Making of crown glass.
In this illustration you can see a craftsman making crown glass or "bullion" glass. This was one of the earliest forms of window glass and many will be familiar with images of diamond quarry and rondel windows utilizing many small pieces of glass leaded into window. One of the characteristics of this glass is the appearance of concentric striations produced while the molten glass is being spun into shape.
Crown glass was an early type of window glass and can be seen in medieval windows and is a traditional glazing that is still in limited use today. In this process, glass was blown into a "crown" or hollow globe. It would then be flattened by reheating and spinning out the bowl-shaped piece of glass (bullion) into a flat disk by centrifugal force, of various diameter. The glass was then cut to the size required.
The thinnest glass was in a band at the edge of the disk, with the glass becoming thicker toward the center. Sometimes referred to as bullseye glass, the thicker center area around the punty mark was used for less expensive windows. In order to fill large window spaces with the best glass, many small diamond, or quarry shapes would be cut from the edge of the disk and these would be assemble with lead came and fitted into the window frame.
Crown glass was one of the two most common processes for making window glass until the 19th century. In the United States you might find examples of this type of glass in the earliest colonial buildings but generally it is a less common form of glazing. The more prevalent would be the mouth blown cylinder glass which I will discuss in my next entry.