Crown glass was an early type of window glass. In this process, glass was into a "crown" or hollow globe. This was then transferred from the blowpipe to a punty and then flattened by reheating and spinning out the bowl-shaped piece of glass (bullion) into a flat disk by centrifugal force, up to 5 or 6 feet (1.5 to 1.8 metres) in diameter. The glass was then cut to the size required. The thinnest glass was in a band at the edge of the disk, with the glass becoming thicker and more opaque toward the center. Known as a bullseye, the thicker center area around the punty mark was used for less expensive windows. In order to fill large window spaces with the best glass, many small diamond shapes would be cut from the edge of the disk and these would be mounted in and fitted into the window frame. Crown glass was one of the two most common processes for making window glass until the 19th century. The other was blown plate. The process was first perfected by French glassmakers in the 1320s, notably around Rouen, and was a trade secret. As a result, crown glass was not made in London until 1678.