Glass blowing had been practiced for over 500 years by the start of the Middle Ages. The technique, which had been developed by the Phoenicians and refined by glassmakers from the Roman Empire, was further developed and perfected in the regions of Europe which are now the nations of Germany, France, and Belgium.
During the early Middle Ages, the Franks began experimenting with various molds and methods of decoration, which elevated the art of glass blowing to a new level. Most of these new techniques were used in the making of drinking vessels and other glassware intended for ceremonial purposes. These examples of glassmaking were characterized by glass of a green or brown color; a result of impurities in the silica and potash used to manufacture glass. However, as the Middle Ages progressed, so did the art of glassmaking.
Even before the Middle Ages, Italy was recognized as one of the glassmaking centers of the world, and during the mid to late Middle Ages, artisans from Venice and Murano began producing more elaborate pieces of glass, such as lamps and “cristallo”, a finely crafted type of glassware that is still highly sought after today. Along with Italy, the Germanic lands along the Rhine River were renowned for glass production during this time.
Glass blowing was paired with the cylinder and crown methods of glassmaking to produce window panes in the early 11th century. However, archaeologists have uncovered evidence that glass windows were in use as early as 100 AD in Alexandria. These examples of the earliest windows were rather crude by today’s standards; they were made with cast glass which did not have the optical abilities of glass windows produced using the glass blowing techniques of the Middle Ages.
The 11th century saw the development by German glass craftsmen of a technique which was later utilized by Venetian glassmakers in the 13th century for the production of glass sheets, which were used for windows. These Middle Age windows were made by blowing a hollow glass sphere and swinging it vertically, where gravity would pull the glass into a cylindrical pod. While the glass was still molten, the cylinder would be cut into shape and laid flat.
By the 13th century, glassmakers were forced to leave their workshops in Venice because of the many fires which were caused by the furnaces. As a result, Italian glassmaking shifted primarily to Murano, although some glassmakers still operated out of Genoa. It was these Genoan glassmakers who, from the 14th to 16th centuries, further refined the practice of glass blowing by finding ways to remove impurities. These developments were then perfected by the glassmakers from Murano, who by the end of the Middle Ages, had discovered how to use quartz sand to manufacture pure crystal.