This early technique for making flat glass involved blowing a glass bubble, transferring the glass bubble from the blowpipe to a punty, opening the end of the bubble, and then spinning the glass so that centrifugal force turned the glass into a large spinning disk. That disk of glass - called a bullion -- was then annealed and cut into (usually rectangular) sheets. Bullions are also referred to as crowns.
The center of the bullion, where the punty was attached, was typically uneven and had concentric, ripple-like ridges. This center section is called the "bull's eye" and, though it eventually became a desirable piece of glass for windows, was originally considered waste.