Front of old townhouse in Old Town Alexandria, Virginia near Washington DC. Wavy glass is depicted. PHOTO: S E Brendel, Wikimedia
Wavy glass is the cool-looking glass commonly found in older window panes, doors, and furniture built prior to the early 1900s.
Early manufacture of glass involved single sheets of glass manufactured by a craftsman by blowing through a tube. Generally the further back in history you go, the wavier the glass is. Early glass also has tiny bubbles called seeds. As craftsmen improved their methods over time, the wave and distortion became less apparent. As a result, glass produced in the 1700s tends to have more distortion than glass produced in the 1800s. In the early 1900s, increasing industrial advances led to machine-produced glass. This glass, while less wavy, still had imperfections and was widely used in the United States cities in the early 1900s.
Crown Glass
Spinning crown glass. Image copyright Pioneer Glass.
Crown Glass was used in Europe starting in the mid-1300s. A glass-smith, using a hollow tube, would blow liquid glass which made a hollow globe called a "crown". This was attached to a rod which was then spun to make thin sheets of glass which were cut into window panes. Because the glass was spun in a circular motion, the "waves" were more rounded. The center piece had an end-knob imperfection (commonly called a crown or bullion) which was sold as part of the window or more frequently as a small piece of decorative glass over or alongside windows and doors. Today this is referred to as a bulls-eye. In the mid 1600s cylinder glass started to gain market share from crown glass. Many people viewed crown glass as a superior product to cylinder glass due to its brilliance. Despite the higher cost, crown glass continued to be manufactured into the 1800s.
Cylinder Glass
Making cylinder glass. Image copyright Pioneer Glass.
Cylinder-blown glass was also started by a glass-smith blowing through a hollow tube. The glass was then swung in a trench rather than spun. It then cooled before it was put onto sheets and reheated. This allowed much longer sections (panes) of glass to be manufactured and lowered production cost. Due to the swinging of the glass during manufacture, the waves in cylinder-blown glass were less curved and more parallel. The advantages of cylinder blown glass were price, uniformity of thickness, larger sheets, and less manufacturing waste. Cylinder glass became the standard window glass panes in the 1800s. The glass still had many waves and bubble pockets. An early version of cylinder blown glass called broad sheet was generally not used for window glass.
Machined Cylinder and Drawn Glass Era
Looking through glass that underwent the Fourcault process in Santiago, Chile.
In the early 1900s, a mechanized method of producing cylinder glass was invented. This glass still had some distortions but could be made into even larger panes and produced in much larger amounts with less manpower. This was a time of major building, so this kind of production was much needed. This type of glass is still very widespread today in American cities. It is very similar to cylinder-blown glass with a somewhat less distortion and is known for its vertical lines.
The Fourcault process and Colburn processes for making vertical drawn glass in the second decade of the 1900s further mechanized glassmaking. This glass could be produced in wider sheets at cheaper costs. These advances led to even less distortion but by no means perfect glass.
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