1 Nov
2011
1 Nov
'11
8:55 a.m.
Jonas Otter wrote:
Heat-worked glass - cast or blown - is a lot like worked carbon or
stainless steel. There will be localized stresses after working and if
it's not tempered can crack or shatter spontaneously, with no more
external stimulus than a few degrees' temperature change.
Cast glass is more homogeneous than worked or blown, but it still
needs tempering. I'd be very surprised if the casting or blowing spec
for a CRT tube didn't include detailed specifications for the cool-down
rate.
Doc Shipley
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
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.