Crown glass is a type of hand-made window glass produced by blowing a bubble of molten glass, transferring it to a pontil rod and spinning it rapidly until centrifugal force opened it into a large, flat disc. The disc, known as a table, was then cut into panes, leaving a thickened centre where the pontil had been attached, known as the bullion or bullseye.
Crown glass was the principal window glass in Britain from the seventeenth century until the mid-nineteenth century, when it was gradually replaced by cylinder glass and later by cast and polished plate glass. It has a brilliant, fire-polished surface with gentle curved ripples, slight variations in thickness and occasional seed bubbles, which give old windows their characteristic shimmer. Because panes cut from a disc were limited in size, Georgian sash windows were divided into many small lights by glazing bars. The bullseye centres were originally cheap offcuts used in less important windows, and they are now associated with picturesque cottages and inns. Surviving original crown glass is precious in historic buildings, and conservation officers generally encourage its retention when windows are repaired.
