HP screen screen rot

That would seem likely since the glass was once melted in order to make the CRT. Whether continuous arcing damages the glass permanently or not is another question. I would guess that the heat in itself does nothing irreversible, but arcing does. I found this in a text about types of glass and manufacturing procedures for CRTs. It did not mention whether the damage was reversible or not. Your observation about connecting a glass rod across the mains and heating it certainly supports their warnings about reduced dielectric strength. On the other hand, I would suppose that severe localised heating could cause the glass to crack due to local expansion creating excessive stresses, so I would still be very careful with the soldering iron. /Jonas