove the devitrified glass that was accumulating around the slot of the débiteuse and on the inner surfaces of the drawing kiln. Whereas the former devitrified material was causing draw lines, the latter changed the flow rate and flow pattern toward the débiteuse. In addition, it was impossible to maintain a completely stable throughput because of bubble formation at the beginning of a drawing cycle and draw‐line problems and instability toward the end [1, 3–6].
Figure 2 Sketch of Fourcault process in cross section. The molten glass flows up through the débiteuse slot and is drawn upward [3].
3.2 Colburn
At the same time the Fourcault process was being developed, the American inventor I. W. Colburn (1861–1917) was experimenting vertical drawing without a débiteuse. His first patent was taken in 1902 but the company he founded in 1906 to produce glass went bankrupt five years later. Working thereafter for the Toledo Glass Company, which had bought his patent, Colby was eventually successful in 1913. With his process, the molten glass introduced into a shallow drawing chamber was drawn upward from the free surface, its edges being gripped and driven by pairs of knurled rolls, and cooled immediately. After being reheated by a gas burner, the formed glass was brought horizontally by a bending roll and conveyed to a horizontal annealing lehr (Figure 3). Since the surface condition and flatness of the bending roll directly determined the quality of the glass sheet, the choice of an appropriate metal as well as the surface treatment and temperature control of the bending roll were crucial. Typically, two drawing chambers were mounted on one glass tank. The 0.9–6 mm thickness range obtained was similar to that of the Fourcault process, but devitrification on the débiteuse was avoided and a much larger width of up to 4.2 m could be obtained, thanks to the horizontally conveying process. But the price to be paid was a lower glass quality because of thickness variations, optical distortions, and surface defects [1, 3–6].
Figure 3 Sketch of Colburn process in a bird's‐eye perspective. The molten glass is drawn upward from the free surface and bended horizontally by a bending roll [6].
3.3 Pittsburg Pennvernon
A process similar to that of Fourcault was developed and introduced by the Pittsburg Plate Glass Company in 1926. In this Pittsburg Pennvernon process, the molten glass was not drawn through a débiteuse but upward from the free surface right above a drawbar, which was a long and thin refractory part immersed below the glass surface. The ribbon width was kept constant because the glass was cooled by edge folks and coolers. After being annealed and cooled in the drawing tower, the glass ribbon was cut off at the top of the tower (Figure 4). The drawbar served to anchor the drawing point and to ensure uniform temperature and glass flow rate across its width. In addition, ell blocks served to homogenize the drawing temperature by keeping the glass melt covered. The operation cycle was much longer than that of the Fourcault process with a better surface quality and without devitrification complications. Typically, the thickness range was 1–8 mm with a width of up to 3.2 m, but the disadvantages were thickness variations resulting from temperature fluctuations and inhomogeneities in chemical composition caused by drawing of the glass directly from its surface [3–6].
3.4 Asahi
The most recent updraw process has been developed by Asahi Glass Company around 1970 to overcome in a new way the disadvantages of the Fourcault process [5, 6, 8]. With it, a pair of hourglass‐shaped rolls, called “Asahi blocks,” is immersed into the molten glass instead of a débiteuse (Figure 5). The trick then is to make the Asahi blocks rotatable to renew the parting line where the glass leaves from the refractory and devitrification takes place. As a result, much longer drawing periods of up to 2–4 months can be achieved. An additional advantage is that thinner sheets down to 1.1–0.7 mm can be produced, especially for electronics applications, with a width of 1.5–2 m, thanks to the forming stability derived from the Asahi blocks.
4 Roll Out Process
The continuous double‐roll process was developed in the United States in an effort led by the Ford Motor Company to meet a growing demand from the automotive industry. As delivered from the forehearth, the molten glass was pressed to a given thickness, cooled rapidly by a water‐cooled pair of rotating rolls, and then conveyed into a horizontal annealing lehr. The thickness was determined mainly by the gap between the rolls, whereas the output was fixed by the rotating speed of the rolls.
As made by Pilkington Brothers in the 1920s, this process was then improved to manufacture plate glass through online grinding after annealing, followed by polishing of the cut plates. The process was further developed by Saint‐Gobain in the 1950s to grind and polish on line the glass ribbon (Chapter 10.9). Along with a waste of about 20% of the glass, very high investment and operating costs were major disadvantages of these mechanical methods, however, which in fact prompted Pilkington to develop the float process as described in Section 5.
Figure 4 Sketch of the Pittsburg Pennvernon process in cross section. The molten glass is drawn upward from the free surface right above the drawbar immersed below the glass surface [3].
Figure 5 Sketch of the Asahi process in cross section. The rotatable Asahi blocks are immersed into molten glass instead of the débiteuse, and enable the parting line to be renewed where devitrification takes place [6].
Because the float process was not designed at all for patterned and wire‐reinforced glass, the continuous roll out process, with which these products have been produced since the 1920s, has escaped oblivion (Chapter 10.9). Thanks to its versatility and facility for customization, it has even found new special applications, for instance, to make cover glasses for solar cells with excellent light diffusion through patterned textures on the surface. Usually the pattern is impressed on the lower surface by the lower roll, which is engraved. Generally the thickness range is 2–7 mm for the patterned and 8–25
