Six Finger Guild
The glassblowers guild uses three principal methods of forming window glass.
Broad Sheet glass is created by first blowing an elongated cup shape. The glassblower shapes the cup into a rough cylinder. When the cylinder reaches the proper size, shears are used to cut away the cone portion. The resulting cylinder is cut from end to end. The cut cylinder is unrolled onto an iron table to cool. The broad sheet glass has many imperfections and is not optically clear. Due to the limited size of the Broad Sheets, multiple glass panes are latticed together to form larger windows.
Crown glass is created by taking a gather of glass on the end of a blowpipe. The glassblower creates a bubble or globe. The glassblower opens up the globe just as if he was making a bowl. He then spins the bowl until centripetal force has fattened and stretched the glass into a circular plate of 5 or 6 feet in diameter. Once the glass cools it is cut to the size required. The band at the edge of the disk was both the thinnest and clearest glass. In order to fill large window spaces with the best glass, many small diamond shapes are cut from the edge of the disk and these are mounted in a lattice work and fitted into a window frame.
The cylinder process creates very large sheets of glass. The process starts similar to Broadsheet glass. The glassblower starts by forming a large bottle. He then swings the bottle in a deep trench to make the bottle elongate to about the height of a man. The long glass bottle is then allowed to cool and removed from the blowpipe. The bottle ends are cut off and a long cut in made down the length of the cylinder. The cylinder glass is then placed into an oven and re-heated. As the cylinder heats up it is unrolled to form a flat glass sheet. The result is much larger panes and improved surface quality over the broad sheet process. The cooling and reheating also leads to a more transparent glass.