It is highly likely that wherever you are in the world, if you are gazing through a window, or even staring at your computer screen, you are looking at float glass produced by the Pilkington Method.
Float glass is the most widely produced form of glass, with a multitude of commercial applications. Due to both its high quality and structural flexibility during production, it can easily be shaped and bent into a variety of forms while in a heated, syrupy state. This makes it ideal for a variety of applications such as:
- Windows and doors
- Automobile glass (e.g. windshields, windows, mirrors)
- Mirrors
- Furniture (e.g. in tables and shelves)
- Insulated glass
Float glass is made by floating molten glass on a bed of molten metal of a low melting point, typically tin. This method gives the sheet uniform thickness and a very flat surface. Most forms of specialized glass such as toughened glass, frosted glass, laminated safety glass and soundproof glass consist of standard float glass that has been further processed.
The process was invented by Sir Alastair Pilkington and Kenneth Bickerstaff of the UK’s Pilkington Brothers at their production site in St Helens, Merseyside between 1953 and 1957. There they developed the first successful commercial application for forming a continuous ribbon of glass using a molten tin bath on which the molten glass flows unhindered under the influence of gravity. The success of this process lay in the careful balance of the volume of glass fed onto the bath, where it was flattened by its own weight. Full scale profitable sales of float glass were first achieved in 1960, and then during the 1960s the process was licensed throughout the world, replacing previous production methods.
Pilkington has more recently developed a self-cleaning coated float glass product, called Pilkington Activ. This self-cleaning glass has a coating which uses a method of photocatalysis to break down organic dirt with sunlight. The dirt is then washed away by the rain during a hydrophilic process.
Manufacture in detail
Float glass uses common glass-making raw materials, typically consisting of sand, soda ash (sodium carbonate), dolomite, limestone, and salt cake (sodium sulfate) etc. Other materials may be used as colourants, refining agents or to adjust the physical and chemical properties of the glass. The raw materials are mixed in a batch process, then fed together with suitable cullet (waste glass), in a controlled ratio, into a furnace where it is heated to approximately 1500 °C. Common float glass furnaces are 9 m wide and 45 m long and have capacities of more than 1200 tons of glass. Once molten, the temperature of the glass is stabilised to approximately 1200 °C to ensure a homogeneous specific gravity.
The molten glass is fed into a “tin bath”, a bath of molten tin (about 3–4 m wide, 50 m long, 6 cm deep), from a delivery canal and is poured into the tin bath by a ceramic lip known as the spout lip. The amount of glass allowed to pour onto the molten tin is controlled by a gate called a tweel.
Tin is suitable for the float glass process because it has a high specific gravity, is cohesive, and is immiscible with molten glass. Tin, however, oxidises in a natural atmosphere to form tin dioxide (SnO2). Known in the production process as dross, the tin dioxide adheres to the glass. To prevent oxidation, the tin bath is provided with a positive pressure protective atmosphere of nitrogen and hydrogen.
The glass flows onto the tin surface forming a floating ribbon with perfectly smooth surfaces on both sides and of even thickness. As the glass flows along the tin bath, the temperature is gradually reduced from 1100 °C until at approximately 600 °C the sheet can be lifted from the tin onto rollers. The glass ribbon is pulled off the bath by rollers at a controlled speed. Variation in the flow speed and roller speed enables glass sheets of varying thickness to be formed. Top rollers positioned above the molten tin may be used to control both the thickness and the width of the glass ribbon.
Once off the bath, the glass sheet passes through a lehr kiln for approximately 100 m, where it is cooled gradually so that it anneals without strain and does not crack from the temperature change. On exiting the “cold end” of the kiln, the glass is cut by machines.