Traditionally, window glass was made by hand by either the crown
or cylinder process. In the crown process a gob of glass was blown
out and one side of the resulting globe was flattened. A solid iron
rod was attached to the flat part and the blowing pipe detached. The
globe was then reheated and rotated until it formed a flat disc about
one meter (3 feet) in diameter. Panes of glass were cut from the disc
after it was slowly cooled. The part attached to the rod was the
"bull's eye," which can still be seen in some older windows. In the
cylinder process the blower made a large cylinder that was then split
open and flattened. Cylinder glass was also made by machine.
In the early part of the 20th century the Fourcault and Colburn processes for drawing sheet glass directly from the glass melt were developed. When used in conjunction with a continuous glass-melting furnace these processes are capable of producing large quantities of flat glass of reasonable quality. Until recently, plate glass of the highest quality was made by flowing the glass from the furnace through rollers. The rough-surfaced glass is then ground and polished by large automatic machines. This process requires a large capital investment but is economical since it produces large quantities of glass continuously. The ground and polished plate glass is very flat but is more expensive than the sheet glass that is drawn directly from the melt. The sheet glass surface has a fine fire-polished finish, but shows some surface distortion because of variations in processing conditions as the glass is drawn from the melt.
In the 1950s an ingenious new method of making relatively inexpensive flat glass of high quality was developed in England by Alistair Pilkington, of the Pilkington Glass Co. In this float process a continuous strip of glass from the melting furnace floats onto the surface of a molten metal, usually tin, at a carefully controlled temperature. The flat surface of the molten metal gives the glass a smooth, undistorted surface as it cools. After sufficient cooling the glass becomes rigid and can be handled on rollers without damaging the surface finish. The glass can be formed at high speeds and is much less expensive to produce than similar quality glass made by grinding and polishing. As a result, many glass manufacturers have converted to the float process, and today most flat glass is made by this process.