I've been printing PLA directly on the glass with some smears of Elmer's purple glue sticks. I run the bed at 60c for parts that I think are likely to lift (long, thin arms) and I run it cold on parts that I expect to stick well. Everything has been flawless. For parts with a small base, I give a quick twist and tug to remove them and for anything that is fairly well-stuck I remove the glass, support it on my lap and give it a quick swipe with a drywall scraper and it pops right off.
A few days ago I tried printing ABS for the first time. I used the default settings for extruder and bed (235c, 110c). Once again I printed directly on the glass with Elmers. The print came out fine. I allowed the bed to cool, then removed it from the printer and gave my standard swipe with the scraper. The part popped off, but a piece of glass from the build platform came with it (stuck VERY firmly to the part). I'm pretty surprised by this. Anyone experienced this before? I haven't done much printing on glass with other printers and I've really been enjoying how easy and maintenance-free this is compared with other options (blue tape, vinyl, etc.). Is the glass supposed to be consumable? Have I done something wrong? You can see the missing piece in the middle of the "M2" logo in this picture.

cheers,
c