Valve in a valve?

[Nick]

I read that glass retains a lot of the stress of manufacture and that's why it flies when broken.  Maybe those stresses become more brittle with age and UV.  Certainly trying to cut old window glass never worked even with a diamond scribe.

Glass is a complex & subtle material, however after being hot-worked, it's annealed - see https://en.wikipedia.org/wiki/Annealing_(glass)

The videos I've seen of the Blackburn works valve production lines certainly show an annealing process as part of the production line (*), so internal stress should not be an issue, even for early valves as this is a well known problem.

(*) there are normally several annealing phases, one after each major hot-glass procedure.