The glassmaking technique of spinning a molten gather into a circular sheet produces crown glass.
In the traditional process, a glassblower gathers molten glass on the end of a blowpipe, opens it into a disk, and rapidly spins it. Centrifugal force spreads the hot material outward into a roughly circular plate. The finished disk is cut into panes, often leaving a central thickened or distorted area known as the bullseye.
Crown glass was widely used for windows before industrial flat-glass methods became dominant. Its variations in thickness and optical distortion are part of the visual character of many historic windows, including older stained-glass installations.
It is different from cylinder glass, which is blown into a long hollow cylinder before being cut and flattened. Float glass, by contrast, forms on molten tin and is designed to be exceptionally flat.