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. To achieve such a seal, two properties must hold:

1. The molten glass must be capable of wetting the metal, in order to form a tight bond.

2. The thermal expansion of the glass and metal must be closely matched so that the seal remains solid as the assembly cools.

When one material goes through a hole in the other, such as a metal wire through a glass bulb, and the inner material's coefficient of thermal expansion is higher than that of the outer, it will shrink more as it cools, cracking the seal. If the inner material's coefficient of expansion is slightly less, the seal will tighten as it cools, which is often beneficial. Since most metals expand much more with heat than most glasses, this is not easy to arrange.