Bubbles in old glass bottles and windows are actually air pockets that became trapped during the manufacturing process. Crude glass almost always contains bubbles, which often adds to its appeal and value among collectors.
According to AntiqueBottles.com, bubbles are rare in glass produced after 1920, so the presence of a bubble may help to date a bottle or window.
AntiqueBottles.com also states that collectors do not view bubbles as "damage." Some even prefer bubbles because they add to the visual appeal of the glass.
A group of small bubbles clustered together are called "seed bubbles."
While interior bubbles are considered desirable, an "open bubble" usually is not. Open bubbles are close to the surface of the glass and appear as a divot, indicating that the bubble's outer edge was broken over time.
People restoring old homes prefer glass with bubbles in it to keep with the historic flavor of the architecture. In 1992, Steven Jayson, vice president of S. A. Bendheim, told the New York Times people were willing to pay a premium for glass that contains bubbles.