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Fernico describe a family of metal alloys made primarily of iron, nickel and cobalt. The family includes Dumet. The name is made up of the chemical symbols of its constituent three elements. Dumet is a portmanteau of 'dual' and metal', because it is a heterogeneous alloy, usually fabricated in the form of a wire with an alloy core and a copper cladding. These alloys possess the properties of electrical conductivity, minimal oxidation and formation of porous surfaces at working temperatures of glass and thermal coefficients of expansion which match hard borosilicate glass closely. These requirements allow the alloys to be used in glass seals, such that the seal does not crack, fracture or leak with changes in temperature.
Valve pins and lead-out wires use Dumet wire. Made of a piece of iron wire which is copper plated a few millimetres long. At the upper end is a nickel wire spot welded and at the lower end a copper wire as connection to the base-pins of octal like valves or pins for all-glass valves.
The copper plated wire adheres to the lead-glass of the pinch and the iron has the same temperature coefficient like the glass so that no crack occurs which may result in a bad vacuum later.
Dumet wire has a red colour in the pinch or glass foot. Today Dumet is still used in light bulb production and thus available.
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