<P> One of the earliest methods of glass window manufacture was the crown glass method . Hot blown glass was cut open opposite the pipe, then rapidly spun on a table before it could cool . Centrifugal force shaped the hot globe of glass into a round, flat sheet . The sheet would then be broken off the pipe and trimmed to form a rectangular window to fit into a frame . </P> <P> At the center of a piece of crown glass, a thick remnant of the original blown bottle neck would remain, hence the name "bullseye". Optical distortions produced by the bullseye could be reduced by grinding the glass . The development of diaper latticed windows was in part because three regular diamond - shaped panes could be conveniently cut from a piece of Crown glass, with minimum waste and with minimum distortion . </P> <P> This method for manufacturing flat glass panels was very expensive and could not be used to make large panes . It was replaced in the 19th century by the cylinder, sheet, and rolled plate processes, but it is still used in traditional construction and restoration . </P> <P> In this manufacturing process, glass is blown into a cylindrical iron mould . The ends are cut off and a cut is made down the side of the cylinder . The cut cylinder is then placed in an oven where the cylinder unrolls into a flat glass sheets . </P>

What do you call a sheet of glass
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