Float glass

Lore & Background

Until the 16th century, window glass was generally cut from large discs of crown glass. Larger sheets were made by blowing large cylinders which were cut open and flattened. Most window glass in the early 19th century was made using the cylinder method, which limited the width of panes. Attempts to form flat glass on a bath of molten tin were made, including by Bessemer, but the process was unworkable at the time. Before float glass, larger sheets of plate glass were made by casting a large puddle of glass on an iron surface and then polishing both sides, a costly process. From the early 1920s, a continuous ribbon of plate glass was passed through inline grinders and polishers, reducing losses and cost. Lower quality drawn glass or broad sheet glass was made by drawing upwards from a pool of molten glass a thin sheet, held at the edges by rollers. This process continued in use for many years after the development of float glass. The success lay in the careful balance of the volume of glass fed onto the bath, where it was flattened by its own weight.

Reader's Guide

Float glass revolutionized the glass industry by enabling the production of large, flat sheets of glass with uniform thickness and perfectly smooth surfaces without the need for grinding or polishing. Before its development, producing high-quality flat glass was expensive and labor-intensive, involving casting, rolling, or drawing methods that required extensive finishing. The float glass process, pioneered by Pilkington in the 1950s, uses a bath of molten tin—one of the few liquids denser than glass that remains calm at high temperatures—to allow the glass to spread and flatten under its own weight. This innovation dramatically reduced costs and improved quality, making float glass the standard for modern windows, automobile glass, mirrors, furniture, and many specialized glass products such as toughened, laminated, and soundproof glass. Most float glass is soda–lime glass, though specialty borosilicate and flat panel display glass are also produced using the process. The method's success led to worldwide licensing in the 1960s, replacing earlier production methods and establishing float glass as the most widely produced form of glass today.

Did You Know?

• The float glass process is also known as the Pilkington process, named after the British glass manufacturer Pilkington.
• Molten tin is used because it has a higher density than glass, so the molten glass floats on it, and its boiling point is higher than the melting point of glass.
• To prevent oxidation of the tin, the tin bath is provided with a positive pressure protective atmosphere of nitrogen and hydrogen.
• Most float glass is soda–lime glass, although relatively minor quantities of specialty borosilicate and flat panel display glass are also produced using the float glass process.