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.
source/image: Corning Museum of Glass
Compared with the bottles, the tubes seem markedly primitive in technique.n cross section, the tubes are noticeably out of round. There appears to be a vestige of a seam running lengthwise on each example./read more (renvenetian).
Thus, it looks as if a somewhat thick, flattened mass of hot glass was rolled up, reheated, and drawn out to make a section of tubing.
After the glass was annealed (cooled), one end of a tube was reheated, perhaps by holding the tip inside a small furnace or in a flame, just as modern flameworkers might do. In this process, as the glass softens, surface tension causes the glass to thicken. Eventually the hole closed. Air was immediately blown forcefully into the other end of the tube, and the still soft glass was inflated to form a bulb.