Pure glass shrinks less in high temperatures than metal does. The mismatch causes metal to crimp, compressing the seal. That has both advantages and disadvantages.

Leak tests on the glass-to-metal seal come in at less than 2 X 10 10 atm-cc/sec of helium. This is significantly lower than conventional sealing methods. FloMet LLC 810 Flightline Blvd. DeLand, FL ...

A process that takes place at temperatures to 1,000°C hermetically seals optical glass to an alloy frame. A key point: the glass and alloy have the same coefficient of thermal expansion.

They are assembled using a glass-to-metal seal process combined with gold germanium brazing, resulting in a rugged and reliable package. The packages can handle temperatures up to 360 degrees Celsius.