Float glass

Float glass is a specific type of flat glass used widely in modern manufacturing. It is produced by floating molten glass on a bed of molten metal. This metal is usually tin, though lead was used in the past. This unique method creates a sheet with a very flat surface and uniform thickness. Because the surface is so smooth, the glass does not require extra polishing. This makes the process much more efficient than older methods. Most float glass is soda–lime glass, which is a common type of glass. However, manufacturers also use this process to create specialty borosilicate and flat panel display glass.

The production process begins with a batch of raw materials. These materials typically include sand, soda ash (sodium carbonate), dolomite, limestone, and salt cake (sodium sulfate). Workers may also add materials to act as colorants or refining agents. These ingredients are mixed together and fed into a large furnace along with cullet, which is waste glass. The furnace heats the mixture to approximately 1,500 °C. Common furnaces are quite large, measuring about 9 meters wide and 45 meters long. These machines can have capacities exceeding 1,200 tons of glass. Once the mixture melts, the temperature is stabilized to about 1,200 °C to ensure a homogeneous density.

After the glass is molten, it moves into a specialized tin bath. This bath contains molten tin that is about 3 to 4 meters wide and 50 meters long. The tin is about 6 cm deep. The molten glass is poured into this bath through a ceramic lip called a spout lip. A gate known as a tweel controls the amount of glass that pours onto the tin. Molten tin is ideal for this process because it is denser than glass, allowing the glass to float. Additionally, tin has a higher boiling point than the melting point of glass. To prevent the tin from oxidizing into tin dioxide, which is known as dross, a protective atmosphere of nitrogen and hydrogen is used.

As the glass flows along the molten tin, it forms a floating ribbon. This ribbon has perfectly smooth surfaces on both sides due to the liquid metal. The temperature of the glass ribbon gradually decreases from 1,100 °C as it moves. When the temperature reaches approximately 600 °C, the sheet is stiff enough to be lifted. Rollers pull the glass ribbon off the bath at a controlled speed. By adjusting the flow speed and the roller speed, manufacturers can create glass sheets of different thicknesses. Top rollers can also be used to manage the width and thickness of the ribbon.

Once the glass is off the bath, it must be cooled carefully. It passes through a lehr kiln, which is about 100 meters long. Inside the kiln, the glass is cooled gradually to allow it to anneal. Annealing is a process that prevents internal strain so the glass does not crack from temperature changes. When the glass reaches the "cold end" of the kiln, machines cut it into pieces.

Humans have attempted to create flat glass through many different methods throughout history. Before the 16th century, glass was often cut from large discs called rondels. Later, makers blew large cylinders, cut them open, and flattened them into panes. In 1848, Henry Bessemer patented a system to produce a continuous ribbon of glass using rollers. However, this was expensive because the glass still required polishing. In the early 1920s, plate glass was passed through inline grinders and polishers to reduce costs. Another method involved drawing thin sheets upward from a pool of molten glass. These methods were often difficult or resulted in surfaces that were not perfectly uniform.

The modern float glass process was pioneered in the 1950s. Between 1953 and 1957, Sir Alastair Pilkington and Kenneth Bickerstaff developed the first successful commercial application. They worked at the Cowley Hill Works in St Helens, Merseyside, in the United Kingdom. Their success relied on balancing the volume of glass fed onto the tin bath so gravity would flatten it. Profitable sales of float glass began in 1960. By the 1960s, the process was licensed worldwide and replaced almost all previous production methods.

Float glass is essential to many industries today. It is used for automobile glass, including windshields, windows, and mirrors. It is also used to make mirrors, furniture like tables and shelves, and insulated glass for windows and doors. Because the glass is flexible while in a heated, syrupy state, it can be shaped into many forms. Many specialized products, such as toughened, frosted, or soundproof glass, start as standard float glass. Even the color of the glass can change based on its thickness; thicker sheets often appear green due to Fe2+ impurities.