how is window glass made so it comes out so flat

Glass comes in many shapes & forms. At one time most of the glass manufactured in the United States was plate glass. Plate glass was made by a process of grinding & polishing. No longer made in this country, plate glass has been replaced by float glass.

Float glass is a term that refers to a process of making glass that was perfected in 1959 by Pilkington Brothers, Ltd. of England. Float glass is made by pouring the molten glass from a furnace into a chamber that contains a bed of molten tin. The atmosphere inside the chamber is carefully controlled. The glass floats on the tin & 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 length of the glass from the furnace to the cutter is about a mile. 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, & has smooth, parallel surfaces. Automatic cutters trim the edges & cut the glass to length.

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