Does the diellectric strength increase back to
its original value on
cooling again? If so, then this is not really a problem if you happen to
heat a CRT part (pin, bit of the envelope) with a soldering iron while
working on the device.
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
I think an arc can actually puncture the glass, in which case the vacuum
will leak out. Not a Good Thing
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
Incidentally, some other materials that you would think are good insulators
show the same behaviour. Things like calcium oxide, zirconium oxide, and
some other rare earth oxieds. Look up 'Nernst Lamp' (or 'Nernst Glower').
Quite an intersting devices that was seriously proposed as an alternative
for the metal filament in an intert atmospehrre (or a vacuum) bulb at one
point. It wasn't much good for that, bnt it was used as an IR light
source doe a time, the point being that the glowing oxide rod could run
in air, you didn't need a surrounding glass envelope that could absorb
some of the IR.
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.
I think anyone who soldered aroudn a CRT with the power on deserves all
they get (and a damaged CRT is likely to be the elast of their problems).
But of course a cracked seel will damage the CRT even when the power is
off, so it's something to be aware of. I would never solder directly to
the pins of a CRT , or solder to the socket contcats without removing
it from the CRT. If the CRT has side pins, e.g. for deflection plates,
the same applies.
 There is one eception to this. If the CRT has a separate base
moulding -- that is int's not na 'all glass' base with the pins sealed
into the glasss envelope,. it can be worth trying to resolder the pins if
you have what appears to be an open-circuit electrode connection. But use
a hot ireon (so it soldered qucikly) and take care.