The making of a small glass tube begins with the pulling of molten glass to form a tube. The tube is then reheated with a torch, the end is closed, and air is blown to create a small bubble. Learn more in The Techniques of Renaissance Venetian Glassworking by William Gudenrath http://renvenetian.cmog.org/chapter/earliest-inflated-glass
The Venetian glass industry enjoyed a golden age during the Renaissance. By the early 1500s, the wonders of Venetian glass were well known throughout western Europe. Not well known, however, was how these objects were made. Until now. The result of 30+ years of research by William Gudenrath, The Techniques of Renaissance Venetian Glassworking – http://renvenetian.cmog.org – presents 35 complete reconstructions of Venetian glassmaking techniques through detailed 360˚ photography and high-definition video.