What glue was used in vacuum flourescent displays?

[Refrigerator]

It surprised me when i saw that a VFD is just glued together.
I have three salvaged VFDs, one made in Japan, one in USSR and another one of unknown origin, yet they all share one thing in common - they are all glued together with a grey glue.
VFDs need a high vacuum to operate and normally when you think of high vacuum stuff like expensive dumet wires, hydrogen leak detectors and glasswork with a gas torch come to mind.
Yet the VFDs i have are stuck together like an aquarium and seem to hold vacuum just fine as evidenced by the getter showing good health in all three.
And for the longest time i've wanted to play with some high vacuum stuff but couldn't because i didn't have the tools to work with glass and dumet wires were hard to get.
But if i could just glue some pieces together then it would be so much easier.

But the question remains, what glue did they use to stick these VFDs together? Does anyone have a clue?

[retiredfeline]

You're sure the glue is not just filler and there is a vacuum sealed unit behind? Game to take it apart?

[Gyro]

It is almost certainly glass 'frit' seal, as used in ceramic ICs...

https://en.wikipedia.org/wiki/Glass_frit_bonding