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What is Float Glass & How Is It Made?

Float glass is extremely smooth, distortion-free glass used in many window applications. It also provides the material for many other forms of glass, including tinted glass (heat absorbing) and laminated glass.

Float glass is made by pouring the molten glass from a furnace into a chamber that contains a bed of molten tin. The process is sometimes call the Pilkington Process. The atmosphere inside the chamber is carefully controlled. The glass floats on the tin and forms itself in the shape of the container. It spreads 90 to 140 inches wide at a thickness determined at the time of manufacture.

The upper surface of the glass is called the air side or score side. It is polished with fire. The lower surface is called the tin side. It is not fire-polished. From the chamber, the glass enters an oven, called a lehr. There it is slowly cooled at a specific rate. This process, called annealing, relieves the glass of internal stresses. The rate of cooling is crucial to the success of the final product. The glass emerges from the lehr at room temperature as a continuous ribbon. It is flat, fire-finished on the top, and has smooth, parallel surfaces. Automatic cutters trim the edges and cut the glass to length.

Plate Glass vs. Float Glass

At one time most of the glass manufactured in the United States was plate glass. Plate glass was made by a process of grinding and polishing. No longer made in this country, plate glass has been replaced by float glass due to the safety hazard it presents when broken and poor energy efficiency. Float glass is a term that refers to a process of making glass that was perfected in 1959 by Pilkington Brothers, Ltd. of England.

Types of Glass From the Float Process

There are two types of glass made by the float process, clear glass and tinted glass. Most of the flat glass made by the float process is clear glass. As its name implies, clear glass is transparent and colorless. Depending upon its thickness, clear glass allows about 75 to 92 percent of the visible light to pass through.

Tinted glass (also called heat absorbing glass) is made by adding coloring agents to the batch mix. These agents include bronze, gray, green and blue. As the glass gets thicker, the density of the color also increases. This causes the glass to transmit less visible light. The light transmittance of tinted glass varies from 14 to 83 percent depending upon its color and thickness.
