Before 1952, the best way to make large sheets of plate glass was to pour molten glass over a hot iron plate and let it solidify. This created a plate of glass with uneven surfaces with poor thickness control, and it had to be ground to a flatter, smoother surface. The grinding process required the glass to be polished afterward, and the whole process was extremely expensive. Another method involved drawing glass vertically out of a furnace, but this produced plates with uneven sides.
According to lore, in 1952 an Englishman named Alastair Pilkington was washing his dishes and observed a plate floating on the smooth surface of the water. This supposedly inspired him to wonder if smooth glass could be produced by floating molten glass on the smooth surface of some other liquid. He worked for a company called Pilkington Brothers (no relation) at the time and was fortunate to receive an incredible amount of support for his idea. Despite taking over seven years and £7 million to develop the idea into something workable, the board of directors continued to fund his research, seeing it through until the end. The idea was realized in 1959, and proved economically viable in 1963, when it was patented. The original process was only able to create glass 6.8mm thick, and by coincidence and luck this was able to provide for most of the market demand for flat glass at the time. Sir Alastair Pilkington was knighted in 1972, I assume for developing the float glass process. He died in 1995.
The float glass process, also called the Pilkington process, creates large, flat sheets of glass in thicknesses from 0.4mm to 25mm, up to three meters wide with almost perfectly parallel top and bottom surfaces and no further processing, grinding, or polishing required. It is a very economical process that has proven so successful that 90% of all the flat glass in the world is now made this way. Worldwide, over 260 factories produce 800,000 tonnes of glass per week using this process, licensed from Pilkington Brothers (who only own about 25 factories themselves). A single factory can produce up to 6,000 tonnes of glass per week.
Production begins with the molten glass, which is mostly silica (sand) with a few other chemicals mixed in, such as limestone (calcium oxide) and soda ash (sodium oxide), which make the product more durable and easier to work with. The glass is melted at high temperatures until it becomes a highly viscous, white-hot liquid which is then poured down a pipe onto a shallow pool of molten tin. The lighter, viscous glass floats on top of the heavier liquid tin, using the smooth surface of the tin to create an almost perfectly flat bottom surface for the glass. The top surface flattens on its own as the molten glass spreads out over the tin. This takes place in a heated tunnel filled with a nitrogen atmosphere to prevent the tin from oxidizing. The side of the glass in contact with the tin is called the tin side, and the other side is called the air side. Since some residue from the tin remains on the glass, it is possible to identify the tin side with a UV light.
The continuous pouring of glass from the furnace over the surface of the tin pushes it forward, down the tunnel, which gradually drops in temperature from 1,100°C at the beginning to 600°C at the end, where it is solid enough to be lifted off the molten tin and onto rollers. In this way the glass is formed as a continuous ribbon, the thickness of which is controlled by the rate at which it is poured from the furnace. Once on the rollers, the glass is slowly cooled in a carefully controlled annealing process – it would crack if cooled too quickly. From there it is inspected with lasers for defects. Defects in the ribbon reflect the laser beams while good glass allows them to pass through. Finally it is cut to customer specifications with diamond edged tools, packaged, and shipped.
Several kinds of defects in the process are possible. Tremors or vibrations create ripples in the molten tin, which then puts ripples in the tin side of the glass. Unmelted silica particles can become trapped in the glass. Air bubbles and voids are likewise unacceptable. Sometimes bits of tin become stuck to the tin side of the glass and solidify during the annealing process. All of these defects are easily caught with the laser sensors and cut out and disposed of during the cutting phase.
Molten tin works extremely well for this process for several reasons. Tin has a relatively low melting point of just 230°C and a boiling point of about 2,600°C, which allows the tin to remain in a liquid state while the glass cools from liquid to solid. Molten tin remains dense enough that the liquid glass floats easily on the surface, and it is chemically inert to silica. Its only real disadvantage is that it must be kept in a nitrogen atmosphere to prevent oxidation.
The manufacture of glass is one of the most important industries in existence, some would say second only to the manufacture of steel. These materials, along with concrete and plastic, are the very building blocks of our cities and industrialized civilization. It is difficult to imagine how much we would lose without a transparent solid, and the ability to manufacture it affordably in large sheets has clearly transformed the face of architecture forever.