mi sembra di aver capito che si tratta di un difetto abbastanza comune. sono perdite (let goes) di aria o vapore che provacano appunto le bolle nel vetro.
dai un'occhiata ai link che ti mando, ci sono anche foto.

During the production of stemware glass, a glass defect bubble in stem appears uncontrolled. This bubble is in most of the cases a vacuum bubble. It is estimated that the reject due to this defect is about 4-6%. Aim of this project is to reduce this reject level.

Air bubbles, voids Air entrapment in and between plies; noninterconnected spherical voids. Whipping action during mixing. Poor or dirty surface. Air bubbles not rolled out during lay-up.

Identification of following bubble sources:
Bubbles

Stones

Cords


MASS SPECTROMETER GIA 707
• Batch Melting
• Reboil
• Refractory Outgassing
• Electrolysis
• Mechanically Entrapped Bubbles
• Redox Change Causing Bubbles
• Water Leakage
Identification of bubble sources is based
on following analytical data:
• Bubble gas composition. The following gases are analyzed:
  O2, N2, CO2, Ar, H2, COS, H2S, CO, H2, (H2O). Some other
  gases can be analyzed.
http://www.tpd.tno.nl/smartsite363.html