Glass to metal seals are what intimately and hermetically seals metal parts within a glass housing or tube. The glass and the metal alloy need to be carefully selected to have the nearest coefficient of liner expansion. A gradual heating cycle is then used to heat up the assembly so that the glass part is melted to form around the metal parts. The Nickel Iron alloy blades of a reed switch are sealed into the glass tube is such a way and this is what makes the reed switch contact sealed away from the effects of the atmosphere.
Glass-to-metal seals are a very important element of the construction of vacuum tubes, electric discharge tubes, incandescent light bulbs, glass encapsulated semiconductor diodes, reed switches, pressure tight glass windows in metal cases, and metal or ceramic packages of electronic components.
Properly done, such a seal is hermetic (vacuum tight, good electrical insulation, special optical properties e.g. UV lamps). To achieve such a seal, two properties must hold:
Thinking for example about a metal wire in a glass bulb sealing, the metal glass contact can break if the CTEs (coefficient of thermal expansion) are not well aligned. For the case that the CTE of the metal is larger than the CTE of the glass, the sealing shows a high probability to break upon cooling. By lowering the temperature, the metal wire shrinks more than the glass does, leading to a strong tensile force on the glass, which finally lead to breakage. On the other hand, if the CTE of the glass is larger than the CTE of the metal wire, the seal will tighten upon cooling since compression force is applied on the glass.
According to all requirements that need to be fulfilled and the strong necessity to align the CTE of both materials, there are only a few companies offering specialty glass for glass-metal sealing, such as SCHOTT AG and Morgan Advanced Materials.