The history of glass production dates back to about 5000 BC. Glass beads discovered in ancient Egypt and early Roman sites bear witness to a long tradition of drawing and moulding techniques used in glass production. For centuries, however, individual craftsmanship dominated manufacturing processes that ranged from using blowpipes and cylinder blow-moulding techniques to the crown-glass method. These manual production methods resulted in small quantities and small windowpanes, which were almost exclusively used in stained glass windows in churches.
Demand for glass during the seventeenth century rose because in addition to master church builders using glass in church windows, builders of castles and stately townhouses were now discovering how to use glass to enclose spaces as well. French glassmakers first developed a glass rolling process that produced 1.20 x 2 m glass panels, a size that until then had seemed impossible. Glass production did not become industrialized until the twentieth century when 12 x 2.50 m sheets of glass later began to be mass produced on a large scale using the Lubbers and Fourcault methods of glass production, advancing to the more recent technologies developed by Libbey-Owens-and Pittsburgh.
But all these methods had one distinct disadvantage: manufactured glass panels had to be ground and polished on both sides to obtain distortion-free and optically perfect mirror glass, a process that was extremely time-consuming and expensive.
1.2 Float glass
Industrial glass – which today would be glass used in the automotive and construction industries – was originally manufactured using a system known as “float glass.” This floating process, which reached its peak in 1959, revolutionised glass production methods. Until this float process was developed, glass panes were produced by drawing or moulding molten glass, and then polishing it.
This new method let the glass “float”, that is, the molten glass spread out evenly over the surface of a liquid tin bath. Due to the inherent surface tension of the liquid tin, and the fact that glass is only half as dense as tin, the molten glass does not sink into the tin bath, but rather floats on the surface, thereby evenly moulding itself to the surface shape of the liquid tin. This method creates absolute plane parallelism, which guarantees freedom from distortion and crystal clear transparency. Reducing the temperature in the tin bath from approx. 1000 °C to approx. 600 °C turns a viscous mass of molten glass into a solid glass sheet that can be lifted right off of the surface of the tin bath at the end of the floating process.
Tin is ideal for shape forming because it remains liquid throughout the entire shape-forming process and does not evaporate, thanks to its low vapour pressure. In order to prevent the tin from oxidizing, the floating process takes place in a protective gas atmosphere of nitrogen with a hydrogen additive.
The molten process precedes form shaping by floating glass in a tin bath. This process begins with an exact proportion of the raw materials that is based on about 60% quartz, 20% soda and sulphate, and 20% limestone and dolomite. These materials are crushed in huge agitators and processed into a mixture. A blend comprising approx. 80 % of this mixture and 20 % of recycled scrap glass is fed into the furnace and melted at about 1600 °C. The result is a chalk-natron-silicate glass that is in accordance with EN 572-2.
After gassing the molten mixture, which is referred to as refining, the molten glass is fed into the conditioning basin and left to cool to approx. 1200 °C before flowing over a refractory spout into the float bath. This mixture is constantly fed, or “floated” onto the tin surface, a method that can be likened to a tub that overflows due to constant water intake. An infinite glass ribbon of about 3.50 m width is lifted off the surface at the end of the float bath.
At this point, the glass ribbon is approx. 600 °C and is cooled down to room temperature using a very precise procedure in the roller cooling channel to ensure that no permanent stress remains in the glass. This operation is extremely important for problem-free processing. The glass ribbon is still approx. 50 °C at the end of the 250m-long cooling line and a laser “inspects” the glass to detect faults such as inclusions, bubbles and cords. Faults are automatically registered and scrapped when blanks are later pre-cut.
Pre-cuts are usually 6 metres or less, with the glass being cut perpendicular to the endless ribbon. Both edges of the ribbon are also trimmed, generally producing float glass panes of 3.21 m x 6 m, which are then immediately processes or stored on frames for further processing. Longer plates of 7 or 8 m are also produced.
An average float glass line is about 600 m long and has a capacity of approx. 70,000 m² with a thickness of 4 mm.