Broad sheet is a type of hand-blown glass. It is made by blowing molten glass into an elongated balloon shape with a blowpipe. Then, while the glass is still hot, the ends are cut off and the resulting cylinder is split with shears and flattened on an iron plate. (This is the forerunner of the cylinder process). The quality of broad sheet glass is not good, with many imperfections. Due to the relatively small sizes blown, broad sheet was typically made into leadlights.
According to the website of the London Crown Glass Company, broad sheet glass was first made in the UK in Sussex in 1226 C.E. This glass was of poor quality and fairly opaque. Manufacture slowly decreased and ceased by the early 16th Century. French glass makers and others were making broad sheet glass earlier than this.