Float glass

Float glass is produced by pouring molten glass onto a bath of molten metal with a low melting point—usually tin, though lead was used historically. Because the glass floats on the metal, it spreads into a sheet with consistent thickness and exceptionally smooth surfaces on both sides. This manufacturing technique is called the Pilkington process, after the British company Pilkington, which first developed it during the 1950s at its factory in St Helens, Merseyside. Float glass now accounts for the majority of glass produced worldwide and is used in countless commercial products.

Before the 16th century, flat glass for windows came from cutting large discs of crown glass. Later, larger panes were made by blowing cylinders, opening them, and flattening them—a method that dominated window-glass production in the early 1800s. Those cylinders measured 6 to 8 feet long and 10 to 14 inches wide, which limited pane sizes and forced windows to be divided by transoms. Henry Bessemer patented the first automated flat-glass system in 1848, using rollers to form a continuous ribbon. But the resulting surfaces needed polishing, which was costly. If glass could be set on a perfectly smooth, calm liquid surface, costs would drop. Bessemer and others tried using molten tin—one of the few liquids denser than glass and stable at the required high temperatures—but their attempts were not practical at the time. Before float glass, larger plate-glass sheets were made by pouring glass onto an iron surface and polishing both sides, an expensive process. From the early 1920s, a continuous ribbon of plate glass passed through a long series of grinders and polishers, reducing waste and cost. Lower-quality drawn glass, or broad sheet glass, was made by pulling a thin sheet upward from a molten pool, held at the edges by rollers. Its surfaces were less smooth and uniform than float glass, and the method remained in use long after float glass was developed. Between 1953 and 1957, at the Cowley Hill Works in St Helens, Lancashire, Sir Alastair Pilkington and Kenneth Bickerstaff of Pilkington Brothers created the first commercially successful float-glass process. They fed molten glass onto a bath of molten tin, where it spread flat under gravity. Success depended on carefully balancing the volume of glass fed onto the bath. Profitable full-scale sales began in 1960, and during the 1960s the process was licensed worldwide, replacing older methods.

The raw materials for float glass are common: sand, soda ash, dolomite, limestone, and salt cake, plus possible colorants or refining agents. These are mixed in batches and fed into a furnace with a controlled amount of cullet, then heated to about 1,500 °C. Typical furnaces are 9 meters wide and 45 meters long, with capacities over 1,200 tons. The molten glass is stabilized to around 1,200 °C for uniform density. It then flows from a delivery canal through a ceramic spout lip onto a tin bath—roughly 3 to 4 meters wide, 50 meters long, and 6 centimeters deep. A gate called a tweel controls the flow. Tin works because it is denser than glass, has a boiling point above glass’s melting point, and has low vapor pressure at process temperatures. But tin oxidizes in air to form tin dioxide (dross), which sticks to the glass. To prevent this, the tin bath is kept under a positive-pressure atmosphere of nitrogen and hydrogen. As the glass floats on the tin, it forms a ribbon of even thickness with smooth surfaces on both sides. The temperature is gradually reduced from 1,100 °C to about 600 °C, at which point the sheet is lifted onto rollers. The ribbon is pulled off the bath at a controlled speed; varying the flow and roller speeds produces different thicknesses. Top rollers above the tin can also adjust thickness and width. After leaving the bath, the glass passes through a lehr kiln about 100 meters long, where it cools slowly to anneal without strain or cracking. At the kiln’s cold end, machines cut the glass.

Today, float glass is the most common type of glass, used in countless commercial applications. Its high quality requires no additional polishing, and it offers structural flexibility.