How Flat Glass Is Made

We have a rough idea of the ingredients that make up glass, but how do these powders turn into large sheets of glass? The answer is a long production line known as a float glass process.

The raw materials are mixed together in a batch blending process. Then they are fed into a very large furnace. These furnaces are 140 feet long and 30 feet wide. In the furnace, the ingredients are heated to about 2700° F at which point they form into molten glass. The glass melt is then moved into a delivery channel where it is brought to a uniform 2200° F. The melt passes through a control gate and then is poured onto a shallow pool of liquid tin. This part of the process is the really cool part.

The glass melt floats on the pool of tin. This creates very smooth and uniform glass and is a huge improvement over past glass manufacturing techniques. Tin is really an optimum choice as the liquid for the flotation pool. Tin is relatively dense which insures that the glass melt will float on the surface of molten tin rather than sink. Glass and tin are immiscible. This means they will not mix together.

A byproduct of tin, tin dioxide, will adhere to the glass which is problematic. If tin is in the presence of a source of oxygen, air for instance, tin dioxide readily forms. Glass manufacturers keep oxygen away from the tin by using a pressurized mixture of nitrogen and hydrogen gas.

As the layer of glass floats across the surface of the pool, it begins to cool and solidify. Once the glass becomes solid, it is lifted off the bath and onto rollers at 1100° F. The rollers mark the beginning of a 350 foot long annealing chamber, a lehr kiln, that allows the glass to slowly enough to avoid cracking or otherwise damaging the glass. After the glass leaves the lehr kiln section of the manufacturing process it is ready to be cut into smaller panes.