Can't really see if the defects are nearing the surface or fully embedded in the glass. I had some success looking at issues in tempered multi-layered glass for windshield, using standard Scanning Electron Microscopy Techniques.

If on the surface, just metal coat and look/analyze with EDS and you will at least see if you have a relative abundance of some element that is too low or too high, or unexpected elements (inclusions).

If embedded, you'll need to embed a small piece to produce a cross section and polish until you reach the defect for the same type of analysis. Might also be able to use a broad ion beam polisher, a FIB-SEM, a Laser-FIB-SEM if you can get your hands on one.

One alternative is to go for TOF-SIMS and "dig" into the glass using SIMS ion milling and see what you get as you reach the inclusion.

Dear Pierre Caulet ,

Thanks for your kind response, actually there are some embedded defects in glass. I'll check the cross-section as you mentioned.

Identifying glass defects involves various methods, such as visual inspection, non-destructive testing(NDT), and laboratory analysis, Visual inspection can reveal surface defects, While NDT methods like ultrasonic testing or radiography can detect internal flaws. Laboratory techniques like spectroscopy or microscopy help analyze chemical and structural aspects. Combining these methods provides a comprehensive approach to identify and understand glass defects.