When did glass stop having bubbles in it?

Time Frame. 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.

What does it mean if glass has bubbles in it?

Bubbles can occur in glass either because the glass-maker intends it, as part of the design, or because the techniques used were so crude that bubbles remain, as sometimes found in very old glass or in the work of novice glassmakers. The Italian name for glass with a regular pattern of air bubbles is bullicante.