Float Glass
Float glass is flat, smooth, uniform panels of transparent glass used to manufacture most if not all of the window panes used in the construction, automotive and engineering industries. It is currently made with either borosilicate or soda lime glass, which contain additives to the standard materials that produce glass-sand, limestone, soda ash, dolomite, iron oxide and salt cake. Most glass panes also contain shards of cullet, which is recycled glass, and helps cut down on melting time. Additives for color and refining are also sometimes added. Float glass is manufactured by the Pilkington Process, where melted glass is fed into a molten tin or lead bath. Since it is less dense, the glass floats to the top to form a flat, totally uniform surface.
The Pilkington process starts with heating the glass elements to 1500 degrees C until they have melted, and feeding them into a molten bath through a delivery canal. After the glass forms the top layer, it is cooled to 600 degrees F. Some of the tin is absorbed into the glass on one side, causing an invisible haze that only effects glass if it is colored. This takes place in a controlled environment of hydrogen and nitrogen, which prevents the tin or lead from oxidizing. The glass pane is then moved via rollers to a kiln where it is cooled gradually and annealed without cracking. After it returns to room temperature, the roller dents are trimmed off the edge of the glass pane.